Topic outline

  • General


    Key unit competence: By the end of this unit, I should be able to interpret statistical data, construct statistical graphs, diagrams and maps

    Introductory activity 

    Read the story below and use it to answer the questions that follow:

    Rwanda exported  over 250b frw Agricultural products in 2017. The National Agriculture Exports Development Board (NAEB), says the country’s agricultural exports generated revenues amounting to 304.6 million frw from January to September 2017 compared to 232.65 million earned in the same period in 2016. In the first quarter of 2017/18, agricultural exports generated 116 million frw against 82,809,581 frw in the first quarter of 2016/17. Agricultural crops locally produced and exported like coffee, tea, and pyrethrum generated 108 million frw from January to September of 2017, compared to 89 million in the same period in 2016.  The volume of exported vegetables was 18.98 million kilograms from January to September 2017, compared to over 15.61 million kilograms in the same period of 2016, meaning that the quantity of exports increased by only 21.56%. The value of exported vegetables was 8 billion frw from January to September 2017, against 4.2 billion frw in the same period of 2016, implying an increase of 98.65 percent, and a rise in price per unit of the vegetables.

    Now answer the following questions: 

    a. Name the data which are represented in the passage?

     b. Using the past studies in both ordinary level, senior four and personal research:

         i. Identify other methods to represent the data displayed in the passage.

         ii. Discuss the importance of the geographical data identified in (a) above and make a class presentation, if possible using ICT tools, or flip charts or any other resource available.

        iii. Comment on the Rwanda’s exports between January 2017 to Oct 2017 and account for the variations in the trend of exports.

    1.1. Definition of statistics and importance of statistical graphs and diagrams in geography

    Learning activity 1.1 

    Read the story below and use it to answer the questions that follow: Micomyiza Jeanne and Habinshuti Gilbert made their field study on ‘The effects of human activities on the physical environment in Twubahane village. They used various methods of data collection. Jeanne was interested in writing down every observed aspect in a descriptive manner while Gilbert filled his note book with tables and charts that he would interpret after the field trip. Some other students who had also attended the fieldwork study, used various methods of noting down data, such as line graphs, bar graphs with description foot notes. They could look at the graphs and charts drawn and easily understand the content contained therein. The collected numerical data revealed that 12 farmers were using poor methods of farming, 20 farmers had applied agro-forestry and tree planting, 2 farmers were engaged in charcoal burning, 7 people had large herds of cattle whose impact on the environment was so evident. All data collected were presented and recorded on a chart.

    a. Explain why most students were interested in using the statistical way of data recording. 

    b. Does the passage above represent statistics? Explain your answer.

    c. Using the example of Habinshuti, examine the importance of statistical graphs and diagrams in Geography.

    1.1.1 Definition of Statistics

     The term statistics is defined in many ways by various authors. The term holds its origin from the Latin word “Statisticum collegium” that stands for council of state, later it changed into the Latin word “Status” that means a group of numbers or figures, the Italian word “Statista” (statesman) and German: “statistic” that means (Analysis of data about the state). This shows that originally statistics were used for governmental purposes. However, later the usefulness of statistics in the real-world experiences increasingly became important. The term statistics therefore, refers to the science of collecting, analyzing, interpreting and presenting various data. It is a science of how data are collected, analyzed and interpreted. Statistics deals with data. It is very necessary to understand the meaning of the term data. Data is a word used to mean the information used as a foundation for reasoning, discussing and calculating. This implies that statistics and data go hand in hand.

    1.1.2: Importance of statistical graphs and diagrams in geography

     Statistical graphs and diagrams play a key role in geography. They are needed in various aspects of human activities in this 21st century. They are important in the following ways:

    1. Statistical graphs and diagrams help students to summarize huge and a wide range of information for future analysis. Diagrams and graphs hold huge information in accurate way.

    2. The geographers can identify and interpret the relationship existing between various variables. For example, the relationship between the increase of prices for commodities and increase in cost of fuel.

     3. Facilitate easy understanding of various geographical variables and clearly show the trend and variations of production, trade, among many others. 

    4. Statistical diagrams and graphs enable the learners to acquire various skills such as analytical skills, interpretation and presentation of geographical data. Such skills can be used in other areas after school.

     5. Most of statistical diagrams and graphs enable the students and geographers to save time as data is being recorded down. 

    6. Statistical graphs and diagrams provide a good and attractive visual impression. Therefore, arousing the interest of the reader or geographer in what is portrayed by the graph or diagram. 

    7. The data contained thereon the statistical graphs and diagrams, are used in making effective decisions. Therefore, logical judgmental conclusions are made based on factual data. 

    8. Statistical diagrams and graphs facilitate easy memory of the reader or students/geographers. Data and geographical information presented on diagrams and graphs are easily remembered than the data presented in a descriptive manner.

     9. They help geographers to predict the trend of geographical events of various phenomena. For example, if a given graph shows a positive increase trend of coffee in country X, for some considerable years, it becomes easy to make predictions.

    Application Activity 1.1 

    Using first hand experiences and skills acquired in statistics, explain how statistics is important in your daily life and show how it would help to understand geography.

    1.2. Line and curve graphs

     Learning activity 1.2.1

     Land use in country X from 2015 to 2017

    Using the knowledge and skills acquired from Mathematics and the data provided in the table above, answer the following questions: 

    a. Draw a simple line graph to represent the above information

    b. Use the same data indicated in the table above, to construct a simple curve graph

     c. Using both the data and the graphs drawn, comment on the trend of land use in country X from 2015 to 2017. 

    d. Discuss the advantages and disadvantages of simple line graph

    There are several types of line and curve graphs. They range from simple to more complex depending on the composition and nature of the statistical data to be represented or portrayed. Under this category of statistics in geography, the following are the main line graphs:

     i. Simple line and curve graph

     ii. Group or comparative line graph 

    iii. Compound line graphs 

    iv. Divergence graphs

    1.2.1. Simple line and curve graphs

     A line graph is one which shows plotted points connected by a series of short straight lines. This distinguishes it from the simple curve graph in which plotted points are joined by a single smooth line. The illustrations 1.1 and 1.2 display the sample examples of the simple line and curve graphs used in Geography. With line graphs plotted points are observable while with curve graphs points are not noticeable. Also, with a simple line graph, the line suspends without touching the two-vertical axis while in a simple curve graph the smooth line drawn touches the two-vertical axis.

    Construction of a simple line graph 

    The following are steps to be followed while constructing a simple line graph. The same applies to the drawing of a simple curve graph. These steps are stated as follows

    1. The independent variables are indicated on horizontal axis. (Independent variables mean that when there is a change in the independent variable or variables, this results into a direct reaction of the dependent variables.) 

    2. The dependent variables are represented on the vertical axis. (Dependent variables are items that are reliant on the independent variables. That is to say; these are measurable responses from the reaction caused by a change in the independent variables). 

    3. The vertical axis should start from Zero. While the top part be slightly greater than the maximum value to be indicated or recorded on it.

     4. The length of the horizontal line is determined by the number of the independent values in relation to the scale.

    5. Draw two vertical lines on both sides and exact numbers of values of constant relationship recorded on the two axes. However, this is not a must be done condition.

     6. When plotting points in correspondence to both independent variables and dependent variables, it is appropriate to put the dot in the middle of the two vertical lines as shown below:

    Worked example

     Table 1. 1: The table showing temperature recorded at station B

    Use the data presented in the table above to answer the following questions that follow: 

     a. Construct a simple line graph to portray the above statistical data. 

    b. Draw a simple curve graph to illustrate the above statistics shown on the table.

    Advantages of simple line and simple curve graphs

     1. They are simple to draw or construct. 

    2. When neatly drawn, they provide a good visual impression.

     3. They are easy to interpret. 

    4. Less time is needed to construct them.

    5. Simple errors or error free because of the simplicity nature of the methods. 

    6. Simple calculations are involved.

     7. They can be used together with other statistical methods and diagrams such as dot maps in a super-imposition manner. 

    8. They have a multi-purpose service. For example, they are used in representing several geographical aspects such as the trend of crop production, variation in temperature, minerals, etc.

    Disadvantages of simple line and simple curve graphs

     1. There is no accurate interpretation because of some points are plotted basing on estimation of values. 2. There are difficulties in determining the appropriate scale especially when large figures or values are used. 

    3. The curve graphs are less accurate in comparison to simple line graphs. 

    4. It is not easy to use the simple line and curve graphs to compare various categories of data. 

    5. Changing scale of either axes, can greatly change the visual impression of the graph.

     6. They are only used while dealing with continuous data.

    1.2.2. Group or comparative graphs

    Group or comparative line graphs are also known as multiple line/curve graphs. This form of statistical method is used to portray more than one dependent variable. It uses several lines of which each represents a given specific variable of individual item. For group or comparative line graphs, the table has many dependent variables as shown below:

    The above table shows several dependent variables as well as the independent variables. There are 5 years and three crops such as coffee, tea and wheat. When critically analyzed, you find that for each independent variable in the left column (Crops) can be represented by a single simple line graph. The difference comes with group graph or comparative graph, each crop will be represented on the same graph.

    As earlier said, a comparative line graph has several lines as shown by the sample below:

    Construction of group or comparative line/curve graph

     Below are steps or guidelines to be followed while drawing a Group or comparative/ multiple line/curve graphs: 

    1. Several lines are constructed on the same graph. 

    2. The lines drawn representing individual variables or items should be easily distinguishable from one another. It is advised to use varying colours.

     3. Avoid congesting the graph with many lines. The recommendable maximum number of lines is five. However, if the data given is having more than five independent variables that determine number of lines to be drawn.

     4, The lines/curve should not cross each other unless the data given requires it. If it happens therefore, strictly varying colours or shading must be used. 

    5. Lines should not touch the vertical axis. 

    6. Names of variables being represented should be written directly on each line.

    Worked example:

    It is important to realize that lines are not crossing each other. The graph drawn shows that there was an increase in cultivated areas for all cash crops with exception for the area under fruits where a decreasing trend was observed.

    Advantages of group/comparative line graph

     1. It is a simple method as it does not involve a lot of calculations.

     2. It gives a clear impression of the different variables presented on one graph. 

    3. It can be used to show comparison of different items.

     Disadvantages of group/comparative line graph

     1. The presence of many variables on the same graph makes their interpretation more difficult. 

    2. The values for each period are not easy to determine.

    1.2.3. Compound line graph

    A compound line graph is also known as a cumulative line graph. It is the type of graph where a variety of variables are presented as subsequent lines. Therefore, a compound line graph displays cumulative totals. It is drawn in a way that lines are super-imposed above each other; creating several segments or components.

    Construction of a compound line graph

     The following are the steps that must be followed while constructing a compound line graph:

     1. Make a cumulative table representing progressive or cumulative totals of the provided variables.

     2. Select the appropriate scale for both dependent and independent variables. 

    3. Follow the steps used when a simple line graph is being drawn. It should be the variables with large or biggest values to begin.

     4. The lines constructed for each variable should connect onto the two vertical lines on both sides.

     5. Super-impose other lines on top of the first line beginning from each segment or component. Indicate the name of the variable in each section shaded.

     6. The lines drawn must begin from or attached to the vertical axis as earlier emphasized.

    7. Since plotting of points is dependent on the calculated values in a cumulative manner, lines therefore, must not cross each other. In case they do so, then, the data given, or the calculated cumulative totals will be having a problem. 

     8. If the statistical data is in large numbers, it is very advisable to convert the value of each variable into percentage. Thereafter, calculate the cumulative totals. Such calculations should be indicated.

    Worked example:

    The above cumulative totals can now be used to construct a compound line graph; as shown below:

    Advantages of compound line graph 

    1. They are easy to interpret. 

    2. They provide a good visual impression. 

    3. They are simple to draw.

     4. Less space is used.

     5. They can be used to present a variety of variables. 

    6. They are suitable for comparison purposes. 

    Disadvantages of compound line graph

     1. When large numbers are used, selecting a suitable scale becomes more challenging. 

    2. It or they involve calculations such as converting the values into percentages and then determining the cumulative totals. 

    3. The use of cumulative totals while constructing the compound line graphs makes it difficult to find precisely the actual values of the variables.

    1.2.4. Divergence line graphs

    Use the diagrams above to answer the following questions:

    a. Identify the differences existing between the two graphs presented above.

     b. Research on advantages and disadvantages of the graph showing the anomalies in mean rainfall at Kigali weather station.

    Divergence line graphs are statistical graphs that directly portray variations in values. When there is a rise in values; a plus sign (+) is used in the figure while a fall or decrease in values is portrayed using a negative sign (-). This type of graph is used while dealing with variables whose values are highly changing spatially and temporally. 

    Construction of a divergence line graph 

    There are six steps to follow when one is drawing a divergence line graph. These are:

     1. Draw two vertical lines (axis) and leave a reasonable space for the baseline; on which the independent variables are to be indicated. 

    2. Draw the average line commonly known as the zero line. This should join the center of the two-vertical axis. 

    3. Draw a basement line for independent variables; joining the bases of the two vertical lines. The length of the horizontal line is determined by the number of independent variables and the horizontal scale used.

     4. Determine the average values of all the values indicated in the table. This is obtained using the following formula:

    5. Deduct the average value from each value. The positive and negative signs should be indicated or shown.

    6. The figures or values with positive sign (+) are plotted above the Zero line or average line. While the negative values are plotted below or beneath the zero or average line. 

    7. While determining the scale, the vertical scale should be determined basing on the calculated divergences. The horizontal scale is chosen normally depending on the independent variables provided. 8. Thicken or darken the zero line/average line.

    Worked example: 

    Table 1.7: The table of monthly pineapple production in different areas

    The average value = 3,910.11 (in 000 tons)

     Therefore, the divergence table will be constructed below: 

    Table 1.8: The table of monthly pineapple production in different areas

    A divergence line graph showing monthly pineapple production in different areas

    Advantages of divergence line graph 

    Below is an outline of the main advantages of divergence line graph: 

    1. It is easy to construct 

    2. It provides a good visual impression. 

    3. It is easy to interpret 

    4. It is an appropriate method of representing items that need comparative analysis. 

    5. It requires limited space.

     Disadvantages of divergence line graph 

    1. It involves several calculations for example the total value, getting average or finding divergences. 

    2. The actual figures are not shown, only divergences are indicated instead. 

    3. There are some difficulties while determining the scale mostly when the divergences are composed of bigger values and very low ones. 

    4. Its construction is time consuming. 

    5. The method allows some inaccuracy while being drawn. This is because if large values are used, decimal points are neglected.

    6. To be used, it requires the reader to have the original raw data or a table of values.

    Application activity 1.2 

    1. Collect data on the students’ performance in any subject of your choice and use the scores to construct the following line graphs: simple and curve line graph, comparative, compound and divergence line graphs. 2. Comment on the distribution of statistical data displayed on the constructed graphs.

    1.3. Bar graphs

    A Bar graphs is one of the statistical methods used to show, portray or represent data. It is a graph constructed using rectangular bars. Bar graphs differ from line graphs because the lines used under bar graphs join to form single and individual rectangular bars. This means that bar graphs stand for a graphic way of numerical (quantitative) comparison by rectangles with heights proportional to the measure of data in question.

    The following are different types of bar graphs:

     Simple bar graphs 

    Compound bar graphs 

    Divergence bar graphs 

    Age-sex graphs

     Dispersion graphs

     Circular graphs

    1.3.1. Simple bar graphs

    A simple bar graph is created following the same procedures as a simple line graph. However, instead of using a line to represent the variables, parallel bars are used. The independent variables are indicated on the horizontal axis and dependent variables shown on the y-axis or vertical line. This form of statistical graph is constructed when a single set of statistical data is used.

    Construction of a simple bar graph 

    The following are the main steps followed to construct a simple bar graph: 

    1. Create a frame of both vertical and horizontal axes. That is, drawing the X and Y axes. The length of these two lines will be determined by the scales used and the figures to be plotted. 

    2. Indicate the independent variables on the X axis and dependent variables on Y axis. 

    3. Select appropriate scale: Both vertical and horizontal scales. This is determined as studied while dealing with line graphs.

     4. The horizontal scale will automatically determine the size of the bars. It is recommended that the size of each bar should not exceed 1 cm or go below 1cm in width. Too small and huge bars make the graph lose its primary appearance. 

    5. While plotting points, use tiny dots to mark the required points. This is because with bar graphs, dots are not supposed to be seen.

    6. The volume, percentage or value of the dependent variable is represented by the height of each independent bar. 

    7. A reasonable small space should be left to separate the bars from the vertical axis.

     8. There should be uniformity in terms of the size of bars, separating space existing between bars. 

    9. The bars should be attached only when a single or similar dependent variable is being dealt with. But this is not a golden ruling. For example, Rainfall, production of coffee or any other variable.

    Worked example

    The data used to construct the above simple bar graph deals with single dependent variables like rainfall and that is why the bars are attached to each other. However, the separated bars are used to represent independent variables as shown on the figure below.

     Table 1.10: Table showing the profits made in a period of 5 years by Umubanomwiza company

    Advantages of simple bar graphs

    1. The following are advantages of simple bar graphs:

    2. They are easy to interpret.

    3. They summarize a large amount of data in a visual form.

    4. They display trends easier to highlight or notice than simple line graphs.

    5. They can be super-imposed on other statistical diagrams. That is, they can

    be used together with other methods.

    6. They are easy to construct.

    Disadvantages of simple bar graphs

    1. They can easily be manipulated to provide false impressions.

    2. When large figures are used, determining a suitable scale is more


    3. Time consuming especially when several variables are dealt with.

    4. They do not provide an adequate explanation. Therefore, for them to be

    rewardingly understandable, an additional explanation is required.

    5. Simple bar graphs do not show key assumptions, causes, impacts and


    1.3.2 Group bar graph

    Group bar graphs also known as multiple bar graphs. Group or multiple bar graph, is a statistical technique used to represent data that is made up of several dependent variables, that can hardly be shown using simple bar graph. A group of bars is constructed following the steps involved in the drawing of a simple bar graph. The only difference is that group bar graph is constructed by attaching a set of bars representing individual variables.

    Construction of a group bar graph

    The following are the main steps involved in construction of a group bar graph:

    1. The way of constructing a group bar graph is the same as that of simple bar


    2. The bars are drawn in the form of a set and touching each other for each

    independent variable. This implies that bars are grouped but separated from

    another group by a space.

    3. The bars are drawn in a vertical manner.

    4. The independent variable that has the largest values should be started with

    and drawn on the left and continuing in a descending order towards the right.

    5. The arrangement of the first group should be maintained throughout.

    6. The width of all bars must be similar. That is, ensure that all bars display equal


    7. Shade differently each individual bar and maintain the same shading or

    colouring up to the last group of bars.

    8. A suitable title as adapted from the title of the table used should be indicated

    on the top of the graph. In the title, the type of the statistical method used

    must be stated.

    Worked example: 

    Table 1. 11: Table of Irish potatoes production in ’000 tons in selected areas

    Advantages of group bar graph

    1. It is easy to interpret.

    2. It favours comparative analysis of the statistical data being dealt with.

    3. It is simple to draw.

    4. The calculations involved are simple and easy.

    5. It provides a wonderful visual impression when it is coloured.

    6. It can be used together with other diagrams in a super-imposed way.

    7. It is suitable for a wide range of variables.

    Disadvantages of group bar graph

    1. It requires a large space.

    2. It is challenging while determining the scale.

    3. It may be difficult to interpret when it is congested.

    1.3.3 Compound bar graph

    Compound bar graphs are also called cumulative or divided bar graphs. This form of statistical graph is used to show how the total in any one bar is divided up between several subtotals basing on the calculated cumulative totals. This implies that the totals used to plot the variables are progressive in nature. A compound bar graph used when there is more than one variable and a variety of the items are to be portrayed using one bar. Basing on cumulative totals as calculated depending on the raw data on the table provided, a given bar can be divided into segments. The size of each segment depends on the value of variable being represented.

    Construction of a compound bar graph

    The following are the main steps followed to construct compound bar graph:

    1. It is constructed by using cumulative progressive totals.

    2. It requires selecting a suitable scale that can favour the plotting of both big

    and small values.

    3. The plotted data are displayed in percentages by starting with the greatest

    value and ending with the smallest figure.

    4. The width of each bar is determined basing on the scale used.

    5. The number of bars will be determined by the number of variables to be

    represented in each set of bars.

    6. The varying sheds or colors are used for each individual segment or

    component of every bar

    Worked example 

    Table 1. 12: Coffee and tea production in ’000 tones for selected regions in 2017

    1.3.4. Divergence bar graphs

    The primary purpose of the divergence bar graph is to represent the way values of various variables deviate from the average/mean value. Therefore, this statistical method shows the fluctuation of the variables.

    Construction of divergence bar graphs

    1. The steps to be followed while constructing divergence bar graph are:

    2. Construct the two vertical axis lines.

    3. Join the two vertical lines with the horizontal line at the base line.

    4. Draw the average line which is known as a zero line at the center of the

    two vertical lines. Write the average against this line at the end points

    where “0” is indicated.

    5. Find the total of all the value of the variables and divide it by the number

    of the items to get the average. Subtract the average obtained from each

    value, to get the divergences or deviations.

    6. Select the suitable scale basing on the divergences calculated.

    7. Plot the negative values below the average line and the positive ones above the same line.

    Worked example

     Table 1. 13: Coffee production in ‘000 tons for country X from 2010 to 2017

    The following divergence bar graph shows the country’s coffee production in ‘000 tons from 2000-2008 which is drawn using the data presented in Table 1.14 above.

    Advantages of divergence bar graphs

    1. The following are advantages associated with divergence bar graphs:

    2. They provide a good visual impression.

    3. They are appropriate for comparison purposes.

    4. They are easy to construct.

    5. They are less complex therefore, easy to interpret.

    6. They use limited space.

    Disadvantages of divergence bar graphs

    1. There are several calculations involved.

    2. The method is only limited to a single item or commodity.

    3. Determining the scale especially when the values have a wider amplitude

    is challenging.

    4. The plotting of values is only based on divergences. Therefore, the actual

    values are not seen. 

    1.3.5. Age and sex graphs

    Activity 1.3.6 

     Statistical diagrams showing population structure in two different countries:

     Refer to the figures presented above to answer the following questions:

    a. Name the diagrams indicated above.

    b. Differentiate the two diagrams represented above based on their


    c. Research on the advantages and disadvantages of such diagrams.

    An age-sex graph is also known as a population pyramid. It is a type of bar graph that represents the structure of the population of a country or area basing on age and sex. An age-sex graph forms a structure of pyramid as the population grows.

    It is a graphical drawing that displays the distribution of several age groups in each population of a country or part of the world, which makes the shape of a pyramid as the population grows. It typically involves two back-to-back bar graphs, with the population plotted on the horizontal line and age on the vertical line. The left side of the population pyramid should indicate the Male and the right for Females. It is usually in age groups of five years for example 0 - 4 years; 5 - 9 years; 10 - 14 years and so on. The age groups are indicated or plotted in the middle.

    Therefore, the primary purpose of the population pyramid is to portray the population structure and age groups. The number may be in a raw form or converted into percentages. Population pyramids are often regarded as the most actual way to graphically represent the age and sex distribution of a population. This is so, because of its clear visual interpretation and direct display of the true picture of the real structure of the population.

    Construction of an age-sex graph

    When drawing an Age-sex pyramid, the following steps should be observed:

    1. It is constructed on two X- axis; one to portray male and another for


    2. Males are represented on the left and females on the right. The bars

    representing each sex are separated by a space. It is in this space that age

    groups are indicated in a progressive manner.

    3. The data provided should be grouped using the interval of 5 years as

    follows; 0 - 4; 5 – 9; 10 – 14; 15 – 19; 20 – 24; 25 – 29; 30 - 34 and so on.

    4. After forming the age groups; the lowest group which forms the youngest

    is indicated on the base of the graph.

    5. The left horizontal line should have figures for males and the one on top

    right showing the figures for females.

    Worked example:

     Table 1.15: Population Structure (age-sex) of country X

    The following age-sex bar graph of country X is drawn using the data presented in the table above.

    Advantages of an age-sex graph

    The following are the main advantages of an age-sex graph:

    1. It provides instant display of the composition of the population being

    portrayed with consideration of the age and sex.

    2. It portrays the general shape of the population structure that facilitates

    analytical purpose.

    3. The features associated with the age-sex pyramid can indicate factors

    affecting an area or region it presents.

    4. It gives a good visual impression that captures the attention of the reader

    or interpreter.

    5. It is easy to interpret

    6. It has limited or sometimes no calculations involved, therefore, it is easy to


    7. Age-sex pyramid is suitable for comparison purposes. For example, the

    number of females and males in each area.

    Disadvantages of an age-sex graph

    1. It is only used to represent the population structure.

    2. It is only used alone and hardly super-imposed on other methods.

    3. There is loss of important information due to the use of figures that are in


    4. It cannot show the population distribution (does not describe or show the

    trend of the population growth).

    5. It is only used in population geography.

    6. The actual figures may not be seen since in most cases percentages are


    7. It takes a lot of time to construct.

    8. It takes a lot of space.

    9. It is challenging to determine a suitable scale. 

    1.3.6. Dispersion graph

    The dispersion graph is a statistical representation method that displays the tendency of data scattered over a range. It is also called scatter graph. It shows the distribution of the dependent variables in relation to independent variables. Hence, it is used to show the relationship existing between two variables.

    Construction of a dispersion graph

    Below are the main steps followed while constructing a dispersion graph:

    1. Draw the vertical axis and horizontal axis.

    2. Select the suitable scale for both axes.

    3. Determine the variable that qualifies to be independent variable and the

    dependent variable.

    4. Indicate the dependent variables along the vertical axis and the

    independent variables on the horizontal axis.

    5. Plot the points using visible dots that are thickened. 

    Worked example: 

     The table below shows the examination scores in Dihiro G.S in Bugesera District in relation to the hours assigned.

    It is important to mention that dispersion graphs can display both negative and positive correlations. For instance, there is a positive correlation between studying hours and students’ examination scores because the students’ examination scores increase with an increase of the hours of studying. However, the following graph shows a negative correlation between altitude and temperature.

    The figure above shows that there is a negative correlation between altitude and temperature as far as the temperatures decrease with altitude.

    Advantages of a dispersion graph

    1. A dispersion graph displays the relationship between two variables;

    2. It tests how reliable the data collected is;

    3. It is easy to draw;

    4. It provides a good visual impression;

    5. It easily portrays the anomalies associated with data.

    Disadvantages of a dispersion graph

    1. Some information that may be important is not displayed.

    2. It is more reliable when a wide data is used but when the data used is

    small, the conclusion may not logically be reliable.

    3. Much time is required when huge information is used.

    4. It is easily constructed using ICT (Excel) and hard to construct manually.

    1.3.7 Circular graph

    Circular graphs are known as the polar graphs. These are statistical and graphical diagrams that take a circular shape as the face of a clock. They are called polar graphs because they display the layout as the one made by the longitudes radiating from the poles.

    Construction of a circular/polar graph

    1. Construct a circle of a convenient size. It is advisable to use large circles to

    avoid congesting the graph with information.

    2. In the middle of the bigger circle, draw a small circle where additional

    information such as activities carried out can be portrayed.

    3. Partition the big circle into 12 equal angular parts all touching on the

    inner small circle. Make sure that each segment represents 30 degrees. To

    get these 30 degrees, 360 degrees are divided by 12 segments.

    4. Labeling of the circular graph begins from twelve o’clock which marks the

    month of January. Thereafter, other months follow, taking the clock wise

    order up to December.

    5. The scale is determined in degrees centigrade for temperature and bars

    are plotted basing on the radii portraying rainfall totals or amounts.

    6. Plot points for temperature and join each with a continuous curve.

    7. Avoid writing words that are upside down or sharply tilted. The writing on

    the graph should enable the reader not to tilt or turn around the graph.

    8. Indicate all the characteristics of a good statistical diagram such as title, key, accuracy and neatness among many others.

    Worked example: 

    Table 1. 17: Table showing agricultural activities through the year and received rainfall

    Advantages of a circular graph

    1. They have a good visual impression.

    2. Compared to the information dealt with, little space is used.

    3. It portrays a wide range of items such as rainfall, temperature, and

    agricultural activities.

    4. They do not involve complicated calculations.

    5. Displays the relationship or influence of climate on human activities.

    Disadvantages of a circular graph

    1. Sometimes it is very difficult to label the graph because of its circular


    2. They consume a lot of time especially when many items are to be


    3. It is sometimes hard to compare some items such as rainfall values since

    they are displayed apart and hardly lie on the same baseline.

    4. They are more challenging while drawing. 

    Application Activity 1.3 

    There is cutting down of trees in Ubwiyunge village. Then senior five students conducted a field work study in the area and they got the following findings:

    1000 trees were cut down

    549 trees were just recently planted.

    1400 tree seedlings

    290 trees had reached maturity.

    After gathering the information from the farmers and local authorities, they passed a word of thanks to everyone that had assisted them. The respondents were so happy to see students of such high respect and values. Use the information presented above to answer the following questions:

    a. Identify the environmental concern associated with Ubwiyunge


    b. Explain the environmental conservation measures you would teach

    the citizens of the above village.

    c. Give a description and interpretation of the constructed graphs. 

    1.4. Statistical charts

    Divided statistical charts may either be divided circles or rectangles. Divided circles are also known as Pie charts. These are graphs that have a shape of a circle, with varying segments that portray a percentage, value or degree of a given component in relation to the general total to be represented. The chart is called a ‘Pie’ chart because when looked at, each segment that forms it, displays a shape of piece of ‘Pie’. It is constructed basing on the degrees that are skillfully measured to partition or divide a circle into various sections with correspondent angle. This implies that each segment or portion represents a given quantity, or value, amount or percentage of the statistical data represented. Divided rectangles are a statistical graphic representation of data, using rectangle that is equaled to the general total of the variable. However, even though the area of the rectangle represents the total, it is partitioned into segments each representing an individual variable. The subdivision of the rectangles in this case stands for individual components (variables).

    The following are the main types of divided circles:

    1. Simple divided circles

    2. Proportional divided circles

    3. Simple divided rectangles

    4. Compound divided rectangles

    1.4.1 Simple divided circles

    A circle is drawn to represent statistical total data given. The circle is then divided according to the value of the variables.

    Construction of a simple divided circle

    The following are steps that are followed in constructing a simple divided circle:

    1. Draw a circle of a reasonable size. Avoid using small or very big circles.

    2. Determine the degrees proportional to the quantity, value, amount, of the

    item or component to be portrayed.

    3. Start with the component with the biggest degree. Place this to the right

    of 12 o’clock.

    4. Arrange the smallest portions or segments to be plotted in one part. That

    is, start from the biggest to the very tiny segment.

    5. Labeling on a pie chart should take a horizontal order. Where need be,

    write the words or names outside the chart using a pointing arrow for the

    small segments.

    6. Shed each segment differently and use the key for referral purpose. If

    colours are available, use them. 

    7. The segments or components should not go beyond 8 in number. Beyond this figure, the chart becomes over crowded or jam-packed.

    Worked sample: 

    Table 1.18: The exports of country X (1)

    The data presented above can be used to construct a simple divided circle. The presentation of the data displayed in table 1.18 above is done using a simple divided circle. This requires calculating the values of each data in degrees. These calculated degrees can be also converted into percentages.

    Advantages of a simple divided circle/pie chart 

    Simple divided circles or pie charts are increasingly becoming a common tool used in the representation of statistical data. This is because of their advantages as shown below:

    1. When the wedges /portions are clearly shaded or coloured, they provide a

    good visual impression.

    2. They are effective and reliable when two or more variables are to be

    comparatively analyzed.

    3. When well-labeled, they are easy to interpret.

    4. They don’t require specific skills to be understood as it is the case for

    dispersion graphs or divergence graphs, among many others.

    5. They involve simple calculations which make them easy to draw.

    6. Pie charts can be super-imposed on other statistical methods such as


    7. They can be used for a wide range of purposes in geography and other


    8. Pie charts are used as a summarizing tool, where vast data is set in a visual


    9. The divided circles require minimal additional explanation or description.

    Disadvantages of a simple divided circle/pie chart

    1. They deal with degrees and sometimes percentages. Therefore, exact

    figures may not be easily noted by the reader.

    2. Pie charts are not suitable for tracking the trend of a given variable. This

    implies that several pie charts must be used.

    3. Pie charts can easily be maliciously manipulated or intentionally altered,

    therefore, displaying incorrect information. Hence, the reader may make

    erroneous conclusions.

    4. They are constructed without basing on a specific scale, this leaves a gap

    in the exactness of the method.

    5. They provide errors due to lack of accuracy, especially when tiny degrees

    are plotted. The thickness of the pencil affects the perfection of the


    6. Labeling the pie chart sometimes is challenging especially when small

    degrees are plotted.

    7. Pie charts or divided circles take a lot of time to construct. This is because

    of the calculations, measuring, drawing, and shading.

    8. Large wedges/portions for bigger values tend to over shadow the small


    9. In case there are several portions of almost matching size, it is challenging

    and confusing or difficult to interpret and assimilate the data.

    10. The reader may find it challenging to comparatively analyze nonadjacent segments. That is, the reader will keep twisting and turning the

    chart to have a suitable angle of observation. 

    1.4.2. Proportional divided circles

    Learning activity: 1.4.2

    Use internet, text books and other sources of information to research on the


    a. Description of proportional divided circles.

    b. How proportional divided circles are constructed.

    c. Advantages and disadvantages of proportional divided circles.

    Proportional divided circles are also called comparative divided circles. They are used when more than one variable is dealt with. They are used for comparison purpose.

    Construction of proportional divided circles

     The techniques involved in the construction of proportional divided circles are like those of a simple divided circle especially how the varying segments are being portioned. However, the proportional divided circles are constructed using circles of varying sizes which are used once their totals are not uniform. Steps involved in the construction of comparative divided circles are described here under:

    1. The number of the circles to be constructed will depend on how many

    variables are being dealt with. For example, if one is comparing imports

    and exports, then the comparative divided circles will be two.

    2. In case the totals of the independent variables are the same, therefore, the

    circles to be drawn will be of the same size.

    3. The circles will have varying sizes, if the totals of the independent variables


    4. Circles should be constructed near each other to facilitate the comparison.

    5. Find the square root of each total of the independent variable.

    6. The size of each circle should be proportional to the totals. Therefore,

    determine the radius of each circle that is obtained by use of the square

    root of each total.

    7. While dividing each circle into wedges or portions, follow the steps

    involved in drawing a simple divided circle.

    8. There must be uniformity in the arrangement of the segments for all the


    Worked example:

     The table below shows the exports and imports valued in Rwandan francs for selected regions 

    Table 1.20: The exports and imports in FRW for selected regions

    The proportional divided circle was constructed below using the data provided in table above. The variation of totals implies that the proportional divided circles will be of differing sizes. Therefore, the radius for each circle is determined as follows:

    Calculation for exports:

    Area = the totals of exports

    Advantages of proportional divided circles

    1. Comparative divided circles capture the attention and interest of the

    reader because of the wonderful visual impression they offer.

    2. They are suitable statistical graphical methods for the data that require

    comparative analysis and interpretation.

    3. They provide an instant visual interpretation of the data represented.

    4. Once the segments are determined in terms of degrees, the method is

    easy to draw.

    5. There are simple calculations involved.

    Note: This statistical method shares directly most of the advantages of a simple divided circle as earlier studied.

    Disadvantages of proportional divided circles

    1. They are not constructed on a scale which affects the effectiveness and

    exactness of the data.

    2. Small degrees sometimes are hard to plot, and such provides erroneous

    effects in the interpretation of data.

    3. When small degrees are involved, writing or labeling becomes hard and

    over-crowding of the chart occurs.

    4. The determining of the radius, degrees and finding the totals, makes the

    method time consuming.

    1.4.3. Simple divided rectangles

    Learning activity 1.4.3

    Use internet, text books and other sources of information to research on the


    a. Description of divided rectangles.

    b. How simple and compound divided rectangles are constructed

    c. Advantages and disadvantages of simple and compound divided


    Divided rectangles are a statistical graphic representation of data, using rectangle that is proportional to the general total of the variable. However, even though the area of the rectangle represents the total, it is partitioned into segments each representing individual variables. Two methods can be used; these include the following: simple divided rectangle and compound divided rectangle. A rectangle is subdivided to indicate the constituent parts. The simple divided rectangle represents the total value or area of the components.

    Construction of simple divided rectangle

    Simple divided rectangles are constructed in the following ways:

    1. A rectangle is drawn whose area is proportional to the quantity or value of

    all parts.

    2. The rectangle is then divided into strips of uniform height.

    3. The variation in the values of the constituent parts are represented by

    making subdivisions along the horizontal scale.

    4. The vertical axis shows a constant of any convenience, but most suitably and commonly recommendable is 100%.

    5. The quantity or values are indicated along the horizontal axis. The height

    is determined by using a given scale of any choice.

    6. Divide the rectangle into segments each representing a given value or

    quantity. But at the end all should add up to make a complete rectangle.

    7. The portions or wedges should be shaded differently or coloured in a

    varying manner.

    Worked example:

    The table below shows crop production in country X per province.

    Table 1. 23 showing crop production per province in a country X

    Advantages of simple divided rectangles

    The following are some of advantages of simple divided rectangles:

    1. When the rectangles are clearly shaded or coloured, they provide a good

    visual impression.

    2. They are effective and reliable when two or more variables are to be

    comparatively analyzed.

    3. When well-labeled, they are easy to interpret.

    4. They involve simple calculations which make them easy to draw.

    5. They can be used for a wide range of purposes in geography and other


    6. They require minimal additional explanation or description.

    Disadvantages of simple divided rectangles

    1. They deal with sometimes percentages. Therefore, exact figures may not

    be easily noted by the reader.

    2. Simple divided rectangles can easily be maliciously manipulated or

    intentionally altered, therefore, displaying incorrect information. Hence,

    the reader may make erroneous conclusions.

    3. They are constructed without basing on a specific scale. Such leaves a gap

    in the exactness of the method.

    4. They may provide errors, due to lack of accuracy; especially when tiny

    degrees are plotted. The thickness of the pencil affects the perfection of

    the method.

    5. Large wedges/proportions for bigger values tend to over shadow the

    small values.

    1.4.4. Compound divided rectangles

    This statistical method is like a simple divided rectangle. The difference is that, with compound divided rectangle, each segment is partitioned basing on the general total, it is further sub-divided to represent the sub-divisions of the segments as illustrated by the practical example.

    Construction of compound divided rectangle

    Steps taken for the construction include:

    1. Get the data to be represented and calculate the total value. This is where

    there is the addition of all the absolute values given.

    2. Draw a rectangle which is proportional to the total quantity calculated in

    step 1 above.

    3. Estimate the vertical scale and this should be constant. This is usually

    given in percentages to simplify data presentation.

    4. Subdivide the rectangle into strips of uniform heights but with different

    subdivisions using the horizontal scale which must be proportional to

    their values.

    Worked example

    The table below shows how the land is used in country X.

    The data displayed in table 1.24 were used to construct the simple divided rectangle as shown below: It should be noted that, in this case the total area (000) km2 is used to determine the suitable horizontal scale, while the % against each independent variable, will be used to portion the segments.

    Advantages of compound divided rectangles

    Compound divided rectangles are very important statistical methods of data

    representation as it is explained below:

    1. It conveys much more statistical information when compared with other

    graphs like compound bar graphs;

    2. It occupies little space compared to circular graphs;

    3. It gives clearly the total values of an entity in question;

    4. It is very simple to construct because it does not involve a lot of

    mathematical calculations compared to divided circles;

    5. It gives a good visual impression when well-drawn and shaded.


    1. It is challenging to find a convenient scale;

    2. They are not applicable to the representation of statistical data that are

    locational in nature;

    3. This method does not give absolute individual values.

    4. Shading consumes a lot of time if many subdivisions are involved.

    5. It is limited to the use of location purposes.

    1.5. Repeated symbols

    Activity 1.5.1

    Using various sources of geographical information research on:

    a. Repeated symbols used in statistics to represent geographical

    information on charts.

    b. Advantages and disadvantages of each method used to represent

    geographical information on charts using repeated symbols.

    Repeated symbols are a form of statistical method used to represent the data that emphasizes locational aspects. That is, some geographic information may require representing symbols of the same size and nature on a map and clearly locating the symbol in its almost exact place. Such symbols are repeated all over the map being super-imposed in relation to where they are intended to be located.

    For example, when dealing with livestock, one may say, one symbol of cow  represents 260 cows. Therefore, the total number of the symbols of a cow will be equal to 260 X total number of symbols. Suppose, on map X, there are 100 symbols of a cow, the reader will count them to get the number:

    100 symbols indicated on Map X

    One symbol of a cow = 260 cows.

    The actual number of cows represented will be 100 X 260 = 260,000 cows.

    There are different kinds of charts which are represented using repeated symbols.

    These include: proportional circles, squares, cubes and spheres.

    1.5.1 Proportional circles

     Circles of different circumference are drawn based on the quantity of the item supposed to be represented. It involves mathematical calculation, and the use of square roots is required to obtain the radii of the circles required. It is constructed like the proportion squares diagram

    Advantages of proportional circles

    1. They display relative proportions of multiple classes of data;

    2. The size of the circle can be made proportional to the total quantity it


    3. They summarize a large data set in visual form;

    4. They are visually simpler than other types of graphs;

    5. They permit a visual check of the reasonableness or accuracy of


    6. They require minimal additional explanation.

    Disadvantages of proportional circles

    1. They do not easily reveal exact values.

    2. Many proportional circles may be needed to show changes over time.

    3. They fail to reveal key assumptions, causes, effects, or patterns.

    4. They may be easily manipulated to yield false impressions.

    1.5.2. Proportional squares

    The proportional squares are almost used in equivalent manner as proportional circles. The general total is equivalent to the area of the square. This implies that the quantity in question will be directly represented equivalently by the size of the square, whose length will be determined by the square root of the total.

    Construction of proportional squares 

    There are mainly five steps followed while drawing proportional squares. This requires remembering the skills acquired in drawing squares. These steps include the following:

    1. Find the length of the sides of squares. This is obtained by calculating the

    square roots of the totals of variables.

    2. Draw the squares for each variable. This means use the square roots to

    determine the length in centimeters.

    3. If the proportional squares are to be super-imposed on maps or any other

    statistical diagram, the south-west corner of the square must touch the

    point of location or representation.

    4. Proportional squares can be drawn alone without being super-imposed

    on maps.

    5. The location or the position of the area being represented can be read by

    looking where the south-west corner connects or touches.

    Worked example:

    Use the table1.25 below to answer the questions that follow:

    Table 1.25 showing the crops product in ‘000 tons in selected areas 

    The data presented in table 1.25 above were used to construct proportional squares

    shown below following the recommended steps.

    Calculation made to determine the square root and length in cm of each crop is

    presented in the table below:

    Table 1.26 showing square root and length in cm of each crop derived from the data

    shown in table 1.25

    The square roots are multiplied by 1000 because the raw data in the table are reduced by ‘000 in tons. The square roots after being multiplied by 1000 each, the figures become big. However, they are reduced by a uniform figure of 1cm: 10,000; to get at least simple figures that are easily measurable.

    The following are proportional squares constructed based on the above length in Cm:

    Advantages of proportional squares

    1. Easy to construct;

    2. They promote a good visual impression;

    3. They can accommodate several figures;

    4. They can be used together with other diagrams such as maps;

    5. They involve few calculations.

    Disadvantages of proportional squares

    1. They are sometimes difficult in interpretation due to over-lapping.

    2. It is hard to analyze small figures reflected between totals.

    3. It shows directly the figures on the square, if a slight difference exists

    between figures it is hard to tell the accuracy of the figures.

    4. They occupy large space, hence are less economical.

    1.5.3. Proportional cubes

    They are representing quantitative distribution of the objects. The side of the cube is directly related to the cube root of the quantity.

    Construction of proportional cubes (cuboids)

    The following are the major steps to be followed while constructing proportional


    1. Calculate the cube root of the value or quantity or total to be represented

    by the cube.;

    2. Place the drawn cubes along the same straight for the purpose of

    comparison. This is applicable where they are not placed to the base map.

    3. Show the key and all elements of a good statistical diagram.

    4. Once used on the base map, maintain the same pattern of constructing

    the individual cubes.

    5. Indicate the quantity in terms of cube roots along the cubes (on the face

    of the cube).


    The data presented in the table above are used to construct a proportional square The cube roots for each variable to be used in the construction of a proportional square are calculated below:

    These cube roots are too big but can be used when they are reduced using the scale

    indicated above (1cm: 20 cm). The reduced centimeters are now used to construct

    the cubes.

    Beans= 4 cm

    Bananas = 3.2 cm

    Peas= 2.9 cm

    Sweet potatoes 2.3 cm

    Advantages of proportional cubes

    1. They are easy to construct.

    2. They promote a good visual impression.

    3. They can accommodate several figures.

    4. They can be used together with other diagrams such as maps.

    5. They involve few calculations.

    Disadvantages of proportional cubes

    1. They are sometimes difficult in interpretation due to over-lapping.

    2. It is hard to analyze small figures reflected between totals.

    3. It shows directly the figures on the cubes, if a slight difference exists

    between figures it is hard to tell the accuracy of the figures.

    4. They occupy large space, hence are less economical.

    1.5.4. Proportional spheres

    Proportional spheres serve the same purpose as the cubes. The volume of the spheres should be proportional to the quantities they represent. The radius of the sphere is determined by calculating the cube root of the quantity to be represented (the volume of a sphere is 4/3/r3). The sphere is then drawn in its correct position on the map. Proportional spheres are not normally used except on a locational basis.

    Advantages of proportional spheres

    1. Easy to construct.

    2. They promote a good visual impression.

    3. They can accommodate several figures.

    4. They can be used together with other diagrams such as maps.

    5. They involve few calculations.

    Disadvantages of proportional spheres

    1. They are sometimes difficult in interpretation due to over-lapping.

    2. It is hard to analyze small figures reflected between totals.

    3. It shows directly the figures on the spheres, if a slight difference exists

    between figures it is hard to tell the accuracy of the figures.

    4. They occupy large space, hence are less economical.

    Application Activity 1.5

    The table showing enrolled students at selected schools

    Use the above presented data in the table to answer the following questions:

    1. Construct different types of charts using repeated symbols.

    2. Interpret the constructed charts in (a) above.

    3. Suggest how peace and different Rwandan values may be promoted at

    Ineza, Ubworoherane and Kwigira schools.

    1.6. Statistical diagrams

    Statistical maps are used in a situation where the data represented need to reflect the distribution of the variable being portrayed. Therefore, maps are used since it is easier for the reader to see directly the location in relation to the variable represented.

    There are four main types of statistical maps, namely:

    1. Dot maps.

    2. Isoclines maps.

    3. Shading maps or choropleth maps.

    4. Flow maps.

    1.6.1. Dot maps

     A dot map is a map type that uses a dot symbol to show the presence of a feature or phenomenon found within the boundaries of a geographic area. In addition, with dot maps, there is an attempt to show the pattern of distribution within the area by placing the dots where the phenomenon is most likely to occur. In a one-to-one dot map, each dot represents a single recording of a phenomenon. Care must be taken to place the dot in its correct position on the map.

    1. Determine the value of each dot. Take note of the nature of quantity. The

    value of dots on the maps will depend on the smallest quantity (value) or

    the biggest figure.

    If the lowest figure on the table is 2500 for Z and highest 5000 (K) then the

    value of dot can be:

    5000 = 5 dots (K)


    2500 = 2.5 dots = 3 dots (Z)


    Therefore, in area K there were five dots and 3 dots in place Z. It is very important to note that, the dot value should not be unnecessarily exaggerated. Too big or too small dot value provides wrong impression which interrupts with the analytical results of the data.

    2. In case there are halves or fractions, it is necessary to round off that whole

    number of dots. For example, if you calculated and found that there are 3.5,

    4.6, and 5.1 dots, then it is advisable to round up these figures to read 5

    instead of 4.5.

    3. The size of a dot should be reasonable. Not too big or very tiny. It should be

    able to enable the reader to be able to use his/her eyes to count the dots.

     4. All dots used on the map should have equal size.

    5. Plot all dots using a pencil so that in case of a mistake, it becomes easy to

    correct it.

    Worked sample

    Table 1.28. Population density of Rwanda in 2012 by district

    The data presented above are used to construct a dot map of the population density of Rwanda in 2012

    Advantages of dot maps

    1. Dot maps provide a good visual impression.

    2. They are suitable for the representation or portrayal of the data of spatial

    distribution such as population distribution.

    3. They don’t involve difficult or challenging calculations.

    4. It is very easy for the reader to immediately compare the distribution of

    what is represented, e.g. population distribution.

    5. It can be conveniently used for the portrayal of a varying range of items

    such as distribution of crops, population, volume, area, etc.

    Disadvantages of dot maps

    1. There is a risk of giving false impression especially when dots are evenly


    2. It involves more calculations such as determining the population density,

    dot value, etc.

    3. They provide unclear impact due to the congestion of data in densely

    populated areas.

    4. Drawing with free hand dots of equal size is difficult and challenging.

    5. Identifying and locating are exposed to personal subjective decision.

    6. In areas where there are many dots near each other, it is difficult to find

    the number without making errors.

    1.6.2. Isoline maps

    Learning activity 1.6.2

    The table showing different lines that join places of equal values

    Use the information presented in the table above to answer following questions:

    a. Fill in the missing information.

    b. Describe how isoline maps are drawn and used in geography

    An isoline or isopleth map is a map with continuous lines joining points of the same value. For instance, the lines joining equal altitude (contour lines), temperature (isotherms), barometric pressure (isobars), wind speed (isotachs) and wind direction (isogon), etc.

    Construction of isoline maps

    The construction of an isopleth map is done as follows:

    1. Obtaining an outline base map and the appropriate necessary

    geographical data.

    2. Marking in the points and their values on the map.

    3. Deciding on a suitable interval of units.

    4. Drawing curved lines joining all places with equal values; always starting

    with the highest value.

    5. Making sure that the lines do not cross or touch each other.

    6. Numbering the isopleth lines.

    Advantages of isoline maps

    1. They are useful in showing the distribution of geographical phenomena at

    a large scale.

    2. They are useful in illustrating the distribution of even and uneven


    3. It is easy to obtain values on any point of the map.

    4. Isoline maps are used for comparing variables.

    5. They provide a clear impression of population density.

    6. They can be used together with other statistical diagrams in a superimposed way.

    7. They don’t involve calculations.

    8. There are easy to construct and easy to interpret.

    9. They provide a good visual impression.

    Disadvantages of isoline maps

    1. With isoline maps, the interpretation may not be easy especially when the

    isopleth lines are not clearly shaped.

    2. It may be difficult to calculate isoline interval on the map.

    3. Isopleth maps are not suitable to present the population distribution.

    4. In drawing isoline maps; the administrative boundaries are not taken into


    5. Isopleth lines may be difficult to draw, especially where there are points to


    6. Isoline maps provide unreliable impression, on assumption that the gap

    between two adjacent isoline lines is uniform.

    7. It is very hard to determine the distance area on isoline maps.

    1.6.3. Shading or choropleth maps

    Learning activity 1.6.3

    Using internet, textbooks and other sources of geographical information research


    a. The difference existing between choropleth and isoclines map.

    b. Description of how choropleth maps are constructed.

    c. Advantages and disadvantages of choropleth maps.

    Choropleth maps are thematic maps in nature described by a series of varying shading patterns, each representing proportionally the measurement of a given statistical variable being portrayed on the map. The choropleth map provides an easy way to visualize how a measurement varies across a geographic area or it shows the level of variability within a region. A choropleth map is a map which shows regions or areas with the same characteristics.

    Construction of choropleth maps

    A choropleth map is made as follows:

    1. Drawing a base map showing administrative units such as provinces or


    2. Calculating the average densities, ratios or percentages for each

    administrative unit.

    3. Choosing and drawing grades or scales of densities to be used.

    4. Indicating on the map the grade of shading or coloring to be used for

    each administrative unit.

    5. Shading or coloring the map but leaving boundary lines

    6. Dividing the range of values into groups.

    7. Including a key showing individual boxes.

    8. Showing increases or decreases in population density, average crop yields,


    Worked example:

    Table 1.29: Population density of Rwanda in 2012

    The data presented above are used to construct a choropleth map showing the population density of Rwanda in 2012

    Advantages of choropleth map

    1. Most often choropleth maps represent the typical value for the region not

    spread uniformly within the region.

    2. Choropleth maps are used for phenomena that have spatial variation that

    coincide with the boundaries of the spatial area used for map.

    Disadvantages of choropleth map

    1. Choropleth map is suitable for mapping discrete phenomena.

    2. In most cases it is not easy to use absolute numbers in choropleth map.

    3. To make phenomena comparable for administrative units it should be

    quite often standardised.

    1.6.4. Flow maps

    Flow maps are statistical methods used to represent diagrammatically the movement of goods from one area to another. They are commonly used to represent the flow of imports and exports. They are again used in other ways, such as the flow of traffic by water, air or rail.

    Construct of a flow map

    1. The line drawn show the direction of flow.

    2. The width of the line represents the quality of goods imported or


    3. Write the amount of goods directly on/alongside the line of lines.

    4. The drawn lines are colored to avoid congestion. In this case, a key can be

    of a good help.

    5. Determine the width of the lines by using a suitable scale.

    6. Lines should not be too big or small. It is advisable to use a scale expressed

    in millimeters but not in centimeters.

    7. Show all elements of a good statistical diagram, namely, Title, Scale, and

    Key among many others.

    Worked example

    Study the table below showing exchange of agricultural products in ‘000 tons from

    one district to other districts in Rwanda.

    Table1.30 showing exchange of agricultural products in ‘000 tons from one district

    to other districts in Rwanda.

    Advantages of flow maps

    1. They are the only suitable method for portraying the movement of goods.

    2. They are easy to interpret.

    3. They can be used together with other maps and statistical diagrams.

    4. They provide a good visual impression.

    5. They are suitable for comparison purpose.

    6. They do not involve difficult calculations.

    Disadvantages of flow maps

    1. They take a lot of time to draw.

    2. They hardly provide immediate interpretation.

    3. They are more challenging to draw.

    1.6.5. Wind rose

    A wind rose is another statistical tool used to portray diagrammatically the average occurrence and direction of wind associated with a specific area. The wind rose is used by meteorologists to summarize data about the wind in relation to specific peed, location, and time. There are two types of wind rose namely simple and compound wind roses. It is important mentioning that there are two types of wind rose namely simple and compound wind rose.

    Construction of a wind rose

    1. Draw a circle of any convenient size. This marks the central point of wind


    2. Show or indicate the calm days inside the circle.

    3. Construct a wind rose following the main 8 points of a compass can also

    be applied.

    4. The days are indicated by the length of the columns commonly known as


    5. Determine a suitable scale of your convenience.

    Construction of compound wind rose

    1. Apply all the steps involved while constructing a simple wind rose.

    2. The width of the columns represents the speed of the wind. When there

    is a rise in the speed of wind, the width of the column is expected to

    increase too.

    3. Divide the columns into segments following the Beaufort Scale.

    4. Draw a key to represent the different segments of each column.

    Advantages of wind roses

    1. They are easy to interpret.

    2. They provide a good visual impression because of the way they display

    polygon shape and colours.

    3. The reader can have clear and faster information replaced.

    4. They portray a wide variety of aspects such as, wind direction, speed, and


    5. They facilitate the central comparison of the climatic data.

    Disadvantages of wind roses

    1. There are challenges involved while determining the scale for the


    2. They do not emphasize seasonal patterns and the movement of winds.

    3. They require the reader to have skills and specialty in interpretation. 

    End unit assessment 

    Landslides and floods hit several parts of Rwanda between 7 and 8 May 2016 after a period of heavy rainfall. Government officials say that at least 49 deaths have been recorded so far. Some of the victims drowned in flood water, others died after houses collapsed under the heavy rain and landslide.

    The worst hit areas are the districts of Gakenke and Muhanga. As many as 34 people have died in Gakenke, 8 in Muhanga, 4 in Rubavu and 3 in Ngororero. Around 26 injuries have also been reported. Reports from the Ministry for Disaster Management and Refugee Affairs reported that over 500 houses have been destroyed. Therefore, use the statistical information presented above to do the following:

    a. Extract statistical raw data mentioned in the story.

    b. Use appropriate statistical diagrams, graphs and charts to display the

    portrayed data in (a) above.

    c. Identify and describe the geographical phenomena that are highlighted in

    the news print.

    d. Explain how you would use the data collected and statistical diagrams,

    graphs and charts constructed to advise the people and the government

    on the environmental challenges to be addressed.

    e. Assess the environmental challenges to be addressed in the area and show

    measures of controlling them.

    Files: 2URLs: 4

    Key unit competency: By the end of this unit, I should be able to measure the bearings and the directions, calculate distances and areas on a map.

    2.1. Location of places using the grid reference

    Activity 2.1

    a) A map has several lines printed on it. Identify the names given to the vertical and

    horizontal lines printed on the map.

    b) What does grid reference mean?

    A grid is a series of straight lines drawn vertically and horizontally on topographical maps. Where the lines cross each other to form squares of equal sizes. The lines running from north to south (vertical lines) are called Eastings. This is because their numbers increase towards the east from the south western corner. This corner is known as the grid origin and it is from it that all readings start. There are those drawn horizontally, and their numbers increase towards the north. These are called the Northings.

    The lines shown on the illustration above, intersect forming grid squares. These are formed by both vertical lines (Eastings) and Horizontal lines (Northings). These are called grid references or geographical coordinates. They are the ones that are used to locate places and features indicated on the map. This is illustrated hereunder:

    The value of the easting (vertical gridline) is read first, followed by the value of the northing (horizontal Gridline). The values of the Eastings and northings are known as coordinates. The coordinates are expressed as a single continuous figure without decimal points or commas, for example, 646 504 and not 64, 65, 04. They are plain numbers, without units of measurement.

    The coordinates are given in two ways:

    - Four figure grid reference

    - Six figure grid reference

    2.1.1 The four-figure grid reference

    The four-figure grid reference has four digits. It gives the grid square in which a position is found. The four-figure grid reference of Yellow Square in figure 2.31 below is found as follows:

    – Read the easting first and record its value. In this case it is 11.

    – Next, read the northings and record its value. In this case it is 81.

    – Put the two values together.

    – The four-figure grid reference for the yellow square is 1181.

    2.1.2 The six-figure grid reference

     This reference has six digits. It is more exact than the four-figure grid reference. To get the six-figure grid reference for red square in figure 2.32 below, the following is done:

     Read the easting first and record its value. In this case it is 62.

    – Subdivide the area between easting 62 and 63 into 10 equal parts.

    – Record the value of red square out of 10 from easting 62. In this case, it is 5. This

    forms the third digit of the easting.

    – The value of the easting for red square is therefore 625.

    – Read and record the northing. In this case, it is 33.

    – Subdivide the area between northing 33 and 34 into 10 equal parts.

    – Record the value of red square out of 10 from northing 33. In this case, it is 3.

    This forms the third digit of the northing.

    – The value of the northing for the red square is therefore 333.

    – The six-figure grid reference for red square is therefore 625 333.


    2.2: Stating directions and bearings on topographic map

    2.2.1. Direction

    Direction or orientation is important for finding the way and its relative position

    or direction of something. All directions are based on the cardinal points of the

    compass shown below:

    Direction is the relative position of a place from another using the points of the compass. The main cardinal points of a compass are north, east, south and west. The first letters of these directions (in capital) are used in place of the full names. These are N—North, E—East, S—South and W—West.

    2.2.2. Bearing 

    Bearing is an accurate way of giving the direction of one place in relation to another. It is more accurate than direction because it has 360 points compared to the 16 points of a compass. Instead of saying, for example, that place A is north east of place B, we use degrees. So, we would say that place B is situated at 045° from place A. The bearing of a point is the number of degrees in the angle measured in a clockwise direction, from the North line to the line joining the centre of the compass with the point. This is illustrated below :

    A bearing is used to represent the direction of one-point relative to another point. For example, the bearing of A Form B is 245˚. The bearing of B from A is 065˚. Bearing is also the direction to something measured as an angle relative to the north. It increases towards the East, with North=0 degrees, East=90 degrees, South=180 degrees, and West=270degrees.

    2.2.3. Measuring true bearing

    True bearing (TB) is a bearing where the true-north line is taken as 0˚. It is the

    measurement of the angle between the true-north line and the line joining the two

    places in question.

    A true bearing is measured using a protractor from True North, as shown below:

    – Draw a north-south line through the starting point.

    – Use a straight line to join the two places given.

    – Draw an arrow from the True North line to the line joining the two places in a

    clockwise direction. Then measure the angle with a protractor.

    In this case, the bearing of B from A is 135 degrees. It is therefore, very important to

    understand how to use the protractor.

    i) Centre the protractor over the starting point and orient 0˚with true north.

    ii) Draw a line on the map from the starting point towards the destination.

    iii) Read the bearing where the line intersects the protractor. Remember that

    line is the direction of travel to your destination.

    2.3: Measure the distance on the maps: straight line and curved line

    Learning activity 2.3

    Read this conversation between Paul and Sarah students in S1 A and answer

    the questions:

    Paul: Let us visit my uncle at Kayonza district.

    Sarah: Can we walk from GS St Aloys Rwamagana to Kayonza?

    Paul: I don’t know, let us look at our map. It has a ratio scale of 1:50 000. On the map, it is

    about 30 cm from our school to Kayonza district.

    Using your map reading skills that you have acquired in your previous studies:

    a) Calculate the actual distance on the ground

    b Can Sarah and Paul walk within two hours from GS St Aloys Rwamagana to

    Kayonza? Support your answer.

    A map is a representation of the actual ground on a piece of paper. It is usually drawn to scale. Distance on the map is measured between two points, e.g. between a school and the museum or any other feature. Then, the distance measured on the map is converted into the actual distance on the ground. The distances can either be straight or curved. To measure the distance requires:

    - To identify the two places and then calculate the length between them in

    either centimetres or millimetres;

    - To convert the length into the units required such as Kilometres or miles of

    the actual area (ground) referring to the map scale. 

    A map scale refers to the relationship (or ratio) between distance on a map and the corresponding distance on the actual ground. The map scale tells you about the comparative size of features and distances displayed on the map.

    2.3.1. Straight line distance

    The shortest distance between two points is sometimes known as “the crow flies’.

    - Use a ruler and measure the distance between two points in centimeters.

    - Convert the centimeter reading to kilometers by multiplying by 0.5 km if the

    map scale is (1:50 000) to obtain the kilometers on the ground.

    - For example, the distance as the crow flies from A to B is 9.5 cm on a map;

    therefore 9.5 cm x 0,5 km = 4.75 km on the ground. 

    2.3.2 Curved Distances

    In map reading, there are times when curved or straight areas are used while

    determining the distance. Below are the explanations of how curved distances can

    be determined or calculated:

    - Place the straight edge of paper along the feature to be measured e.g. road;

    - Make a mark where the paper intersects the road;

    - Hold the paper steadily with the point of a pair of dividers.

    - Rotate or swivel paper following the line/road being measured.

    - Mark where paper insects with the line/road.

    - Write down the reading after measuring the line/ road.

    - Check the scale of the map.

    - Now multiply the distance between the two points by the scale on the map.

    - The answer above needs to be converted to the unit for the actual ground

    distances (km). 

    NB – An alternative method is where a piece of string is used to measure the curved

    / winding line. Make sure that the string is not elastic.

    Formula: Actual Distance on the ground = Map distance/Scale.


    2.4. Calculate the areas on topographic maps: regular and irregular shape

    An area is calculated to determine the actual (size on the ground) of a feature / region / demarcated area. The area of a feature can be regular or irregular in shape.)

    2.4.1. Calculate areas of regular shapes

    Getting the area of a regular body on maps is simple. This is because the body may

    be a square, rectangle, triangle or a circle. Once the distances are obtained, the

    formulae used in mathematics are used.

    – Square and rectangle: multiply the length by the width.

    – Triangle: half base multiplied by height.

    – Circle: Pi multiplied by square of radius.

    2.4.2. Calculate areas of irregular shapes

    The following methods are used to calculate the irregular shapes:

    a. The use of grid squares

    On topographical maps of the scale 1:50,000, there are grid squares measuring 2 cm by 2 cm. On the ground, these measure 1 km by 1 km. This means that they have an area of 1 km2. To calculate the area of irregular shapes,

    the following steps are done:

    – Count all the full squares inside the irregular body.

    – Count all the half squares inside the irregular body

    – and divide them by two.

    – Add the total of the full squares and the half squares.

    For example,


    Full squares= 20

    Half Squares=26

    Get 26 halves divide by 2 =13 full squares

    20 +13=33 full squares X the area of one square

    33 x1 square km

    =33 km Squared.

    b. The use of strip method

    Here, strips of equal width are drawn on the irregular body. The length of the strips

    differs from strip to strip. Follow the method used to find the area of regular shapes.

    Calculate the area of each strip. Add the areas of the various strips. This gives you the

    area of the irregular shape.

    c. Use of rectangles and triangles

    The irregular body is divided into a rectangle and triangles as shown below. Their

    measurements are taken. The formula for finding the area are used (known). Then

    get the totals of these areas. 


    Application activity 2.4

    Using your knowledge, identify how you can calculate the area of a farm if the

    measurement got are 45 m length, and 34 m width. 

    2.5: Representation of relief on the map

    The surface of the earth is not flat. It has such features as mountains, valleys, gullies, hills, plateaus and plains. In Geography, this form of landscape is called relief. In other words, relief is the general appearance of the land’s surface. It does not include the aspects covered in human geography. The following are the most common techniques used to represent relief features:

    2.5.1. Altitude

    Altitude is the height above sea level. It is the height of a point in relation to the seal level. Elevation is the vertical distance above the sea level while Height is the vertical distance from the base to the top. The figure below shows how relief is determined.


    2.5.2. Contours

    a. Contour lines

    Contours are lines that join places of the same height above the sea level. A contour is represented in brown lines on the topographical maps. The height of the contour is indicated on each contour line. The difference between one contour line and the next is called contour interval. If only certain contours are numbered, one must find how many contours lie between the numbered contours and work out the contour interval. A contour interval is given on the legend on the topographical maps. From the below illustration, the contour interval is 20 m.


    b. Uses of contours and identifying the shape created by the patterns of


    i. Contours help to recognize land features

    By recognizing land features, we understand our natural environment. This is

    useful and helpful for a wide range of activities including:

    – Planning housing estates, freeway routes and reservoirs;

    – Organizing outdoor recreational pursuits;

    – Managing hazards such as flooding, landsides, etc.

    ii. Contours help to recognize the shape of the land

    By reading the contour lines an understanding of the shape of the land is obtained.

    Land features are identified from the contour lines as follow:

    – spacing (density) of the contours indicates the slope (gradient) of the land;

    – contour lines that are close together denote steep slopes;

    – contour lines that are far apart denote gentle slopes;

    – when there are no contour lines, it means the land is almost flat;

    – when the contour lines spaced further apart as the height of the slope

    increases, the

    – Slope is convex; when the contour lines spaced closer together as the height

    of the slope increases, the slope is concave (broad spacing to narrow spacing).

    iii. Recognizing land features on a map involves identifying the shape created

    by the patterns of contours

    2.5.3. Hill shading

    Hill shading is the method of adding light and dark area or shading to a map to highlight the location of hills or mountains. When light is shone from a given direction, areas with steep slopes are hidden. Such parts can be clearly shown by shading. The thickness of the shade depends on how steep the slope is. Steep relief has darker shading compared to gentle slopes. This method does not show the exact height of the relief feature. Refer to the figure below.


    2.5.4. Hachures

    Hachures are short lines on a map that indicate the direction and steepness of a slope. Hachures that represent steep slopes are short and close together while hachures that represent gentle slopes are longer, lighter, and farther apart.


    2.5.5. Pictorial representation

    In this method, certain symbols are used to show relief on maps. It does not give the heights above sea level. Only a small variety of landforms can be shown using it. The pictures may further hide important details. See the figure below.


    2.5.6. Colouring/ Layer Tinting

    Colouring or layer tinting is a method of showing relief by colour. A different colour is used for each band of elevation. Each shade of colour or band, represents a definite elevation range. A legend is printed on the map margin to indicate the elevation range represented by each colour. However, this method does not allow the map user to determine the exact elevation of a specific point; only the range is identified.

    Application activity 2.5

    Make your own research and find an extract of the topographic map of

    Rwanda, and describe its relief basing on different representation signs used.

    2.6. Calculation of the vertical interval and the amplitude of relief /spot heights

    2.6.1. Vertical interval and amplitude of relief

    The dark lines with reading are index contour lines while thin lines represent intermediate contours. The difference in height or altitude between two places is known as the vertical rise or the vertical interval (V.I.).

    How to calculate the vertical interval?

    There are several steps that are followed when the vertical interval is being

    determined. These include the following:

    1. Locate 2 index contour lines that are labeled with a specific elevation.

    2. Now calculate the difference between the two-selected index contour line

    selected from a map. To take the difference, subtract the higher elevated

    line with the lower elevated line reading.

    3. Now count the number of non-index lines contour lines between the 2

    index contour lines selected for the contour interval calculating in the 1st


    4. The number of lines obtained in the above step is taken and added with 1.

    For example: if the number of lines between 2 index lines are 4. Then add

    1 to 4 that becomes 5.

    5. The final step is the measure of the difference between 2 index lines (step

    2) and the number of lines in between two index lines plus 1 (step 4).

    6. The final answer we get after dividing is the contour interval of the specific

    topographical map.

    2.6.2. Spot heights and trigonometrical stations

    A spot height is shown as a dot and the actual height in metres. Spot heights are mainly used where drawing of complete contours is difficult. This happens on such features as mountain peaks and hilltops. Trigonometrical stations are also known as triangulation points. Surveyors mark those using triangles or circles with a dot at the centre.

    End unit assessment

    Mugisha and Mucyo are traders in Kigali and they export goods made in Rwanda and import some missing commodities not locally produced. Sometimes goods delay as they are being transported from Mombasa port. Suppose you want to travel with these traders, explain how you would use a map to find out the distance from Kigali to Mombasa

    Imagine someone comes from Europe to visit Rwanda, specifically in Muhoza sector, Musanze district (see the map below). At Kigali airport someone gives him a map with a scale of 1:50 000. He/ she needs to know the bearing of Muhoza from Kigali.

    1. Show how you would find the bearing of Muhoza if this visitor asked for


    2. What is the relative position of Muhoza from Kigali?

    3. Basing on your general knowledge describe physical features that

    make Musanze to be known and hence attracting many people all over

    the world. In addition, explain the environmental hazards that use to

    happen in its area of northern part of Rwanda.


    Key unit competence

    By the end of this unit, I should be able to explain the relationship between the

    physical and human activities on maps

    Introductory activity:

    A map is a representation of all or part of existing things on the earth surface on a flat surface. A photograph is a picture of an object or environment taken by a camera at a particular time in a given place. Maps and photographs represent both physical and human features. The aspects represented on map/photographs are shown using several types of signs and symbols. This helps map /photograph users to extract the needed information from a given map /photograph.

    Referring to the map above and the definition above;

    a. Identify the main physical features represented on the map.

    b. Identify the human features represented on the map.

    c. Use the following map to represent approximately the same features as

    those represented on the map above. 

    d. Take a photograph of the environment around your school and then identify all physical and human aspects taken on that photograph.

    3.1. Interpretation of physical aspects from maps/photographs

    Activity: 3.1

    Make a research on:

    a. The major physical aspects which may be represented on a map/


    b. The signs and symbols which may be used to represent physical

    features on a map/photograph.

    Physical features are also called natural features. They include the following:

    – Relief which comprises rocks, slopes, soils, valleys, plains, plateau, hills and


    – Drainage features like rivers, dams, lakes, seas and oceans.

    – Vegetation cover like forests made of several types of trees, crops and ground

    vegetation, wetland vegetation.

    – Climate which is defined through various parameters like rainfall, temperatures,

    relative humidity, cloud cover, wind speed and direction, atmospheric pressure,

    sun shine.

    The following are the most commonly used methods to represent the selected physical features:

    3.1.1. Interpretation of geological features

    The main geological features commonly represented on a map/photography

    include rocks, cliffs and boulders.

    – A rock is a solid matter which is found on the surface or in the interior of the


    – A cliff is a vertical or nearly vertical rock face in mountainous and hilly areas.

    Cliffs are found along the shores of lakes and seas. Cliffs and rocks are shown

    on topographical maps using contours that are close to each other. Boulders

    are large pieces of rock. They indicate weathering and mass wasting taking

    place on the cliff.

    – Flat rocks show the eroded and exposed rock on the seabed where the shore

    is shallow or found the bed of a river or within the dry valley.

    – A crater is a bowl-shaped depression, or hollowed-out area, produced by the

    impact of a meteorite, volcanic activity, or an explosion.

    – A quarry is a place from which dimension stone, rock, construction aggregate,

    riprap, sand, gravel, or slate has been excavated from the ground. A quarry appears like a place where open cast mining is practiced.

    3.1.2. Interpretation of slopes from a map/photograph

    The slopes are classified according to their inclination, constancy or shape. The slopes are represented on the map by help of contours. Contours are lines on a map linking places of the same height above the sea level. They are drawn at fixed intervals, known as vertical interval or VI. Contours are labeled from the lowest to the highest. Where they are too close to each other, the relief is steep. Where they are far apart, the land may be a plain or a plateau. The main types of slopes are: gentle, steep, regular, irregular, convex and concave slopes.

    Gentle slopes: Spaced contours are used to indicate gentle slopes. In that

    case the land may be a plain or plateau.

    Steep slopes: Very close contours are used for steep slopes. The closer the

    contours, the steeper the slope. In that case the landform may be a mountain.

    Regular slopes: Regular slopes are also called constant or even slopes. The

    contours have constant spaces. The slopes can be either gentle or steep slopes.

    Irregular slopes: These are uneven or inconstant slopes. They are represented

    by unequally spaced contours. They can be either gentle or steep slopes. These

    slopes are found mostly in rugged, mountainous or hilly areas. 

    Concave slopes: They indicate that the land is steeper on the upper part and gentler on the lower part. They are drawn using closely packed contours on the upper part of where the slope is steep. The contours are widely spaced on the lower part where the slope is gentle.

    Convex slopes: Convex slopes are gentle at the top and steep at the bottom of the hill or mountain. The contours are closely spaced at the bottom section (steep slopes) and widely spaced at the top section (gentle slopes).

    3.1.3. Interpretation of drainage and landforms from a map/photograph

    i. Interpretation of drainage features from a map/photograph

    The lake, sea and ocean occupy a very large area. They are seen in blue color on a color photograph and in dark color on black and white photograph. However other conventional symbols may be applied on a map to represent a lake, sea and ocean; the most important thing is to put in legend/key the used symbols.

    The rivers originate from mountains or hills and flows on steep slope to end in the depression that are usually found in valleys or low-lying areas. The streams and rivers are seen on a photograph/map as lines and they have different patterns or arrangements which can be detected easily on a map or photograph.


    ii. Interpretation of landforms:

     Valley: A valley is an elongated depression sloping towards a drainage basin like a sea, lake or swamp and which may contain water or not. The valley is normally represented on a topographic map by using the contours having “V shape”.

    Plain: Most of the plains are in low-lying areas but some of them may be raised but the slopes remain gentle. On the topographical maps, a plain is represented by widely spaced contours. The river passing in plain areas may also be shown on the map.

    Plateau: A plateau is an extended landform which is bordered by steep slopes. On the map, a plateau is shown as a wide area surrounded by one or two contours having the same height on both sides.

    Mountain: A mountain is an extended landform with steep slopes. On the map, a mountain is shown as a wide area with very close contours surrounded by one or two wider contours showing the depression areas surrounding the mountain.

    3.1.4. Interpretation of soils on the map

    Assorted colours or symbols are used to show several types of soils on a topographic map. Tiny brown dots called stipples are used to represent a surface covered by sand or mud. The types of soil can be also shown by help of dots having varied sizes according to the texture of the soil. The types of soils can be differentiated on a photograph based on their texture (sizes) and color. Surfaces covered by lava flows are shown by symbols that look like inverted V’S.

    Application activity: 3.1

    a. Identify the physical features in area where you live and describe how

    they are represented on a topographic map.

    b. Draw a sketch of your home area on it name and mark the physical

    features identified in (a) above.

    c. Suggest ways through which the above features can be protected

    and conserved.

    3.2. Interpretation of human aspects on maps

    Learning activity: 3.2

    Make a research on:

    a. The major human aspects which may be represented on a map/


    b. The signs and symbols which may be used to represent human

    features on a map/photograph. 

    Human aspects on a map reflect human activities of a given area represented on a map. These include agricultural development, mining, industry, settlement, etc.

    3.2.1. Agricultural activities 

    Crop plantations are drawn on a topographical map by using light-green shading. A letter may be used over the shade to indicate the name of the crop growing in each area. For example, C for coffee; T for tea.

    3.2.2. Mining and quarrying activities from a map

    Mining refers to all the processes by which minerals are obtained from the earth’s crust. Minerals may be in gaseous, liquid or solid form. Quarrying is the digging of stones, sand or soil from the ground. These are used, for example, in construction. Mining and quarrying activities on a map are shown by symbols as represented on the figure below. The mining activities may be taken on a photograph and different types of minerals/quarries may be differentiated based on their colors.

    3.2.3. Industrial areas

     Industrial area is a geographically localized set of specific industries. Such areas are subject to important production, marketing and other interrelationships.


    3.2.4. Settlements 

    Settlement refers to a place where people live. It also means the process of settling in such a place. Settlement on the map can be shown by dots or rectangles which may be black or grey in colour. Dots indicate rural semi-permanent settlements while rectangles show permanent buildings like those built of stone or bricks, with iron or tile roofing. The dots or rectangles are spaced in relation to the concentration of settlement. Nucleated settlements are represented by the dots and rectangles close to one another while they are much spaced for scattered settlement.

    The settlement is also represented according to its shape, for example ring or linear. The shapes of settlement may be influenced by communication networks like roads, railways, landforms like valley, plain, plateau, mountain among others.

    Application activity:3.2

    1. Use a photograph/map to identify and describe mining/quarrying areas.

    2. Visit the nearest mining/quarrying areas and represent them on sketch

    map using appropriate signs and symbols.

    3.3. Relationship between physical and human aspects on maps photographs

    Physical aspect of an area plays a key role in the activities people do. It also has an influence on the number of people who settle in an area. For instance, the relief affects human activities in the following ways:

    – Mountainous places are less populated because of steep slopes being unfavorable for settlement and for mechanized agriculture. Such landscape makes the establishment of social facilities (hospitals, schools, shopping

    centers) in such places to be difficult.

    – Places that are poorly drained like swamps and marshes are also not suitable

    for settlement.

    – Some plains and plateaus have good soils. These allow growing of crops and

    keeping of livestock. It is also possible to use machines in farming. There is no

    need to make terraces to stop soil erosion. Therefore, more food is produced,

    and more people live there.

    – The depressions or valleys areas are also suitable for agriculture.

    – Fishing is done in seas, oceans and lakes.

    3.3.1. Drainage patterns

     A drainage pattern is a network formed by rivers and their tributaries on the landscape. The development of the drainage patterns is influenced by the gradient of the slope, nature of the bedrock in terms of hardness, structure of the basement rock. The drainage pattern can also result from human activities which may change the original patterns. Man’s activities that have direct impact on the drainage pattern include agriculture, industries, settlements, dam construction, etc.

    The main drainage patterns which can be seen on a topographic map/photograph

    are as follows:

    Trellised drainage pattern: Such patterns are developed in simple folds

    characterized by parallel anticlinal ridges alternated by parallel synclinal


    Dendritic drainage pattern: The dendritic pattern is associated with the

    areas of similar lithology, horizontal or very gently dipping strata, and rolling

    extensive topographic surface having extremely low reliefs.

    The rectangular drainage pattern is generally developed in the regions

    where the rock joints form rectangular pattern. The rocks are weathered and

    eroded along the interfaces of joints, fractures and faults and thus surface

    runoff collects in such long and narrow cliffs and forms numerous small rills.

    Radial drainage pattern also known as centrifugal pattern is formed by the

    streams which diverge from a central higher point in all directions. 

    Centripetal drainage pattern: This pattern is formed by a series of streams

    which after emerging from surrounding uplands converge in a central lowland

    which may be a depression/basin/crater lake

    Annular drainage pattern: The annular drainage pattern, also known as

    “circular pattern”, is developed over a mature and dissected dome mountain

    characterized by a series of alternate bands of hard and soft rock beds.

    Barbed drainage pattern is formed when the tributaries flow in the opposite direction to their master streams.

    Herringbone drainage pattern also known as rib pattern (like the limbs of human beings) is developed in mountainous areas where broad valleys are flanked by parallel ridges having steep hillside slopes.


    Parallel drainage patterns comprise numerous rivers, which are parallel to each other and follow the regional slope. This pattern is more frequently developed on uniformly sloping and dipping rock beds such as cuestas or newly emerged coastal plains.


    3.3.2. Settlement patterns

     Settlement refers to a place where people live. It also means the process of settling in such a place. The settlement patterns of a given area are influenced by both physical and human factors like topography, road network, community services, cultural and economic factors, etc. This results into several types of settlement patterns as follows:

    Nucleated settlement: This refers to the grouping of many houses around a center called nucleus; often around a central feature like a church or a pub. Houses are built close to each other. This type of settlement is mainly influenced by cultural and social factors. For example, the houses in “imidugudu” settlements may have such pattern.


    Linear settlement: This is where houses are well planned. They are built along the road, railway and river or along the coast.


    Sparse or scattered or dispersed settlements: These settlements are associated with houses which spread out over a wide area (houses are built far from each other). They are often the homes of farmers and can be found in rural and mountainous areas.


    Isolated settlement: This is where a few people live away from other people.

    These people could be hunters in a forest.

    Ring settlement: In this type of settlements, houses make a circle. There is an

    open ground at the center.

    Planned settlement: This type of settlement is well organized and planned in specific ways as directed for instance by the government.

    3.3.3. Vegetation

     Human activities are related to vegetation. The distribution of vegetation on a photograph/map in each area is linked with human activities taking place in that part/dwelling. Below are some of the examples that show the interrelationship between man and vegetation.

    In areas with dense forests: The main activities are timber harvesting. The timber is used to make furniture and paper.


    In savannah grasslands: the main activities are livestock keeping and agriculture. This is because in these areas soils are fertile.


    Desert vegetation: Agriculture is only possible in oases. Very few animals can survive in the dry areas like the camels, goats and sheep and reared under nomadic pastoralism.

    In tundra and taiga forests: The low rainfall and temperature affect human activities. Only hunting, fishing and mining are the main activities done in these regions.

    3.3.4. Communication networks 

    Communication networks facilitate the movement of people and commodities from one place to another over a given distance. Communication networks may also refer to the physical facilities which help the transmission of information in the form of news and messages. The communication networks represented on map/ photograph are most of time influenced by both physical and human factors. Some of them are discussed below:

    Relief: Steep slopes make the construction of roads and railway lines expensive. On the other hand, valleys have swamps; they contain water logged soils that are too soft to allow the movement of heavy objects like trailers, lorries and trains.

    Climate: Too much rainfall results into floods and landslides and these disturb land transport. On the other hand, accumulation of fog and clouds reduce visibility hence affecting air transport.

    Drainage: Navigable rivers, lakes, seas and oceans provide natural route-ways used for transportation.

    Economic activities: The economic activities (mining, agriculture, industries, etc) taking place in given areas influence the construct of communication networks.

    Application activity: 3.3

    1. Identify several types of communication networks on any map/


    2. Explain how the identified communication networks are related to

    physical and human features.

    3. Move around your village and identify the existing communication

    networks and explain their relationships with physical and human


    End unit assessment

    1. “As it is difficult to reach all parts of the world; the photographs and

    maps help to explore different physical and human features on the


    a. Identify physical features on topographic map of Rwanda.

    b. Identify the human features on thematic maps of Rwanda.

    2. Show different signs and symbols to be used in representing physical

    and human features on maps/photography.

    3. Discuss the relationships existing between settlement patterns and

    physical features in your district and present them on a sketch map

    using signs and symbols.

    4. From the photographs taken at different places in Rwanda, prepare

    a sketch map of captured physical and human features by using

    conventional signs and symbols.


    Key unit competency:

     By the end of this unit, I should be able to distinguish between the components of

    the universe and the solar system.

    Introductory activity:

    One of the students in senior one moved outside one evening and all of sudden

    saw a running star that disappeared. The moon was fading, being covered by dark

    clouds. He wondered whether there are moons elsewhere and went back to bed. In

    the morning, the sunrise with golden rays replaced the dark and starry night.

    a. Identify the heavenly bodies mentioned in the passage.

    b. Using your experience and the passage above, identify other

    components of the universe not mentioned in (a) above.

    4.1. The universe

    4.1.1. Definition of the universe and Components of the universe.

    The universe refers to all of space and everything in it. It contains everything that exists, from the smallest particles to the largest structures known. The exact size of the universe is not known. Astronomers estimate that it contains about 100 billion galaxies. Astronomers are experts who study bodies in the sky or outer space. A galaxy is a system of stars, together with gas and dust. These are held together by forces of gravity. Each of the galaxies has an average of 100 billion stars. The origin of the universe is explained through the Big Bang Theory, which happened about 13 billion years ago.

    4.1.2. The components of the universe 

    The Universe contains many components, which vary considerably in size. The smallest components are atomic particles followed by atoms (mostly free hydrogen and helium), molecules, dust, space rocks, comets, asteroids, moons, dwarf planets, planets, solar systems, stars, black holes, nebulae, and galaxies. Among these components the solar system is the most known with certainty.

    The table below shows the components associated with the universe.


    4.2. Solar system: sun and planets

    Activity 4.2.

    1. Using the previous knowledge that you have in Geography, explain the

    meaning of solar system and identify the elements or components that

    form it, and share your finding in class.

    2. Using internet, text books and other sources of geographical

    information, research on the characteristics of sun and planets.

    The term solar system is defined as the arrangement of the sun and planets that revolve around it (Sun). The solar system is therefore made up of the following: the sun, planets including the earth, the moon and other heavenly bodies such as asteroids, comets, meteorites, meteors, etc.


    4.2.1. The sun

     The sun as earlier studied, is one of the billions of stars that make up the Milky Way galaxy. It is one of the smallest stars in our universe. However, it is 109 times bigger than Planet earth. It forms the center of the solar system. Its gravitational force keeps planets in their orbital position. All the 8 planets revolve around.


    1. Characteristics of the Sun

    The sun as any other heavenly body, has characteristics that distinguish it from the

    rest of other components of the solar system. These are explained hereunder:

    – It makes or emits its own light/heat.

    – It has the diameter of 139,200 km.

    – Its temperature ranges from 4000-9000 degrees Celsius.

    – Its mass stands at 1.98892 x 1030 kilograms.

    – It has a density of 1.4 grams per cubic centimeters.

    – The sun is made up of hydrogen and helium.

    – Its radius is estimated to be at 695,500 kilometers.

    – It takes 25 days to turn once on its axis.

    2) The influence of the sun on the Earth

    – It holds the earth in its orbital position.

    – It contributes greatly in the balancing of the tidal bulge caused by the moon’s

    gravitational pull and the inertia.

    – The sun is the source of the energy that is used by the earth and all that is


    – It engines the hydrological cycle.

    – Supports life on earth through many ways such as creation of suitable and

    favourable temperatures.

    – The sun influences the general climate at the hand of the solar radiation


    – Contributes to the formation of tides that support in one way or the other

    support ecosystem and man’s activities. 

    4.2.2. Characteristics of different planets and their positions 

    The term planet means the rock solids that are in oval or spherical shape floating in space and rotating on its axis and revolving round the sun. Planets are grouped into categories as shown below:

    Pluto is known as a dwarf planet. It never developed fully. It is very important

    to know that Pluto was removed from the list of planets in 2006, because of the

    following reasons:

    c. Its size is too small to be classified as a planet.

    a. It has no uniform revolution round the sun.

    b. Its revolution is not circular but spherical in nature.

    c. Its movement is too slow when compared with the rest of the planets

    of our solar system.



    Application Activity 4.2

    1. You are asked to address the senior fives from other schools, explain

    what you would tell them about the positioning and characteristics of

    the planets found in our solar system.

    2. “The earth is the only planet that supports life” Explain why it is so and

    show how you would ensure that it continues supporting flora and


    4.3. Earth: Peculiar elements of the earth

    The peculiar elements of the earth are outer parts of the eath. They include the


    1. Hydrosphere or water bodies

    2. Lithosphere or Land and rocks

    3. Atmosphere or Gasses

    4. Biosphere or flora and fauna


    – Hydrosphere: This stands for all the waters found on the earth surface.It

    covers 71% of the earth’s surface.It includes: Lakes, Seas,Oceans,Wetlands ,


    Biosphere: This is known as ecosphere. It is a part of the earth that includes

    the totality of life on the earth (Animals,Plants and Man).

    Lithosphere: This is the solid part of the exterior area of the earth.It is made

    up of the crust and a small percentage of the upper mantle (Land ,Rocks, Soils

    and Minerals).

    Atmosphere: It describes the zone occupied by air or gasses that surround

    the earth.This zone is composed of gasses such as: Nitrogen,Oxygen, Argon

    ,Water vapour, Carbon dioxide, Helium and Methane.

    4.4. Earth’s movements

    Learning activity 4.4

     Read the following passage and answer the questions provided.

     Every day the sun rises in the east and sets in the west. This process led many people in the past to think that the sun is moving, and the earth is fixed. The sun moves around the earth. But with the advancement of science, this has been proved that the sun itself is moving and the earth has also motion. Later it has been revealed that the sun does not move around the earth, rather the earth moves around the sun.

    With the help of your knowledge and skills acquired in previous studies answer

    the following questions?

    a. How many movements does the earth make?

    b. Identify proof for the earth rotation and revolution

    c. Describe effects caused by the above movements

    4.4.1. Rotation of the Earth 

    Rotation of the Earth is defined as the movement of the Earth spinning on its own axis. This movement of the Earth on its own axis is in an anticlockwise direction. The earth takes 24 hours to complete 360˚. At the equator the earth rotates at a speed of 1676 km and zero km at the poles per hour.

    The effects of rotation of the earth

     As the Earth turns around its axis, it affects some processes on the earth’s surface and other associated celestial phenomenon. Some effects of the earth rotation are:

    i. Rotation causes day and night

     Earth’s rotation on its axis creates day and night. The one half of the Earth that faces the sun has day time, while the opposite half facing away from the Sun has night time.

    ii. Rising and falling of ocean water (tides)

    During the rotation of the earth, gravitation force pull of the sun and the moon acts

    on the ocean water to produce tides which may be high or low.

    iii. Deflection of wind and ocean current (Coriolis Effect)

    Rotation causes winds to be deflected to the right in the Northern or to the left

    in southern whenever they cross the Equator. This deflection is called the Coriolis


    iv. Time difference between longitudes

    One round of the Earth is completed after turning 360 .

    This implies that the earth takes 24 hours to complete rotation. Therefore, for the

    earth to cover 15o it is calculated as follows:

    v. Temperature difference 

    Due to the spherical shape, the parts of the Earth located in the tropical areas between 23.5˚ North and South of Equator, get direct sunlight all the year round. Regions located in higher latitude get less rays during the year.

    4.4.2. Revolution of the Earth 

    The revolution of the earth is the movement of the earth around the sun. Earth revolves around the Sun along an oval-shaped path called an orbit. The area of the oval-shaped path is called Plane of the ecliptic, in which the axis of the Earth is tilted at an angle of 23 27’. Earth takes 365 days and 6 hours or one year to complete one revolution, at a speed of 106,260 km/h around the Sun (30 kilometres per second).


    The Leap year is the year with 366 days which come after four years where February has 29 days instead 28 days. This results from adding 6 hours of rotation of each year to make one year after 4 ordinary years.

    The consequences of the revolution

    i. Determination of seasonal variation

    The whole year has been divided into four divisions. Each of such division is known as a season. There are four seasons namely summer, autumn, winter, and spring. The earth’s revolution leads to change in the sun’s overhead positioning. This comes along with the occurrence of solstices (winter solstice, summer solstice) and equinox.

    Solstice is either of two times of the year at which the sun reaches its highest

    or lowest point in the sky at midday, marked by the longest and shortest days

    over the tropics. Solstice occurs on 21st June and 22nd December when the

    sun’s overhead position is either at the tropical of Cancer or Capricorn.

    Equinox occurs two times in the year (around 21st March and 23rd September)

    when the sun is above the Equator, day and night have equal length. 


    The four seasons:

    Summer is the hottest of the four temperate seasons. This occurs immediately after the spring season and before autumn. When it is the summer solstice, the days are the longest and the nights are the shortest. The day length begins to decrease as the season progresses towards autumn. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice-versa.

    Winter is the coldest season of the year in the polar and temperate zones. It occurs after autumn and before spring in each year. Winter is caused by the axis of the Earth in that hemisphere being oriented away from the Sun. when it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere, and vice versa. During winter the days are short and nights have longer hours. However, this changes as the season progresses towards spring.

    Spring is one of the four conventional temperate seasons, following winter and preceding summer. Days become longer, and weather gets warmer in the temperate zone because the Earth tilts towards the Sun. In many parts of the World plants grow and flowers bloom.

    Autumn, this is a season of the year between summer and winter during which temperatures gradually decrease. The fall in temperatures brings direct impact on the growth of vegetation. Therefore, the vegetation begins to decrease. It’s the season when the days get shorter and colder, and everything turns brown and the plants begin to shed leaves.

    ii. Varying length of day and night at different times of the year 

    The revolution causes variations in the length of the day and night over different latitudes. When the sun is in the Southern hemisphere i.e. overhead the tropic of Capricorn, the latitudes in the northern hemisphere receive less hours of the sunlight (daytime) but more hours of night time; while the Southern hemisphere receives more hours of heating hence more hours of daytime.

    During the summer solstice, day time is longer than night time in areas found in higher latitudes. This implies that, latitudes beyond equator will experience increase in hours of day time towards the poles (from 12hrs to 24hrs at the arctic circle and beyond). During the winter solstice, night time is longer than day time at latitude beyond equator. Norway is known as the land of the midnight sun because the sun does not go below the horizon or comes above it on 21st June


    iii. Climatic zone. 

    These are divisions of the Earth’s climate into general climate zones according to average temperatures and rainfall. The three major climate zones on the Earth are: the Polar, Temperate, and Tropical climatic zones. Temperatures in these three

    climate zones are determined mainly by the location, or latitude. The reason why the equatorial zone is hotter than the poles is that sun’s rays fall vertically at the equator and obliquely at the poles. Example, the stations around the equator such as Kisangani, Masaka, Libreville, and Manaus experience hot temperatures while areas such as Alaska, Greenland and Siberia near the poles experience cold temperature.

    iv. A light year

     A light year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. More precisely, one light-year is equal to 9,500,000,000,000 km.

    Why such a big unit of distance?

     Well, on Earth, a kilometer may be just fine. It is few hundred kilometers from Kigali city to Rwamagana; it is a few hundred kilometers from Rusizi to Ngoma. In the Universe, the kilometer is just too small to be useful.

    For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21quintillion km. That’s 21,000,000,000,000,000,000 km. Astronomers use other units of distances in terms of the Astronomical Unit (UA). The AU is defined as the average distance between the Earth and the Sun. It is approximately 150 million km.

    4.5. The Moon

    Activity 4.5.1

    With the help of geographical documents and other relevant sources of

    information, search for the following information:

    a. The natural satellite of the earth.

    b. The characteristics of the moon.

    c. How moon influence the earth.

    d. Types of eclipses.

    e. Identify the effects tides may cause.

    The moon is a natural satellite that moves around the planet (larger natural body) in space. It is the only satellite of the earth which is about 4.5 billion years old. The moon is the natural satellite that goes around the earth and that people can see shinning in the sky at night.

    4.5.1. Characteristics of the moon

    4.5.2. Phases of the moon 

    As the Moon orbits Earth, it reflects light from different angles, which change shape of the moon. These change shapes of the moon are called the phases of the Moon.

    The phases of the Moon are divided into Primary phases and Intermediate phases.

    Primary phase includes the following:

    The New moon phase: This is a period when the moon is between the Sun

    and the Earth. Its sunlight side is turned away from Earth (it is not visible).

    The first quarter phase: Is a phase that occurs 7 days after new moon phase

    when half shape of the Moon appears to be receiving sunlight.

    The full moon phase: The whole side of the Moon receiving sunlight faces the

    Earth, 14 days after new moon. This implies that the Earth, Sun, and Moon are

    almost aligned in a straight line, with the moon in the middle.

    Last quarter phase: A half of the moon appears to be lit by sunlight. The left

    part of the moon is facing the direction of the rising sun. This is because the

    moon is in front of the sun but between the Earth and the sun.

    Intermediate phases

    Waxing crescent is a small part of the moon that receiving sunlight seen from

    the Earth just as a bright crescent in the direction of the setting sun.

    Waxing Gibbous: This is when the moon appears to be having a small part

    that receives sunlight as seen from the Earth. It is just as a bright crescent in

    the direction of the rising sun.

    Waning Crescent: The Moon appears to be partly but less than one-half

    illuminated by direct sunlight. The fraction of the Moon’s disk that is illuminated

    is decreasing. 

    Waning Gibbous: This occurs when more than a half of the lit portion of the moon can be seen and the shape decreases in size from one day to the next. It occurs between the full moon and the third quarter phases.

    4.5.3. Influence of the moon on the earth

    1) Eclipses of the moon

    Eclipse is the obscuring of one celestial body by another, particularly that of the sun

    or a planetary satellite.

    Types of eclipses

     – A lunar eclipse: A lunar eclipse occurs when the moon passes directly behind the earth into its umbra (shadow). This can occur when the sun, earth and the moon are aligned exactly or very closed with the earth in the middle. A lunar eclipse occurs only during night. A lunar eclipse may be total lunar eclipse: that happens only when the sun, earth and moon are perfectly aligned. When its alignment is less than perfection, therefore a partial lunar eclipse occurs.


    Solar eclipse: This is a type of eclipse that occurs when the moon passes between the sun and earth, and the moon fully or partially blocks rays of the sun from reaching the earth. This occurs during daytime.


    (2) Tides 

    The word “tide” is a term used to define the alternating rise and fall in sea level, produced by gravitational attraction of the moon and the sun. The moon tries to pull anything on the earth to bring it closer, but the earth is able to hold onto everything except water. Since the water is always moving, the earth cannot hold onto it, and the moon is able to pull it. Each day, there are two high tides and two low tides. The ocean is constantly moving from high tide to low and then back to high tide. The high tide is called spring and the low tide is named neap.

    Application activity.

    Using specific examples answer the following questions.

    1. Explain why the moon shows one side from the earth.

    2. Explain what happens when the moon is placed between Earth and the


    4.6. Other heavenly bodies

    It is important to note that there are other heavenly bodies. These include the ones

    shown below:

    -Asteroids - Meteoroids - Meteorite - Meteors

    -Comet - Constellations - Galaxies

    Application activity

    Search on internet and use other geographical documents and answer the

    following questions:

    1. Discuss conditions that make our planet habitable.

    2. What are the benefits people gain from some heavenly bodies that land

    on the earth’s surface?

    a. Identify the heavenly bodies shown in the two photographs.

    b. Mention and describe the heavenly bodies found in the universe that

    are not shown in the two photographs.

    c. Explain why the areas shown in the two photographs are all found on

    planet earth yet having different habitable conditions.

    d. Draw a program that can enable man to live in harmony with the

    universe and the earth.

    2. Read the story below and use it to answer the questions that follow:

    Recently John and Mary went to visit their uncle in Rubavu. In the afternoon of their second day, they were taken by their cousins to visit the lake shores. Evening came, and the moonlight was so intense that one could pick the smallest thing at that time. Suddenly, they saw water extending in a rhythmic manner to the place they had made a camp fire.”

    a. How could you use your knowledge to convince John and her sister

    Mary that what happened was connected to the moon, sun and

    earth’s rotation.

    b. Suppose you want to build near the coastline, explain how the

    knowledge and the skills acquired from the lesson on tides can guide


    c. The Indian Ocean experiences many tides, design a project that you

    would sell to the Kenyan government to address the effects of tides

    on the coastal biodiversity.


    Key unit competence:

     By the end of this unit, I should be able to discuss the theories of the origin of the


    Introductory activity

    Using previous knowledge gained in Geography and other disciplines:

    a. Explain how the Earth came into existence.

    b. Identify the major characteristics of the earth.

    c. Describe the internal structure of the Earth.

    d. Find out and describe the various geological eras and periods

    recorded in history. 

    5.1. Theories of the origin of the earth

    The formation of the Earth is imbedded in the whole process of the universe formation. The study of the universe is called cosmology. Cosmologists study the structure and the changes that take place in the universe. The universe contains all the star systems, galaxies, gas and dust, and all the matter and energy that exist. The universe also includes all of space and time. The part of the entire universe that we can see is called the observable universe. It can be seen because light from celestial bodies is able to reach the earth. The earth is part of it. Now the big question is to know: what is the origin of the earth? In other words, how the earth came into existence?

    The origin of the earth has resulted into many theories that were put forward to explain its possible origin. Eleven theories about the origin of the earth are highlighted below. However, the two first ones - Big Bang theory and Creation theory – seem to be the most accepted.

    1. The Big Bang theory

    2. The creation or biblical theory

    3. The dust cloud theory

    4. The Kant-Laplace nebular hypothesis

    5. The Chamberlin-Moulton planetesimal hypothesis

    6. The Cometary collision hypothesis

    7. The Encounter hypothesis

    8. The Tidal theory

    9. The Fission theory

    10. The Accretion theory

    11. The Stellar collision theory

    5.1.1. The Big Bang Theory

     This theory was first suggested by a Belgian priest named Georges Lemaitre in 1920. He hypothesized that the Universe began from a single primordial atom. This theory further states that in the beginning, there was totally nothing. In about 15 billion years ago a sudden explosion happened (big Bang) which produced a speck of matter that was smaller than an atom. From the time of its formation, this small matter has continued expanding and resulting into the formation of the Earth and all that forms it: the atmosphere, hydrosphere, lithosphere, and biosphere. This theory is criticised on one ground; how can nothing lead to the occurrence of something.

    5.1.2. The Biblical or creation theory

     The biblical or creation theory is based on the liturgical narrative as expressed in Genesis chapter one of the Holy Bible. It talks about specific acts of divine creation. The theory is believed in by Christians. According to the creation theory, at the beginning there was nothing. The divine power (God) created heaven and earth and all that are in them. The theory holds that God is the master creator of planet Earth and the universe in general. The theory further shows that God created two great lights that is; the greater light which He called Sun and the lesser light which was named the moon. The sun was to provide light during the day and the moon to provide light at night. The whole creation process took place in six days.

    – On the first day, God created the day and the night.

    – On the second day, He created the sky.

    – On the third day, He created the land which is the earth, the sea and the plants.

    – On the fourth day, He created the sun, moon and stars.

    – On the fifth day, He created the sea creatures and the birds.

    – On the sixth day, He created land animals of all kinds and human beings. 

    Therefore, according to this theory, the Earth and universe were all together created by God.

    Application activity 5.1

    From the theories that you have learnt above;

    a. Explain the difference between the Big bang theory and the Biblical

    theory about the origin of the earth.

    b. Make a research on other theories of the origin of the earth.

    5.2. Characteristics of the Earth

    Activity 5.2

     Read the passage below and answer the questions asked Far back in ancient times, everybody thought the earth was flat. This is because it looks flat. If you are in a boat way out in the middle of the ocean, the top of the water looks flat in every direction and the sky seems to fit over it like an upsidedown bowl. The line where the sky and water meet is called the “horizon.” The horizon looks like a circle with you yourself at the center. If you are on land, the land stretches out to a horizon also. The horizon on land, however, is not even. It goes up and down because of houses, trees, hills, and other things. Some ancient people suspected that the earth went on forever. They thought it might be a huge flat piece of land and sea with no end at all.

    a. What does the story above talk about?

    b. How do you find the shape of the earth?

    c. Find out the evidences advanced to support the most agreed shape

    of our planet.

    d. How big is the planet Earth?

    5.2.1. The shape of the earth is spherical

    1. The shape of the earth

    Determining the shape of the earth was a point of concern for many centuries. It was first believed that the earth was flat. This is because the planet appears to be generally flat (not considering mountains and valleys); but the surface of the earth has a slight curve. Further studies based on modern technology proved that the shape of the earth is not a perfect sphere, but an oblate spheroid also known as a geoid. This is because observing the polar areas reveals that they have a flat landscape. At the same time, the equatorial areas have a bulging appearance as shown below:

    2. Evidences showing that the earth is spherical

    There are several evidences to prove the spherical form of the earth:

    – The circumnavigation: If one travelled across the world along the straight path

    without stopping would come back to the point of origin where the journey

    started from.

    – Images from the space: photographs taken from space by satellites show that

    the earth’s surface is curved (spherical shape).

    – Ride of plane: If on takes a trip, especially for a long destination, 2 interesting

    facts are noticed about the planes and the earth:

    – Plane can travel in a relatively straight line a very long time and not fall

    off any edges;

    – If one looks through the window on the trans- Atlantic flight, in most of

    the time sees the curvature of the Earth in the horizon.

    – The view of other planets: All observations from telescopes reveal that the

    planetary bodies are spherical from whichever angle. Therefore, since the

    earth is one of them, its shape is also spherical in nature.

    – The shadow of the earth during eclipses: The shadow resulting from the eclipse

    of the moon (Lunar eclipse) shows that the earth is round.

    – Day-night and seasonal change: The earth’s tilted axis produces seasonal

    climatic conditions and gives days and night of varying length. If the earth

    was at right angles to the sun, the day and the night would always be of equal

    length, and there would only be one season throughout the whole year.

    – The size and diameter of the earth: The earth has an equatorial diameter of 12,

    751 km and its circumference is 40,080 km. These however, are much bigger

    than those of the polar areas.

    – The International Date Line: if two people started off from the prime meridian

    and one went east while another went west, both would meet at the

    International Date Line which separates east from west and if they continue

    moving, each would end up where they started at the prime meridian.

    – The sun rise and sun set: if the earth was flat, the sun would rise and set at

    the same time in all countries. But, the sun rises and sets at different times in

    different places.

    – The polar star: As one moves towards the poles, the size of the polar star

    increases and when one moves away from the Polar Regions it decreases. This

    means that, the Earth is round. In case it was flat, the size of the polar star

    would remain the same or constant.

    – The changing altitude of the sun at different times of day: When the sun rises

    and sets, it displays a lower sun’s altitude than when it is overhead at mid-day.

    – The ship sailing away from or towards the coast: The lower parts of a ship that is moving away from the coast disappear before the upper parts. On the other hand, on an in-coming ship, the upper parts appear first before other parts of the ship. The figure below helps to understand this evidence:


    5.2.2. The shape of the earth is oblate 

    The earth is slightly flattened at the poles and bulging at the equator. From the outer space, the earth looks perfectly round and smooth. The oblateness of the earth’s shape is more pronounced at the equator due to the earth’s rotation. The bulging at the equator is because of strong centrifugal force at the equator is enough to over-ride the earth’s gravity. Therefore, being able to pull matter away from the centre. The main cause of the flat appearance at the Polar Regions is because of the great gravity existing at the poles due to limited rotational speed. This result into the pulling of matter at the polar regions towards the centre of the earth, hence flattening them.

    The following are more details proving that the shape of the earth is not a perfect


    1. The equatorial diameter is larger than the polar diameter. The diameter of

    the Earth at the polar region is 12,713 km, while at the equator it is 12,756


    2. The polar circumference is less than that of the equatorial circumference.

    The polar circumference is 40,008 km while the equatorial circumference is

    40,075 km.

    3. Latitudes near the equator are longer than those near the polar areas. For

    example, 10 latitude near the equator is 111,926 km, while near the north

    pole it is 109,051 km.

    4. Areas at the equator are far away from the centre of the earth hence they

    have a lower gravity than the areas at the poles. At the poles, the areas are

    near the centre of the central part of the earth. This is the reason for the

    intense gravitational pull at the poles.

    5. Basing on the images taken by the satellites, it has been found out that the

    Northern Hemisphere is smaller than the southern hemisphere.

    5.2.3. The Earth’s size: radius, diameter, circumference, volume and mass

    – The average radius of the earth is 6,371 km.

    – The Earth’s equatorial diameter is 12,756 km while the polar diameter is

    12,714 km.

    – The Earth’s circumference at the equator is 40,075 km. From pole to pole,

    its circumference is 40,008 km.

    – The Earth’s volume is 1.08321×1012 cubic kilometres.

    – The Earth is the largest of the four inner planets, although it is nothing

    compared to the gas giants.

    – The Earth’s mass is 5.9736 x 1024 kg.

    – The Earth’s density is 5.52 g/cm3. It is the densest planet in the solar system

    because of its metallic core and the nature of the content of mantle.

    5.2.4 The earth’s gravity 

    Planets are held in their orbits by the force of gravity. The force pulling the planet is the pull of gravity between the planet and the Sun. when you throw an object into the air, it come back and fall to the ground. This is due to the force of gravity. Normally, the gravity of the earth is approximately 9.8 m/s2. The Earth’s gravity is so intense at the poles because, as mentioned above, such areas are near the centre of the earth than at the equatorial areas. The earth’s gravity at the equator is 9.78m/s2 while at the poles it is 9.832m/s2.  In other words, you weigh more at the poles than you do at the equator because of this centripetal force.

    The influence of the gravity

    The earth’s gravity plays a great role as shown below:

    – The density and weight of the materials that compose the interior of the

    earth are influenced by gravitational force. Without this force, the interior of the earth would be unconsolidated and easily pulled by the forces of other

    heavenly bodies.

    – The earth’s gravity glues all materials that form the earth together, hence

    forming a single mass called planet earth.

    – The gravitational force leads to the occurrence of disturbances in the

    asthenosphere which lead to the mobility of the lithosphere. Hence, facilitating

    the formation of new features of great importance. This means that the earth’s

    surface is always renewed.

    – It plays a great role in holding up the organization structure of the solar system.

    The constituents of the solar system are held together by the gravitational

    pull. O such was not in existence, external gravitational forces could literally

    pull components of the solar system including apart, thus destroying them.

    5.3: The internal structure of the earth and mineral composition

    5.3.1. The internal structure of the earth

     The layers of the Earth’s interior include the crust, mantle, liquid outer core, and solid inner core. Continental crust has both felsic and mafic rock zones, while oceanic crust has only mafic rock. The interior structure of the earth is layered in concentric shells:

    – An outer silicate solid crust.

    – A highly viscous mantle.

    – A liquid outer core that is much less viscous than the mantle.

    – A solid inner core.

    The figure below depicts the main layers of the earth’s interior part and their respective properties.

    1) The crust

     The crust is the outermost layer of the Earth. Various landforms feature like mountains, plateaus and plains, rivers, lakes, sea, oceans and human settlements are found on the crust. It is also referred to as the lithosphere. The crust is divided into two types, the oceanic crust and the continental crust:

    – Oceanic crust (SIMA) is a layer consisting mainly of basalt, averaging 6-10 km.

    in thickness. At its deepest it has a temperature of 1200° C.

    – Continental crust (SIAL) can be up to 70 km thick. The crust separated from

    the mantle by the Moho discontinuity. The crust and the rigid lower layer

    bordering the mantle are collectively known as the Lithosphere.

    2) The Mantle 

    The mantle is composed mainly of silicate rocks, rich in iron and magnesium. The mantle is under the crust. It is composed of rocks that are in a semi-molten state. These have relatively higher density compared to the rocks that constitute the crust. The mantle has a mean density of 4.6g/cm3. They are mainly composed of Ferro magnesium silicate (iron, magnesium) minerals.

    The mantle is divided into three main parts, namely the asthenosphere, the upper

    mantle and the lower mantle:

    – The asthenosphere: It is a soft layer of the upper mantle, beneath the rigid

    lithosphere. It is separated from the crust by the transitional line of discontinuity

    called Mohorovicic discontinuity that is shortened the Moho discontinuity.

    – The upper mantle: It is a layer of less rigid and more plastic rocks. It extends

    from Mohorovicic discontinuity to the depth of 1,000 km.

    – The lower mantle: It goes from 1,000 km to 2,900 km in depth. At this depth

    the lower mantle is separated from the outer core by a discontinuity surface

    called Gutenberg.

    3) The core

    The core is composed of outer core and inner core. They are mostly constituted by:

    – The outer core: Is liquid and kept in a semi-molten state.

    – The inner core: Is solid and made mostly of iron and with some nickel (NIFE).

    The temperature at the centre of the Earth (6 371 km below the surface) is

    about 6200° C (hotter than the surface of the Sun).

    The figure below shows the relative depth of different internal layers of the earth.

    5.3.2. The mineral composition of the Earth 

    The mineral composition refers to both the types of minerals within a rock and the overall chemical makeup of the rock. Earth’s rocks are produced by a variety of different arrangements of chemical elements. A list of the eight most common elements making up the minerals found in the Earth’s rocks is described in the table below.

    Rocks are classified into three categories: Sedimentary rocks, Metamorphic rocks and Igneous (magmatic) rocks. Each category has its particular mineralogical composition as summarized in the table below.

    Application activity 5.3

    Apply the knowledge you have acquired in this lesson to answer the following


    1. Suppose you are asked to describe how Mount Muhabura is related to

    the internal structure of the earth, what would be your response?

    2. The earth’s crust is composed of many minerals. Indicate its most

    common chemical elements.

    5.4. Superficial configuration of the earth: Continents and Oceans

    Superficial configuration refers to the distribution of continents and water bodies especially oceans and seas. Oceans and seas occupy the larger part of the earth’s surface. Therefore, the superficial configuration of the earth means the way parts or elements of the planet earth are arranged on its surface. Superficial configuration deals with the distribution of continents and oceans.

    The size of the land surface of planet earth is 148 million km², while the water surface is estimated to be at 363 million km². This shows that the land surface occupies 29 % of the total area of the globe; while the remaining 71 % is composed of water surface coverage.

    5.4.1. Continents

     A continent refers to the world’s continuous masses of land. The earth is constituted of the following 7 continents: Europe (10,532,000 km2), Asia (44,383,000 km2), Africa (30,330,000 km2), America: North America (24,256,000 km2) and South America (17,819,000 km2), Australia (7,687,000 km2) and Antarctica (14,000,000 km2).

    Some Geographers, like Peter George, defined continents as the big land areas surrounded by oceans. Hence, apart from that common assertion of 7 continents, there can be several ways of distinguishing the continents according to the countries:

    – Four continents: Afro-Eurasia, America, Antarctica, Australia;

    – Five continents: Africa, Eurasia, America, Antarctica, Australia;

    – Six continents: Africa, Asia, Europe, America, Antarctica, Australia;

    – Six continents: Africa, Eurasia, North America, South America, Antarctica,


    – Seven continents: Africa, Asia, Europe, North America, South America,

    Antarctica, Australia/Oceania.

    5.4.2. Oceans

     An ocean is a large mass of saline water. It occupies a basin between continents. An ocean is different from a sea. A sea refers to a relatively large body of the salty water that is completely or partially landlocked. Examples of seas include the Mediterranean Sea, the Black Sea, the Red Sea, the Dead Sea, the Caspian Sea and the North Sea. There are five oceans in the world and they include the following:

    – Pacific Ocean: 155 million km2

    – Atlantic Ocean: 76 million km2

    – Indian Ocean: 68.5 million km2

    – Antarctica Ocean: 20 million km2

    – Arctic Ocean: 14 million km2

    Application activity 5.4

    Suppose that the Government of Rwanda has granted the scholarships in the

    following countries North Korea, Germany, Japan, Brazil, Senegal and New

    Zealand. As you get home your parents ask you:

    1. Where the above countries are located? What will be your answer?

    2. Describe the world large mass of saline water.

    3. With specific examples, explain how water is an important resource.

    Describe how it can be conserve.

    5.5. Geological time scale

    Learning activity 5.5

    Read the passage below and answer the questions that follow:

    A study of fossils (palaeontology), together with other geological and biological evidence, provides information on the history of Earth and the evolution of life. Fossils provide evidence about the relative ages of rock strata, paleo-environments and evolution of life. The fossil record has been used to develop the worldwide geological time-scale. Organisms may leave traces of their existence in the sediments formed during, or shortly after their lifetimes. As you have learnt early in this unit, the earth is old enough. If you take a journey back through the history of the earth you will get much understanding about this. Every discipline tells you its own story. Then:

    1. What does the passage above tell us?

    2. Search on internet or other geographical documents on the meaning of

    fossils mean.

    3. Find out and describe the various geological eras and periods recorded

    in history.

    The geological time scale refers to a scheme or chart that indicates age classification of rocks and associated geomorphological and biological events. The earth’s history is divided into smaller units based on the types of life-forms living during certain periods. The division of earth into smaller units makes up Geological time scale. Some of the division in the geologic time scale are also based on geologic changes occurring at that time.

    The geological time scale is a record of Earth’s history, starting with Earth’s formation about 4.6 billion years ago. The geological time scale is used by geologists, palaeontologists and other earth scientists to describe the timing and the relationship between events that have occurred throughout Earth’s history.

    Subdivision of geological time

     There are three types of subdivisions of geological time. These are: Eras, Periods, and Epochs. Eras are major units (subdivisions) of geologic time scale based on differences in lifeforms. There are 3 major eras indicated on the geological time scale. They include the following:

    I. Cenozoic (recent life).

    ii. Mesozoic (middle life).

    iii. Palaeozoic (ancient life). 

    In table 5.37 The Precambrian time is shown. However, this is not included in eras because it lacks reliable geological evidences.

    – Eras are subdivided into Periods (refer to the figure below). Periods are based on

    the types of life existing at the time and on geologic events, such as mountain

    building and plate movements.

    – Periods may be divided into smaller units of time called Epochs. The figure below shows that only the Cenozoic era is subdivided further into epochs. Why is this so? The fossil and geological events records are more complete in these recent rock layers.

    Application activity 5.5

    Using knowledge and skills acquired from this unit 5, make debate and discussion

    to find answers for the following questions:

    1. How can archaeological research and environmental protection be


    2. Explain the importance of Geological time scale.

    End unit assessment

    Read the following discussion between two students and answer the questions


    1. Two students Mugisha and Uwamahoro were discussing about the

    origin of the earth. Mugisha said that the earth and other celestial

    bodies came into existence in different ways: some were formed due to

    the collision of stars, others by explosion. With confidence, Uwamahoro

    said that everything in the universe was created by God.

    a. Who is right, who is wrong?

    b. Describe four theories of the origin of the Earth

    2. The physical features of the earth are changing day by day due to

    natural reasons and human activities. Suggest ways people can use to

    conserve the nature for its sustainability.


    Key unit competence:

     By the end of this unit, I should be able to examine the internal processes responsible

    for the evolution of different relief landforms.

    6.1. Faulting and processes leading to the formation of different faulted features

    Learning activity 6.1

    Using textbooks, internet, and other sources of geographical information research


    1. The meaning of the faulting and fault

    2. The processes responsible for the formation of different faulted


    Forces causing faulting

     Faulting is the fracturing of the crustal rocks due to the influence of endogenic forces or processes. Endogenic forces are responsible for various types of vertical irregularities that give birth to numerous relief features including mountains, plateaus, plains, lakes, faults and folds. The endogenic forces include faulting, folding, warping, vulcanicity and earthquake. The endogenic forces and related horizontal movements are caused by the existence of convectional currents within the mantle. These lead to lateral earth movements, that are either tensional or compressional in nature. Sometimes there are up and downward movements that cause the crustal rocks to fracture, fold, down warp or up warp. The endogenic forces result in the formation of features or landforms such as plateaus, block mountains, basins and rift valleys.

    Meaning of faulting and fault

     Faulting is the process through which the rocks of the earth’s crust crack or fracture due to tectonic forces as accompanied by the displacement of blocks. The direct effects of faulting include the formation of rift valley, block mountains, fault scarps, tilt block landscape, etc. A fault is a fracture in the crustal rocks where the rocks are displaced along a plane called a fault plane. A fault is also considered as a rupture or fracture of rock strata due to strain, in which displacement is observable.

    Main parts of a fault 

    Processes of faulting: the process of faulting is caused by tensional and compressional forces. When these forces act on crustal rocks of the earth’s crust, they develop stress in them and break along the zone of maximum tension. The rocks are then removed from their original position either upwards, downwards or horizontally.

    When a rock is displaced upward it is called up throw, and the downward displacement is called down throw.

    Application activity 6.1

     Rwanda is made up of six physiographic regions which resulted from different internal processes including faulting. With convincing examples/evidences support this statement.

    6.2. Types of faults

    Faulting leads to numerous types of faults that include the following:

    i. Normal faults/consequent fault lines are formed due to the tensional forces that lead to opposite displacement of the rock blocks. The steep scarp resulting from normal faults is called fault-scarp or fault-line scarp.

    ii. Reverse faults/ obsequent fault lines are formed due to compression forces that lead to the movement of the fracture rock blocks towards each other. The fault plane in a reverse fault is usually inclined.

    iii. Lateral or strike-slip faults are formed when the rock blocks are displaced horizontally along the fault plane. They are called left-lateral or sinistral faults when the displacement of the rock blocks occurs to the left; on the other side of the fault, they are called right-lateral or dextral faults.


    iv. Step faults are formed when many parallel faults occur within the crustal layers of the rocks in a way that the slope of the entire planes of the faults are in the same direction. The series of blocks are thereafter up lifted at different rate, therefore forming step faults.


    v. A thrust fault refers to a reverse fault in which the dip of the fault plane is at a small angle to the horizontal and it is caused by strong crustal compressional forces.

    vi. Anticlinal faults are formed as a result of great compressional forces acting on layers of rocks. Compressional forces further cause stress leading to the development of cracks (faults) on the crest of the anticline.


    vii. Shear or tear faults/strike or wrench faults 

    These are formed as a result of horizontal movements caused by the intense stress coming from opposite directions but acting obliquely to one another. In case there is displacement of blocks during the formation of tear faults, that is, when there is both horizontal movement and vertical displacement, the fault formed will be called Oblique slip fault.


    viii. Trap door fault is a type of fault produced by tension forces when fracturing does not cut through the whole crustal rocks (i.e: semi-diplacement takes place).


    Application activity 6.2

    Figure 6.115: Trap door fault Basing on the knowledge and skills acquired from the above lesson, describe the area where the faulting process occurred.

    6.3. Influence of faulting on landscape and drainage

    Learning activities 6.3

     Kwigira and Mahoro are students from GS Terambere. They visited Lake Kivu and its surrounding area in the western part of Rwanda. Back to school, they were told to give the description of the area they had visited. In the description they included: Lake Kivu and other different relief features around. They said that their teacher called Bumwe who accompanied them explained how Lake Kivu was formed. He said that Lake Kivu was formed in the same way as Lake Tanganyika, Eduard, George, Manyara and Turkana in East Africa. Furthermore, teacher told them that there are other landforms which resulted from faulting in Africa and in the World. Basing on this story answer the following questions:

    1. Describe the impact of faulting on the landscape of the area visited by

    Kwigira and Mahoro.

    2. Examine other landforms produced as a result of faulting not

    mentioned in the passage above.

    3. Assess the impact of faulting on the drainage system.

    6.3.1. Influence of faulting on landscape 

    There are several landforms produced by faulting including: rift valleys, fault scarp, escarpments, block or Horst Mountains, fault guided valleys, titled block/landscape, Rift valley lakes and grabens.

    Rift Valley 

    Rift valley is a trough or hollow/depression (graben) which may result from both tensional and compressional forces. It is formed when two faults are developed parallel to each other. The middle part sinks to form the floor of the rift valley. The outer blocks remain standing to form escarpments.


    There are various theories put forward to explain the formation of the rift valley and they include: Tensional theory, compressional theory, differential up-lift theory, crustal separation theory and relative sinking theory.

    a.Formation of the rift valley by tensional forces (Tensional theory by J.W.Gregory)

    A rift valley is formed when tensional forces move away from each other. These tensional forces produce faults and the block (part of the crustal block in the middle) of between two parallel faults subsides to form a rift valley. The outer blocks remain standing to form escarpments.

    b. Formation of the rift valley by compressional forces (Compressional theory by E.J. Wayland)

    Rift valley is formed when horizontal forces act towards each other. These forces push the crustal rocks or layers towards the centre from all sides. These forces of compression produce two parallel faults and the pieces of land on either side are lifted above the general level of the ground to form a rift valley.

    c. Theory of differential uplift (by Dixey and Troup)

    Rift valley is formed when normal faulting produces several normal fault lines followed by gradual up lift of the faulted area with several step faults. Typical example is Kedong in the part of Nairobi.


    d. Theory of crustal separation/Plate tectonics theory 

    The theory suggests that the rift valley was formed as a result of sea-floor spreading or tectonic movement, where convectional currents within the mantle rise vertically and then divert from each other horizontally. This results in the dragging of oceanic or continental plates. Such results in the breaking of crustal layers that are dragged apart together with continental rafts. As the movement continued, the central block was forced to subside. Example: along the Atlantic oceanic trench, where the plates continue moving apart.

    e.Theory of relative sinking (by Suess)

    This happens when there is contracting of the earth followed by the fault blocks that are slowly settled. It makes some blocks remaining in the original position (standing up) to form horsts. Others,which are settled faster, form graben or rift valleys. A typical example is the Kamasia ridge in Kenya.

    The following are some examples of rift valleys: East African rift valley in Africa; Jordan rift valley in Asia and Rhineland rift valley in Europe.

    Block mountain (horst) 

    A block mountain is an upland bordered by fault scarps on both sides (the block of land between two faults being uplifted because of compressional forces.


    The following are some examples of Block Mountains: Cyamudongo, Mishahi, Muzimu, Cyendajuru in the western part of Rwanda; Usambara and Uluguru in Tanzania; Rwenzori in Uganda; Vosges and Black Forest in Europe and Mount Sinai in Asia.

    Fault step landscape/tilt block landscape is when vertical tectonic forces push a block of crustal rocks or layers upwards and it happens that some middle parts are pushed higher than others. This is usually because of uneven forces that cause the formation of horst titled landscape characterized by several fault carps (step faults). These can also be formed as a result of differential up-lift.


    Fault scarp: An escarpment or cliff/steep slope (a wall of lift valley) formed by a fault that reaches the Earth’s surface. Most fault scarps have been modified by erosion and mass wasting that may reduce the vivid evidences of faulting.

    6.3.2. Influence of faulting on drainage

    – Faulting influences the drainage pattern in a given area. The different drainage

    patterns influenced by faulting include rectangular, parallel and trellised

    drainage patterns.

    – Most of the time, water passes through the valleys, depressions and fractured

    areas hence forming fault guided valleys. A good example is River Rusizi in

    western Rwanda.


    – Faulting leads to river reversal (change of direction). Example of river reversal due to faulting include River Katonga and Kafu in Western Uganda that were joining the Congo river basin and reversed towards Victoria basin. The figure below shows how River Katonga and Kafu changed their flow of direction to Lake Victoria basin as a result of faulting in Western part of Uganda.

    – Rift valley lakes are formed when graben or rift valleys are filled by water. Typical examples include: Lake Kivu, Eduard, Albert, George, Tanganyika in the western arm/branch of East African rift valley, Turkana, Manyara, Nyasa, Magadi in the Eastern arm/branch of East African rift valley.


    – Some of the waterfalls are in faulted areas e.g. Rusizi, Mururu, water falls in

    Rwanda and Mubuku water falls in South Western part of Uganda.

    – Faults give rise to the underground water table along fault planes. Typical

    examples are Mwiyanike and Nyamyumba in Rwanda. 

    Application activity 6.3 

    Explain the influence of faulting on landscape and the drainage system in the context of Rwanda.

    6.4. Impact of faulting on human activities

    6.4.1. Positive impacts

    The positive impacts/effects of faulting to man include:

    – Faulted sedimentary strata are good for oil exploration. A good example is oil

    deposit from Lake Albert in Uganda.

    – Faults give rise to the underground water table along fault planes. This water

    is important for agriculture and it is used in industries and domestic activities.

    – Faulting leads to the formation of depression which, when filled with water

    , form lakes like Kivu, Edward, etc. These lakes are used for fishing, irrigation,

    transportation and mining.

    – Faulting creates lines of weaknesses in the earth’s crust through which the

    underground hot water reaches the surface as hot springs and geysers. These

    are sites for geothermal power production. A good example is Nyamyumba

    hot spring.

    – Faults may also give rise to waterfalls which are used for hydroelectric power


    – Features produced through faulting, for example, fault scarps, rift valleys and

    horsts are major tourist attractions.

    – Faults play a leading role in the weathering of rocks. The courses of small streams

    often follow rock joint systems. This, therefore, facilitates soil formation.

    – Some faulted features like rift valley lakes are good sources of minerals like

    methane gas from Lake Kivu.

    – Faulting exposes mineral bearing layers of rocks, hence facilitating mining. E.g.

    copper mining around Mt. Rwenzori in Uganda.

    6.4.2. Negative impacts

    Below are some of the negative effects/impacts which include:

    – Faulting processes result in destructive earthquakes

    – Faulting processes change the existing landforms to create the new landforms;

    hence deforming the already existing beautiful scenery.

    – Faulting processes may result in volcanic eruption which comes along with


    – Fault scarps form topographic barriers that make the development of

    infrastructure such as roads and railways difficult.

    – Faulted areas are barriers to the development of transport and communication


    – Faulted areas discourage the settlement, agriculture and livestock, industrial

    development, etc.

    Application activity 6.4 

    Conduct your own research to assess both the negative and positive impacts of faulting on human beings.

    6.5. Distribution of landforms associated with faulting

    Learning activity 6.5

    1. Draw a sketch map of East Africa, name and locate faulted areas.

    2. Identify the world areas that are most affected by faulting outside East


    Fault areas are mainly located in the following areas:

    – East Africa: Faulted areas in East Africa extend from Mozambique in the South

    to the Red Sea in the North and from Malawi through Tanzania and Burundi,

    then Rwanda to Uganda and Kenya.

    – Western Europe: Many faulted areas are in deep oceanic parts of the Atlantic

    Ocean in Western Europe.

    – The Rhine Rift Valley extends along the border of North East France and South

    West Germany.

    – The Western Coast of North America, which includes the San Andreas Fault of


    – The Alpine fault region in New Zealand in the Far East.

    – The North Western highlands of Scotland and the Guadalquir valley of Spain.

    6.6. Definition of folding and its process

    Learning activity 6.6 

    Read the passage below and provide answers to the question that follow: 

    The relief of Rwanda is characterized by six topograhic units that include Bugarama plain and Lake Kivu Banks, Congo-Nile crest, Central plateaus, Eastern low lands, Buberuka region and the Volcanic region. Some of these topographic units were formed through faulting process as it was explained in the previous lesson. Those topographic units formed as a result of faulting include Bugarama plain and Kivu banks. On the other hand, volcanicity has been responsible for the formation of volcanic relief in the North, while the Eastern lowlands resulted from warping. Some landforms of Rwanda were formed through folding processes. Refer to the above presented passage to answer the following questions:

    a. Explain the folding processes

    b.Differentiate the folding processes from other internal processes in the


    Meaning of folding and folds

    Folding is a process by which crustal rocks bend due to compressional forces. This results from horizontal movements caused by the endogenic forces originating deep within the earth.

    Folds are the wave-like bends resulting from folding processes. The up-folded rock strata in arch-like forms are called anticlines, while the down folded structure forming trough-like feature is called syncline. The sides of a fold are called limbs of the fold.


    Processes of folding 

    The processes of folding are mainly engineered by the presence of compressional forces that push intensely the crustal layers/rocks towards a common centre. When crustal parts move towards each other under the influence of horizontal or convergent forces (compressional forces), the crustal rocks undergo the process of ‘crustal bending. The process of folding occurs in areas with soft and young rocks and instead of fracturing due to the internal forces or convectional currents that lead to the formation of compressional forces, the crustal layers start to bend.

    Application activity 6.6 

    Compare the process of faulting and folding in the context of Rwanda.

    6.7. Types of folds

    Learning activity 6.7:

     Referring to the knowledge and skills about folding, describe the characteristics and categories of the limbs, synclines and anticlines formed as result of folding process.

    Based on the inclination of the limbs, folds are divided into the following categories:

    Symmetrical folds are simple folds whose limbs incline uniformly. These folds are examples of open folds. Symmetrical folds are formed when compressional forces work regularly but with moderate equal intensity. Symmetrical folds are very rare indeed.

    Anticlinal fold: This is a fold that has the convex appearance with the oldest beds at its core

    A syncline fold: This is a fold with younger layers closer to the folded structure. Synclines are typically a downward fold, termed as syncline (i.e. a trough) but synclines that point upwards or perched can be found when strata have been overturned and folded.


    Asymmetrical folds: These are characterized by unequal and irregular limbs. Both limbs incline at different angles. One limb is relatively larger,with moderate and regular inclination, while the other is relatively shorter with steep inclination. Thus, both limbs are asymmetrical in terms of inclination and length. They are formed when compressional forces acting upon the crustal young layers are unequal. One side is intensely pushed while the other is moderately compressed.


    Over fold: This is a type of fold formed where the compressional forces push one limb in asymmetrical fold over the other limb at a short distance.

    Monoclinal folds: These are folds in which one limb inclines moderately with regular slope while the other limb inclines steeply at the right angle at the slope is almost vertical. Splitting of limbs gives birth to the formation of faults. Also monoclinal folds are formed as a result of unequal horizontal compressional forces coming from both sides.



    Over thrust / Nappes: These are the results of complex folding mechanism caused by an intense horizontal movement and resultant compressional forces. Both limbs of the recumbent fold are parallel and horizontal. The great intensity of the compressional forces on one limb causes the crest to fracture (Faulting). Therefore, one part of the block jumps over the other.


    Isoclinal folds are formed when the compressive forces are so strong that both the limbs of the fold become parallel to each other and not horizontal.

    Open folds are those in which the angle between the two limbs of the fold is more than 90o but less than 180o . Such open folds are formed due to wave like folding because of the moderate nature of compressional forces.

    Closed folds are the folds in which the angle between two limbs of a fold is acute. Such folds are formed because of intense compressional forces.


    Chevron folds are structural features characterized by repeated well behaved folded beds with straight limbs and sharp hinges. These display a set of repeated set of v-shaped beds.


    Dome and basin fold:

     These are types of folds that display dome-like anticlines instead of arching. Therefore, the fold assumes a dome shape that looks like an inverted bowl. The would-be syncline forms basin like appearance instead of having a sinking arch. A typical example is the Mauritania desert.

    Application activity 6.7

     With help of digram, differentiate symetrical fold from other type of folds referring to knowledge and skills you gained about folding.

    6.8. Influence of folding on the drainage and landscape

    Learning activity 6.8

    1. Suggest the impact of folding processes on any landscape you have


    2. Assess the impact of folding on the drainage system.

    6.8. 1. Influence of folding on drainage

    The following are examples of the influence of folding on drainage:

    – Folding led to the formation of many waterfalls in Rwanda and elsewhere in

    the World. Some of the waterfalls are located on the upward folded areas. A

    good example is Rusumo waterfall.

    – Fold Mountains are good catchment areas. A typical example in Rwanda is in

    Gicumbi District.

    6.8.2. Impact of folding on landscape

    There are various landforms resulting from folding. They include:

    Rolling plains: These types of landforms result from down warping movement caused by the folding processes. They have gently sloping anticlines with very wide synclines.

    Ridge and valley landscape: The strong compressional forces create a series of asymmetrical folds. The anticlines form the uplands (ridges) while the synclines form the valleys.


    Fold Mountains: The most common features resulting from the process of folding are the mountains. The mountains rise to different heights depending on the velocity and intensity of compressional forces.


    The following are examples of folded mountains:

    – Highlands located in the Northern region of Rwanda (various mountains

    located for instance in Gakenke, Musanze, Burera);

    – Kigezi highlands in South Western Uganda;

    – Caledonian folds: The folds of Caledonia are found in Scotland, and they

    affected the Caledonia and Scandinavian countries during the primary era (Paleozoic);

    – Hercynian folds, they include: Mountains in Scotland and the Appalachians in


    – Rockies and Andes in America;

    – Insular arcs of the Pacific Ocean;

    – Parts of Asia: Japan, Philippines, Kouriles Islands;

    – Near the Equator: Marianna, New Zealand, Kermadea;

    – Parts of Europe: Alps;

    – Parts of South Asia (Saudi Arabia and India);

    – North Africa: around the Mediterranean Sea.


    6.8.3. Impacts of folding on the physical environment

    The above features formed through folding process influence the human activities

    positively and negatively as follow:

    Positive effects

    – Fold Mountains receive heavy rainfall on the windward side hence favouring

    crop growing while the leeward side receives little or no rainfall. Thus, there is agriculture and livestock keeping on the windward side;

    – Fold mountains are a source of clean water, which is used by human beings;

    – The fold mountainous areas can be used for lumbering activities;

    – During folding, some valuable minerals are brought closer to the earth’s

    surface. This promotes mining activities.

    – Some features resulting from folding attract tourists who bring foreign


    Negative effects

    – The Foëhn winds associated with leeward sides in fold mountain areas

    discourage the growing of crops, hence hindering agriculture.

    – Fold Mountains are barriers to the development of transport and

    communication lines. They hinder air transport due to poor visibility;

    – The steep and rugged slopes of Fold Mountains discourage settlements,

    agriculture and livestock keeping.

    – Fold moutains discourages the development of mining sector since some

    minerals can be taken deeper into the ground and become hard to exploit

    especially in synclinal parts.

    Application activity 6.8

    1. Conduct your own research to identify both negative and positive

    impacts of folding on environment.

    2. Assess the significance of fold mountains to the economic development

    of Rwanda.

    6.9. Meaning, causes and types of warping

    Learning activity 6.9

    Read the passage below and provide answers to the questions that follow:

    The relief of Eastern Rwanda is characterized by low plateaus formed through an

    internal process that operate by gentle compressional forces caused by internal

    convection currents. That process led to formation of various physical features

    namely undulating gentle hills, basin and basin lakes.

    a. Identify the process that has been responsible for the formation of

    features mentioned in the above passage.

    b. Describe how the process of warping occurs.

    c. With specific examples; explain the influence of warping on the

    drainage system.

    6.9.1. Meaning of warping 

    Warping is the downward movement of crustal rocks caused by the sinking of convectional currents within the interior part of the earth. This results in a saucershaped basin. It is produced where only a gentle deformation of crust has taken place over a considerable area and time. It results from the sinking of convectional currents which pull the earth’s crust towards the core. A typical example where warping took place in East Africa is Lake Victoria basin.

    6.9.2. Types of warping

    There are two types of warping: 

    Down warping: This is formed when the sinking of convectional currents drag downwards the lower part of the crust. Therefore, the crustal layers bend inwardly to form basins. This created East African major basins i.e. the depressions occupied by Lake Victoria, lake Kyoga in Uganda and Lake Muhazi in Rwanda are good examples.

    Up warping: During the down warping, the outer parts of the crustal layers tend to move upwards. This is commonly caused by isostatic movement associated with upward movement. It produced up land regions like East African plateaus and other uplands elsewhere in the world.


    Broad warping 

    When the process of up warping and down warping affects larger areas, the resultant mechanism is called broad warping.

    6.9.3. Causes of warping 

    Warping is mainly caused by convection currents that result in lateral compression forces of low intensity or gentle compressional forces. Typical examples are in East Africa where few areas escaped the down warping and up warping processes. The process led to the formation of Great East African Basin, crustal warped lakes, extensive swamps around the lakes, extensive plateaus, reversal of some rivers, etc.

    6.9.4. Landforms associated with warping

    The process of warping led to formation of numerous landforms which include:

    Plateau: it is a large and extensive uplifted part of the earth’s crust which is almost flat at the top. The top of the plateau is mostly flat (i.e. a plateau is a flat toped hill. Example: Eastern plateaus of Rwanda, the Rwandan part affected by Warping). Other plateaus resulted from warping include: Guinea highlands, Jos plateau, Ahaggar, etc.

    Basin: It is a large and extensive depression on the earth’s surface. Most basins are formed due to vertical downward movement of the earth’s crust.


    Examples of basins include: An inland drainage e.g. Congo; Chad; and Amazon basins. When a basin is filled by water; it forms basin lakes like Lake Muhazi and Mugesera in Rwanda, Lake Victoria, etc.

    Plains: These refer to flat areas that are located in lowland areas. Typical examples are generally in coastal regions that were greatly affected by warping process.

    6.9.5. Influence of warping on drainage

     Warping plays a major role in the drainage system

    – Some rivers change their courses due to warping in a given area. A good

    example is the case of hydrography of East Africa where some rivers changed

    direction and other filled the depressions to form lakes.

    – Warping led to the formation of many lakes in East Africa. Good examples

    include: Lake Victoria and Lake Kyoga in Uganda, Lake Muhazi and Mugesera

    in Rwanda.

    – Some of the waterfalls are located on the upward warped areas.

    – The drainage of Rwanda flows from West to East from the up warped features

    of Rwanda. 

    Positive effects

    – Upward warping areas receive heavy rainfall on the windward side favouring

    crop cultivation.

    – The water of river flowing from the warped areas is clean and may be used by

    human beings in various activities;

    – The upward areas may be used for lumbering activities;

    – During warping, some valuable minerals are brought closer to the earth’s

    surface. This promotes mining activities;

    – Some features resulting from warping attract tourists which bring foreign


    – Basin lakes stimulate various activities like fishing, water transport and

    provision of water for irrigation;

    – Hills formed under warping stimulate the development of communication by

    favouring the construction of communication masts.

    Negative effects

    – The destructive Foehn winds are common in warped mountain areas.

    – Warped areas are barriers to the development of transport and communication,

    where they lead to high cost of the construction of roads.

    – The steep and rugged slopes of warped mountains discourage the settlement,

    agriculture and livestock keeping.

    – During warping, some valuable minerals can be taken deeper into the ground

    and become hard to exploit.

    Application activity 6.9

    1. “Some parts of East Africa have been affected by up warping and down

    warping”. With relevant examples support this statement.

    2. Draw a sketch map of Rwanda on it label and name the major landforms

    which resulted from wraping.

    3. Assess the significance of warping on drainage system of East Africa.

    6.10. Processes of vulcanicity and volcanic materials

    The following are the key terms used in vulcanicity studies:

    Vulcanicity refers to the processes from which the molten materials are either

    ejected on the surface of the earth or remain in the earth’s crust.

    Volcanicity refers to the process by which lava through volcanic eruption is

    ejected to the earth’s crust.

    A volcano is a mountain that opens downward to a reservoir of molten rock

    below the surface of the earth.

    Materials of vulcanicity:

    Volcanic materials of various types are ejected through the volcanic pipe known as

    a vent. This is situated in the part of fault line that assisted the escape of magma.

    Volcanic materials include the following:

    Vapour and gases: Stream and vapour include phreatic vapour and magmatic

    vapour. Volcanic gases include carbon dioxide, nitrogen oxides, sulphur

    dioxide, hydrogen and carbon monoxide.

    Magma and lava: Generally, molten rock materials are called magma when

    they are still below the earth’s surface. When they reach the earth’s surface,

    they are known as lava. However, when it is still in the mantle it is refer to as

    molten material. Lava and magma are grouped into two classes. These include

    the following:

    a. Based on silica percentage; lava and magma are divided into two

    groups: acidic magma (high percentage of silica) and basic lava (low

    percentage of silica).

    b. Based on light and dark coloured minerals; lava and magma are also

    classified into Felsic and Mafic lava.

    Fragmental or pyroclastic materials. These are thrown out during the

    explosive type of eruption. They are grouped into three categories:

    – Essential materials - consolidated forms of live lava

    – Accessory materials - dead lava

    – Accidental materials - fragmental materials of crustal rocks.

     Based on the size of pyroclastic materials: They are grouped into four kinds:

    – Volcanic dust (finest particles)

    – Volcanic ash (2 mm in size)

    – Lapilli (of the size of peas), and

    – Volcanic bombs (6 cm or more in size). 

    Application activity 6.10

    1. Describe the main materials derived from vulcanicity.

    2. Name and locate the volcanoes available in Rwanda.

    6.11. Intrusive and extrusive features

    Magma found inside the crust may sometimes reach the surface of the crust through fracture, fissures and consolidate from there. In this case, the features formed are extrusive features. But when the magma fails to reach the earth’s surface and consolidate inside the crust before reaching the surface, the features formed are called intrusive features.


    6.11.1. Extrusive volcanic features

    These features include the following: 

    Volcano: This is formed when molten materials escape from the mantle and solidify on the earth’s surface to form a highland known as volcano.

    Acidic lava cone: This refers to a cone made of viscous lava normally that is ejected out of the earth’s crust and solidifying faster as soon as it reaches the Earth’s surface. It always cools faster than basic lava because it is viscous in nature.

    Basic lava cone which is a cone of basic fluid/lava spread over a long distance. Basic lava cone is characterized by gentle slope. Basic lava cone is also known as shield or basalt volcanoes. Typical examples include Nyamuragira in DRC and Muhabura in Rwanda.


    Crater: This is a volcanic depression on top of the volcano. There can be a ring crater or explosive crater in circular shape when a crater is filled by water, it forms a Crater Lake. Examples of crater lakes in Rwanda are found on Kalisimbi, Muhabura and Bisoke volcanoes.


    A caldera: This is a wide depression that usually forms on top of a volcanic mountain due to explosive secondary eruption. When a caldera is filled with water, a Caldera Lake is formed. A typical example of dry caldera is Ngorongoro Caldera in Tanzania.

    Ash and cinder cone: These are formed when lava is ejected into the air violently and breaks into small particles known as Ash and cinder. These fall back and pile up to form alternating layers of ash and cinder. These have slopes that are importantly concave and asymmetrical in nature.


    Volcanic plateaus: These are formed when there are various fissures or vents and basic lava flowing out and spreading to a wide area. This results into the formation of a flat-topped highland known as volcanic plateau.

    There are two types of volcanic plateaus which include: Lava plateaus which are formed by highly  basaltic  lava during numerous successive eruptions through several vents without violent explosions (quiet eruptions). They form an extensive and flat landform. Typical examples of lava plateaus are found in Musanze, Nyabihu, Rubavu, Burera (North West of Rwanda), Ethiopian highlands, Deccan plateau in India and Bui plateau in Nigeria.

    The second type of lava plateau is known as pyroclastic plateau which is produced by massive pyroclastic flows and is underlain by pyroclastic rocks. Examples include Shirasu-Daichi in Japan and the North Island Volcanic plateau in New Zealand.


    Volcanic plug: This is also called a volcanic neck or lava neck; it is a volcanic feature created when the magma solidifies within a vent and later the soft layers of rocks surrounding it are eroded away. Then, a hard rock in form of a pillar is left standing as a volcanic plug.


    Composite cone: This is a volcano associated with various cones. This is formed when after the formation of the main cone, the magma solidifies in the vent, therefore, blocking it. This forces the active magma for subsequent eruption to find its way out through subsidiary vents along the sides of the cone. This results in the formation of other cones on the sides of the main cone. Examples include, Mt. Kilimanjaro, Mt. Muhabura, etc.


    Lava and ash cone or strato volcanoes: They are formed when the lava is violently blown to high heights, breaking into small particles. These fall in the form of Ash. When the pressure reduces, magma comes out gently to form a layer of lava on top of the ash layer. The typical examples include: Muhabura, Elgon, Longonot, etc.


    Dissected volcanoes: This is a volcanic mountain that is greatly and deeply affected by serious erosion. A typical example is Mt. Sabyinyo between Rwanda, Uganda and DRC.

    Cumulo dome/Volcanic dome/Lava Dome: This is formed when lava is so viscous that it cannot flow away readily. It therefore accumulates around the vent. Hence, a steep sided volcanic dome with no visible crater is formed. Cumulo dome has convex sides.


    Explosive crater: This is a depression formed when the pressure reduces and the magma is now subsiding. This means that the crater is occupied by relatively active magma that is under low pressure. It has a circular shape.

    Hot spring: This refers to natural outflow of superheated water from the ground. This is formed when the underground water is in contact with superheated rocks associated with the magma. The water heats up, and when it finds its way out, it flows out in the form of Hot spring. It contains mineral substances in solution. Hot springs are found in Iceland, (in Europe), Kenya and Ethiopia (in Africa).

    Geysers: These refer to the forceful emission of hot water and steam from the ground to a high level in the air. This is formed in the same way as hot springs, the only difference is that, the super-heated water and steam are realised and ejected out at irregular interval. Geysers are found in Iceland, North Island and New Zealand.


    Fumaroles: This is a volcanic feature associated with the emissions of gases from the vents. These gases are composed of steam that continuously comes out from the Earth’s crust in volcanic region. They are formed when the underground water meets the hot magma and creates steam.


    6.11.2. Intrusive Vulcanic features

     This is a type of vulcanicity where the magma does not reach the earth’s surface but cools and solidifies within the crustal rocks. Intrusive vulcanicity leads to the formation of intrusive features. These features occur beneath the crust and may be exposed to the surface after the overlying rock is removed by erosion. They include: Lava flow, laccolith, Volcano, Dyke, Lapolith, sill, etc.


    Batholith: This refers to large dome-shaped intrusion of the magma extending to great depth within the earth’s crust. Batholiths are formed deep below the surface when large masses of magma cools and solidifies. These may later be exposed because of erosion to form inselbergs.



    Laccolith: It is a dome-shaped intrusion of the magma formed when the magma cools and solidifies in anticline bedding plane


    Phacolith: This is lens-shaped mass of igneous rock formed when the magma cools and solidifies at anticline and syncline in folded rocks. Phacolith is much shallower.


    Lapolith: It is a large saucer-shaped intrusion formed when the magma (molten rocks) cools and solidifies in a syncline bedding plane. Lopoliths form shallow basins along the rock bedding plane.


    Sills are horizontal intrusions of magma which have solidified along the lines of bedding planes.


    Dykes: These are vertical intrusions formed when magma solidifies in a vertical manner or within the vent and subsidiary vents. They cut across the bedding planes of the crustal rocks into which they have been intruded. Dykes often occur in groups where they are known as dyke swarms.


    Application activity 6.11

    With the help of diagrams, differentiate intrusive and extrusive volcanic landforms.

    6.12. Types of volcanoes and their characteristics

    Learning activity 6.12:

    In the previous lesson you learnt that volcanicity leads to the formation of various

    landforms including volcanoes.

    a. Describe volcanoes according to their period of activity.

    b. Make a classification of the volcanoes according to their nature of

    volcanic eruptions.

    A. Classification based on periodicity/ activity of eruptions: 

    Active volcanoes: These are volcanoes which constantly eject volcanic lavas, gases, ashes and fragmental materials. They erupted very recently or are engaged in eruption. Examples are Nyiragongo and Nyamuragira in DRC, Ol Doinyo in Tanzania, Lengai, Etna and Stromboli (in Mediterranean Sea).

    Dormant volcanoes: These are the volcanoes which have taken long without erupting but still show signs of eruption. That is, they are quiet after their eruption for some time. However, they suddenly erupt violently and cause huge damage to human, animal and plant life around them. An example is Muhabura, Karisimbi, Gahinga and Bisoke in Rwanda, Vesuvious near Naples in Italy.

    Extinct volcanoes: These are the volcanoes that have taken a long time without erupting and show no signs of erupting in the future.. A good example is Mount Sabyinyo in Rwanda.

    B. Classification of volcanoes basing on the nature of eruptions

     Based on the nature of eruptions volcanoes are classified into explosive and quiet volcanoes. These include the following:

    Explosive type of volcanoes: They are the volcanic eruptions that occur whereby the magma is violently ejected out of the Earth’s crust through a central pipe (vent). This breaks and blows off crustal surface due to violent and explosive gases accumulated deep within the earth.

    Explosive type of volcanoes are classified into the following: 

    Hawain type of volcanoes: Such volcanoes erupt quietly due to less viscous lava and non-violent gas. A good example is Nyiragongo of DRC, Mihara in Japan, Etna of Italy, Kilauea of the southern Hawaii island.

    Strombolian type: Such volcanoes erupt with moderate intensity. Besides lava, other volcanic materials like pumice, scoria and bombs are also ejected into the sky. e.g: Stromboli in Italy.


    Vulcanian type of volcanoes: Such volcanoes erupt with great force and intensity. The lava is so viscous and pasty and it quickly solidifies and hardens. It crusts over the volcanic vents. e.g: Lipari in the Mediterranean Sea and Sakurajima in Japan.


    Visuvious type of volcanoes: They are more or less similar to the Vulcanian and Strombolian type of volcanoes. The difference lies only in the intensity of the expulsion of lava and gases. There is a violent expulsion of the magma due to an enormous volume of explosive gases. A good example is the Plini volcano in Italy that erupted in 79 AD.


    Fissure eruption type or quit eruption type: These occur along the fracture, fault and fissure and there is slow upwelling of the magma from below and the resultant lava is spread over the ground surface. An example is the Laki fissure eruption of 1783 in Iceland.

    Application activity 6.12

    With specific examples from East Africa,

    a. Identify and describe the extrusive volcanic landforms.

    b. Categorize the volcanoes according to their period of activity.

    6.13. Influence of volcanicity on drainage

    The volcanicity influences the drainage system in a given area in different ways as


    – Some seasonal rivers originate from the craters and flow down slope. A typical

    example is Susa River in Rwanda.

    – Crater or caldera of volcanoes may be filled with water to form lakes. Good

    examples in Rwanda are the lakes located on Bisoke, Muhabura and Kalisimbi


    – Some rivers may change their courses due to volcanicity. For example, before

    the Rwandan volcanoes came into existence, Nyabarongo River was flowing

    northward and then when volcanoes came in place, the river changed its

    course and went southward.

    – It leads to the formation of lava dammed lakes. E.g. Burera and Ruhondo.


    Application activity 6.13

     With the help of a sketch of topographical map of Rwanda, examine the relevance of volcanicity on the drainage system of Rwanda.

    6.14. Impact of volcanicity on humans

    Learning activity 6.14 

    Using your own experience, textbooks and internet, research on the impact of volcanicity on the social economic development of a country.

    Volcanicity has both negative and positive impacts on human life. They include:

    Positive impacts

    – Volcanicity is associated with rich (fertile) volcanic soils that stimulate


    – Volcanicity lead to the production of geothermal heat and geothermal energy;

    – Volcanicity stimulate the mining of metal ore deposits (including gold, silver,

    copper, tin, iron, lead, wolfram and zinc);

    – Volcanicity leads to the formation of crater and lava dammed lakes that

    stimulate fishing and water transport. A typical example is in Burera and

    Ruhondo lakes in North Rwanda;

    – Industrial materials like building stone, Sulphur, pumice and clay are provided

    by volcanic features;

    – Volcanicity leads to the formation of waterfalls that facilitate the generation

    of electricity (HEP). Good examples are Ntaruka and Mukungwa Hydro-power

    plants in North Rwanda.

    – Volcanoes play a great role in the modification of climate which supports

    various human activities such as agriculture.

    – The hot springs are used for medicinal purposes, especially in the treatment

    of skin diseases.

    Negative impacts

    – Volcanic eruptions cause heavy damage to human lives and property through

    outpouring of lava, fallout of volcanic materials, speeding lava flows etc.

    – Too much volcanic materials also lead to the diversion and blocking of drainage

    systems and floods.

    – The poisonous gases produced during the eruptions may cause acid rain;

    – Sometimes volcanic eruptions are followed by heavy rainfall. The heavy rain

    mixes with falling volcanic dusts and gases to cause mudflows or lahar;

    – Volcanic eruptions may generate tsunamis and seismic waves causing death

    to human beings;

    – It contributes to the deglaciation because of increasing temperature. The melt

    water results in the occurrence of floods and their effects.

    – Large quantities of volcanic dust and ashes produced during volcanic eruptions

    have been associated with weather and climatic changes. reduces visibility.

    – Volcanic eruption affects weather patterns. It is possible that increased volcanic

    activity was one of the factors responsible for the little ice Age.

    – It off balances the ecosystems and biodiversity in the areas where vulcanism

    takes place. For example, Scientists believe that volcanic eruptions and fallout

    of dusts and ash, may cause the extinction of several animal and plant species.

    Based on this hypothesis the mass extinction of dinosaurs about 60 million

    years ago has been linked to an increased volcanic activity.

    Application activity 6.14

    1. Examine the significance of volcanicity to the economic development of

    the areas found in volcanic regions.

    2. Assess the role played by volcanicity in the development of the

    following economic activities in Rwanda:

    a. Tourism industry.

    b. Agriculture

    c. Power and energy

    6.15. World distribution of volcanoes

    Learning activity 6.15

    1. Conduct your own research and draw the world sketch map showing

    areas mostly affected by vulcanicity.

    2. Locate the volcanic regions on the East Africa map.

    There are three major belts or zones of volcanoes in the world. These are:

    a. Circum-Pacific belt: volcanic zones of the convergent oceanic plate margins, including the volcanoes of the Eastern and Western coastal areas of the Pacific Ocean. This includes island arcs and festoons off the East coast of Asia and of the volcanic islands scattered over the Pacific Ocean. This volcanic belt is also called “The fire girdle of the Pacific or the Fire ring of the Pacific”

    b. Mid-continental belt, also known as ‘the volcanic zones of convergent continental plate margins’. This belt includes the volcanoes of the Alpine mountain chains and the Mediterranean Sea, and the volcanoes of the faulted zone of Eastern Africa.

    c. Mid-Atlantic belt: This includes the volcanoes along the Mid-Atlantic ridge which represents the splitting zone of plates. In other words, two plates diverge in opposite directions from the mid-oceanic ridge. Thus, volcanoes of fissure eruption type occur along the constructive or divergent plate margins.

    Application activity 6.15: 

    Locate and describe the major volcanoes of the world on the world sketch map.

    6.16. Meaning of Earthquake and its related concepts

    Learning activity 6.16

    Read the passage about earthquakes occurrence and provide answers to the

    questions that follow. 

    Earthquakes occur when masses of rock in Earth’s crust slip and slide against one another. This kind of movement is most common along a fault, a break in a body of crustal rocks that can extend for kilometers or even hundreds of kilometers. When pieces of crustal rock suddenly slip and move, they release enormous amounts of energy, which then propagates through the crust as seismic waves.

    At the Earth’s surface, these waves cause the ground to shake and vibrate, sometimes violently. Geologists classify seismic waves into two broad categories:

    body and surface waves. Body waves, which include Primary and Secondary waves, travel through the Earth’s interior. Primary waves resemble sound waves, which means they compress and expand the materials as they pass. Secondary waves resemble water waves, which means they move the materials up and down. Primary waves travel through both the solids and liquids, while Secondary waves only travel through the solids.

    1. Identify the internal process explained in the above passage

    2. Explain how those processes occur.

    3. Explain the types of seismic waves explained in the above passage.

    4. Conduct a research and find the meaning of the following terminologies:

    – Hypocentre

    – Epicentre

    – Earthquake

    – Focus

    – Tremor

    An earthquake is a sudden tremble or shaking of the ground caused by abrupt release of energy from crustal rocks; a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking buildings and causing gaping fissures in the ground.

    – Magnitude

    – Intensity


    Description of the terminologies associated with Earthquakes:

    a. Focus or hypocentre: It is the place of origin of the earthquake and it is

    always hidden inside the earth. It is also considered as the place of the

    occurrence of an earthquake. The deepest focus has been measured at the

    depth of 700 km from the earth’s surface.

    b. Epicentre: It is a place on the ground surface which is perpendicular to the

    focus. That place is the first to experience seismic event.

    c. Magnitude: It is a measure of the amount of energy released during an

    earthquake. It may be expressed using several magnitude scales such as

    Richter scale and the Mercalli scale.

    d. Intensity: It is a number (e.g. V for Mercalli scale and 5 for Richter scale)

    describing the severity of an earthquake in terms of its effects on the earth’s

    surface, humans and structures such as buildings.

    e. Tremors or temblor is the perceptible shaking of the surface of the Earth,

    resulting from the sudden release of energy in the earth’s crust that creates

    seismic waves.

    f. Measurement of the Earthquakes:

    The intensity or strength of an earthquake is measured using either the Richter

    scale or the Mercalli scale. The Richter scale range between 0 and 9. But in the

    real sense, the scale has no upper limit because it is a logarithmic scale

    The world’s largest and most intensive earthquake recorded had a magnitude of 8.9. The second method of measuring the intensity of an earthquake was invented by Giuseppe Mercalli and it is called Mercalli scale. The intensity is estimated basing on the destruction made by an earthquake. The instrument used in measuring earthquake is seismometer or seismograph.


    Application activity 6.16 

    Through internet, journals, textbooks and magazines search on the earthquakes which affected Western Rwanda in past years. Then answer the following:

    1. Name the epicentre of the earthquake which took place in Rwanda in

    February, 2007 and August, 2015.

    2. Give the intensity on Richter scale of these earthquakes

    6.17. Causes, consequences and measures of earthquakes

    6.17.1. Causes of earthquakes

    Earthquakes are caused by both natural and human factors as follows:

    Natural causes of earthquakes

    Earthquakes arising from natural causes include:

    – Natural earthquakes are caused by natural processes or they are a result of

    endogenic forces. Good examples are the earthquakes which took place in

    Rwanda in February 2007 and August 2015.

    – Volcanic earthquakes are caused by volcanic eruptions of explosive and fissure

    types. Generally, volcanic earthquakes are confined to volcanic areas. The

    intensity and magnitude of volcanic eruptions is usually high. Examples are

    the earthquakes caused by violent explosions of Krakatoa volcano in 1883 and

    Etna volcano in 1963.

    – Tectonic earthquakes occur because of the dislocation of crustal rock blocks

    during faulting. Such earthquakes are severe and disastrous. Examples are the

    1872 and 1906 earthquakes of California (USA); 1923 earthquake of Sagami

    Bay (Japan); 2001 earthquake of Gujarat in India.

    – Isostatic earthquakes are triggered by sudden disturbance in the isostatic

    balance at the regional level due to an imbalance in geological processes.

    Generally, earthquakes occurring in the active zones of mountains fall in this


    – Plutonic earthquakes are deep-focus earthquakes, which occur at greater

    depths. The centres (focus) of these earthquakes are generally deep, ranging

    from 240km to 670km.

    Human causes of earthquakes

    The earthquakes may also result from human activities such as:

    – Pumping water from underground aquifers, oil reserves; deep underground


    – Blasting of rocks by dynamites for purposes of the construction (of dams and

    reservoirs, roads);

    – Nuclear explosions;

    – Storage of huge volumes of water in big reservoirs. 

    6.17.2 Consequences of earthquakes

    Earthquakes are known to have the following consequences where they occur:

    – Loss of life and destruction of property happen when violent shaking of the

    land causes cracks on walls and making the buildings to collapse. Vegetation

    and the landscape are not spared either.

    – Outbreaks of fires occur where the earthquakes destroy oil and gas pipelines.

    – Tsunamis are huge and destructive sea waves caused by Earthquakes.

    – Landslides occur when the Earthquakes happen. These are sudden movements

    of large masses of rock and soil downhill.

    – Displacement of crustal rocks. This is caused by Earthquakes. It takes place

    vertically and laterally, leading to the damage of transport and communication

    lines such as roads, railways, etc.

    6.17.3. Measures to curb earthquake effects

    It is true that earthquakes lead to the loss of lives and property. Though they cannot

    be stopped from occurring, there are several things people can do to reduce such

    losses. Such measures include:

    – Building houses in a way that they are more resistant to earthquakes.

    – During an earthquake and when one is inside a building, he should drop to

    the floor and wait until the shaking is over ; and it is safe to move out of the


    – When one is outside, he hould stay there or move away from buildings and


    – If trapped under a collapsed, structure do not light a match or move about. Tap

    on a pipe or wall so that rescuers may get to locate you.

    – When the shaking stops, look around to make sure that it is safe to move. Then

    leave the building.

    Application activity 6.17

    It has been noticed that the Western part of Rwanda experiences earthquakes at

    a great extent compared to other regions.

    a. Suggest why that part of Rwanda experiences such a disaster.

    b. What do you think are the effects of such a catastrophe/disaster on

    any region where it occurs?

    c. Assume that you are in charge of disasters management and

    preparedness; suggest the measures that should be taken to control


    6.18. World distribution of Earthquakes

    Learning activity 6.18 

    The most active region in the world associated with Earthquakes corresponds to the margins of the Pacific Ocean. Earthquakes with large magnitudes take place along this zone in the Americas from the Aleutian Islands to Southern Chile and from the Kamchatka peninsula in Asia to New Zealand. Besides shallow earthquakes throughout most of this long region, intermediate and deep shocks take place along the margin of Central and South America and on the other side of the Pacific. This include s the long systems of the island arcs (Aleutians, the Kuriles, Japan the Philippines). Another large seismically active region is known as the Mediterranean-AlpineHimalayas region which extends from West to East from the Azores to the Eastern coast of Asia. This region is related to the boundary between the plates of Eurasia (to the North) and Africa, Arabia, and India-Australia (to the South). A third seismic region is formed by Earthquakes located in the oceanic ridges that form the boundaries of oceanic plates such as the Mid-Atlantic Ridges, East Pacific Rise, etc.

    1. Identify the areas affected by Earthquake in the passage above.

    2. On the sketch of world map, mark and label the areas that are mostly

    affected by earthquakes.

    The following are the major seismic zones of the world:

    a. Circum-Pacific Belt or Ring of Fire surrounding the Pacific Ocean.

    b. Mid-Continental Belt representing epicentres located along the Alpine

    Himalayan chains of European and Northern Africa and epicentres of

    East African fault zone.

    c. Mid-Atlantic Belt representing the earthquakes located along the

    mid-Atlantic Ridge and its off-shoots. The high-quality seismicity maps

    showed that narrow belts of epicentres coincide almost exactly with

    the crest of mid-Atlantic (ridge), the east Pacific, and other oceanic

    ridges where plates separate.

    d. Volcanic regions of the convergent Oceanic plate margins. These

    include the volcanoes of the Eastern and Western coastal areas of the

    Pacific Ocean, island arcs and festoons off the East coast of Asia and the

    volcanic islands scattered over the Pacific Ocean. This volcanic belt is

    also called “the fire girdle of the Pacific or the fire ring of the Pacific.

    e. Zones of subduction are the biggest crash scene on the earth. These boundaries mark the collision between two of the planet’s tectonic plates. The plates are pieces of crust that slowly move across the mantle’s surface over millions of years. When two tectonic plates meet at a subduction zone, one bends and slides underneath the other curving downwards into the mantle.

    Application activity 6.18 

    With reference and help of a seismic sketch map of Africa, identify the East African areas that are mostly affected by earthquakes.

    End unit assessment 6.1

    1. Discuss the impact of faulted landforms on the East African landscape

    and drainage.

    2. Explain the significance of folded relief features in the socio-economic

    development of East Africa.

    3. With specific examples, explain the importance of warping on drainage

    systems of Africa.

    4. To what extent has vulcanicity shaped the nature of landscape of East


    5. Critically examine the effects of either Tsunami in Japan or earthquake

    in Haiti on the World economic development.

    6. Assume that you are appointed Director General of Rwanda

    Environmental Management Authority (REMA), what are the measures

    you will take to reinforce the conservation of the basin lakes and

    wetlands found in Eastern province. 


    Key Unit Competence:
     By the end of this unit, I should be able to investigate the different constituents and morphological properties of soil.

    7.1. Soil constituents

    Learning activity 7.1

    Using previous knowledge learnt in Geography textbooks and other sources of

    geographical information;

    a. Define soil.

    b. Name the constituents/components of soil.

    c. Draw a pie chart showing the approximate composition of soil.

    Soil is defined as a superficial covering on the earth’s crust having been derived from both inorganic and organic materials of the Earth’s crust. Soil is not merely a group of mineral particles. It also has a biological system of living organisms and some other components. It is a complex of five components:

    Inorganic materials: A matrix of mineral particles derived from varying degrees of breakdown of the parent-rocks through weathering. These particles vary in shape and size. They include the following:

    a. Silica: In this context we can give an example of sand.

    b. Silicate: This is predominant in clay and as an example we can talk

    about aluminium silicate.

    c. Oxides: These are product of metal rust and mineral oxidation, for

    example iron oxides.

    Inorganic matter provides important plant nutrients; determine soil aeration, soil texture and drainage. It also gives support to the plants. Inorganic matter makes up 45% of the soil.

    Organic matter or humus: It is made up of animal and plant wastes and decomposed animals and plants living in the soil. This forms the humus which is found on the surface of the earth. The humus is black or dark-brown in colour and it is formed from the breakdown of organic matter. Organic matter makes up 5% of the soil. It provides the soil with important benefits which include the following:

    – It enhances the soil’s ability to hold and store water;

    – It improves the soil structure;

    – It reduces eluviation of soluble minerals from the top soil;

    – It helps in soil aeration.

    – Soil water and moisture: This refers to all the water contained in the soil together with its dissolved solids, liquids and gases. Soil water is held by capillary and absorptive forces both between and at the surface of soil particles. Soil water is a dilute solution of many organic and inorganic compounds, which is the source of plant mineral nutrients. The movement of water and dissolved minerals is called leaching, and the water is called gravity water. Water also moves upward in soil by capillary action. The water containing dissolved minerals is called capillary water. The soil water makes up 25% of the soil and it occupies the pore spaces in the soil. Soil water are useful in the following ways:

    – The soil water dissolves various substances for example salts that

    are derived from plant or animal remains forming solutions;

    – The soil water helps plant to absorb minerals from the soils;

    – The soil water washes away highly soluble minerals from the upper to the lower layers (leaching);

    – The soil water brings soluble minerals from the lower to the upper

    horizons of soil through capillarity;

    – The soil water is very important in the sense that it provides a

    medium within which most of the chemical processes of the soil

    formation take place;

    – The soil water provides a medium through which living organisms

    and soil bacteria operate during the decomposition of organic


    The soil air: it occupies the pore space between soil particles, which is not filled with water. The soil is normally lower in oxygen and higher in carbon dioxide content. The soil air includes gases from biological activity and chemical reactions. The air or gases make up 25% of the soil. The air in the soil is important in the following ways:

    – It facilitates plant growth by supplying oxygen to the root hairs;

    – It supports micro-organisms which are found in the soil;

    – It helps in the process of weathering known as oxidation which is

    responsible for breaking down rocks to form soils.

    Biological system or living organisms and bacteria: The living organisms and bacteria help to decompose the organic matter into humus. This is sometimes classified together with organic matter/humus


    Application activity: 7.1 

    Carry out a tour around your school then observe critically the nature of the soil constituents and then describe them.

    7.2. Morphological properties, soil profile, soil catena and fertility of the soil

    Soil properties refer to the chemical and physical characteristics of the soil and these include mainly: structure, texture, colour, porosity, pH and consistency.

    7.2.1. Soil properties

     The main properties of the soil include the following:

    Soil structure: This is the arrangement of the individual soil particles. Soil structure varies in size and shape. On the basis of the shape, the following types of soil structure exist: granular, prismatic, platy, columnar and blocky.

    Granular and crumb: They are individual particles of sand, silt and clay grouped together in small, nearly spherical grains. Water circulates very easily through such soils. They are commonly found in the A-horizon of the soil profile.

    Platy: It is made up of soil particles aggregated in thin plates or sheets piled horizontally on one another. It is commonly found in forest soils, in part of the Ahorizon.

    Prismatic: The soil particles are formed into vertical prism-like particles. Water circulates with greater difficulty and drainage is poor. They are commonly found in the B-horizon where clay has accumulated.

    Columnar: It is similar to prismatic but the particles are rounded at the top.

    Blocky: These are soil particles that cling together in irregular square or angular blocks having more or less sharp edges. Relatively large blocks indicate that the soil resists penetration and movement of water. They are commonly found in the B-horizon where clay has accumulated;

    Soil texture: This is the proportion of sand, silt and clay within a soil. The soil particles can be grouped according to size. Particles that are larger than sand are grouped as stones. Soil texture is important due to the following reasons:

    – Soil texture influences the amount of air and water available within the soil.

    Silt and clay soils have a high water holding capacity. Sandy soils have a low

    water holding capacity.

    – It also influences the amount of water and the ability of the roots to pass

    through it.

    – It determines the easiness with which plant roots penetrate the soil.

    – It determines the soil’s ability to retain humus from being washed away.

    – It influences the aeration of the soil.

    Soil colour: It is the property of the soil that is easily identified through seeing. The colour of a soil depends on the parent material making up the soil. The soil colour is influenced by the parent rock, organic matter, moisture content and minerals. Soils have a variety of colours. They include red, yellow, black, grey, white and brown. A soil that is black or dark brown has high organic matter content. Soils that are reddish brown are well drained whereas grey soils are infertile.

    Soil Porosity/permeability: This refers to the number of pore spaces in the soil. The pore spaces relate to the portion of the soil space occupied by air and water. This is determined by the arrangement of the soil particles. Soils vary in porosity. Soils with large pores, for example sand are porous while clay is non-porous.

    Soil pH (potential for Hydrogen): Soil pH is measured by a pH scale or chart numbered from 1 to 14. Soil pH is an indicator of the acidity or alkalinity of soil. It is also known as soil reaction. Numbers from 1 to 6.9 indicate acidity; number 7 indicates neutral state, while 8 to 14 indicate alkalinity. A soil pH of below 7 shows that the soil is acidic. Acidity of the soil increases from PH 6.9 to 0.A soil pH of 7 shows that the soil is neutral. A PH of above 7 shows that the soil is alkaline. Alkalinity of the soil increases from pH 7 to 14.

    Soil consistency: This is the strength with which soil materials are held together or the resistance of soils to deformation and rupture. Soil consistency also refers to the easiness with which individual particles of soil can be rushed. This is done by the fingers or a cultivation tool. Soil consistency depends on the soil moisture content. Soil consistency is determined using wet, moist and dry soil samples.

    Thickness (depth): In very dry regions, the soils are usually thin. In sub-humid regions soils are generally thick. Thin soils are not good for agriculture. Thick soils are good for agriculture (above 1 m of depth).

    Soil moisture: This is the water in the soil. It is determined by soil texture and structure. Soil water helps in the movement of minerals up and down the layers of soil. Clay soils, for example, keep water for long. They become water-logged. Sandy soils allow water to pass easily. They are well drained.

    Soil temperature: This is the degree of warmth or coldness in the soil. Soil temperature affects the germination of seeds and plant growth. It also influences soil moisture, air and availability of plant nutrients.

    Soil air: This occupies the pore (or open) spaces that are not filled by water. It is controlled by drainage and soil texture. Poorly drained soils have little air. This is because the pores are filled with water.

    Soil nutrients: These are chemical elements found in the soil. They help in plant growth and ensure the soil remains fertile. The three main nutrients are nitrogen (N), phosphorus (P) and potassium (K). Together they make up the trio known as NPK. Other important nutrients are calcium, magnesium and sulphur. Plants also need small quantities of iron, manganese, zinc, copper, boron and molybdenum, known as trace elements because only traces are needed by the plant.

    Soil depth: The depth of soil profile from the top to parent material or bedrock or to the layer of obstacles for roots. It differs significantly for different soil types. It is one of basic criterions used in soil classification. Soils can be very shallow (less than 25 cm), shallow (25 cm - 50 cm), moderately deep (50 cm - 90 cm), deep (90cm - 150 cm) and very deep (more than 150 cm).

    Soil density: It is expressed in two well accepted concepts as particle density and bulk density. In the metric system, particle density can be expressed in terms of mega grams per cubic meter (Mg/m3).

    Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization. Salts occur naturally within soils and water. Salinization can be caused by natural processes such as mineral weathering or by the gradual withdrawal of an ocean. It can also come about through artificial processes such as irrigation.

    7.2.2. Soil profile and catena

    a. Soil profile

    Soil profile refers to the vertical arrangement of the soil in layers from the ground/ surface to the parent rock/bedrock or mother rock. It can be as little as 10 cm thick in some places or as deep as several meters in others. The layers are known as soil horizons. They are marked using letters A, B, C and D.

    i. Horizon O: This layer is also known as the superficial layer. This layer includes organic litter, such as fallen leaves and twigs. These are fresh or partially decomposed organic matter. This zone has two sub-layers:

    O1: This is the uppermost layer consisting of freshly fallen dead organic matter such

    as leaves, branches, flowers, fruits and dead parts of animals.

    O2: This layer lies just below the O1 layer. Here, organic matter is found under

    different stages of decomposition.

    ii. Horizon A: This layer is also known as the top soil. It refers to the upper layer of soil, nearest the surface (Horizon O). This is where biological activities and humus content are at their maximum. This layer is rich in silica or other resistant minerals. It is the zone more affected by the leaching of soluble material and by the downward movement like minerals. Hence, this layer is known as the zone of ‘eluviation’. This horizon is divided into two sub-layers:

    A1: This is dark and rich in organic matter, called ‘humus’. This layer has a mixture of

    finely divided organic matter and the mineral elements.

    A2: This layer is of light colour, with more sand particles and little organic matter. In

    regions of heavy rainfall, the mineral elements are rapidly lost downwards in this

    region. This is also known as podzolic or eluvial (E) or zone of leaching.

    iii. Horizon B: This is called the sub soil. It is the layer below the top soil. Its colour is determined by the parent rock and presence of organic matter. It has fewer living organisms and is rich in clay deposits.

    iv. Horizon C: This is the layer with recently weathered materials. It has low organic matter. It is the zone of deposition.

    v. Horizon D: This is the parent rock. It contains the rock which is resistant to weathering.

    b. Soil catena: 

    Soil catena is a sequence of different soil profiles that occur down a slope. It shows the changes that take place in the soil from the top to the bottom of the slope. A long the slope, different soils develop. These are influenced by climate and angle of slope.

    Upland areas have deep soils. This results from heavy weathering due to high rainfall and temperature. In those regions, the rain water reaches deep leading to a high degree of leaching. This makes the soils to become very mature. Thick layer of humus is developed in areas of thick vegetation. Along the slopes, the soils are thin. This is caused by movement of materials by gravitational forces. The soils are dry and less developed due to high runoff water on the slope. The run off is the rain water that moves freely down the slope. The soil profiles here have no top soil.

    A soil catena consists of three main divisions called complexes namely: Elluvial complex, the colluvial complex and the illuvial complex.

    – The elluvial complex: This is the upper convex slope where weathered materials are washed out downwards. It includes the summit and free face of the hill. Erosion predominates the free face hence; the soils are skeletal and with shallow soil profile.

    – The colluvial complex: This is the lower concave slope where there is gradual deposition of eroded material. The soil of the colluvial complex is thus moderately drained and retains certain moisture hence, supports agriculture and grazing.

    – The illuvial complex: This occupies the valley bottoms where fine materials are washed by seepage out outflow.

    Soil catena is important in the following ways:

    – The soil catena (elluvial complex) is used for settlement;

    – The soil catena is used for rock quarrying for construction especially on free

    face which has no vegetation;

    – The valley bottoms are used for brick lying, e.g Ruliba Clay Works along River

    Nyabarongo in Rwanda;

    – The illuvial complex is used for agriculture due to the deep soil profile derived

    from deposition;

    – The valley bottoms are used for rice growing and growth of vegetables and

    yams, etc.

    7.2.3. Soil fertility 

    Soil fertility is the ability of soil to support plant growth. Fertile soil is the one that is rich in nutrients that plants use to grow. Examples of these nutrients are nitrogen, phosphorus and potassium.

    Factors affecting soil fertility

    The following are the major factors affecting soil fertility:

    – Mineral matter: A matrix of mineral particles derived from varying degrees

    of breakdown of the parent-rocks. The fertility of soil depends on the type of

    mother rock to which the rock was derived.

    – Organic matter or humus: The fertility of soil depends on the amount of organic

    matter or humus available in soil. The higher amount of organic matter reflects

    the higher level of soil fertility.

    – Soil water or soil solution: the required optimum amount of water depends on

    the type of crops to be grown in specific area. Some crops such as rice require

    much more amount of water while the crops like sweet potatoes, cassava

    required low quantity of water.

    – Soil nutrients: these are chemical elements found in the soil. They help in plant

    growth and ensure the soil remains fertile.

    Application activity 7.2:

    1. Basing on the knowledge and skills acquired from the above lesson,

    differentiate soil porosity in Eastern Rwanda from those of Northern


    2. With the help of diagrams differentiate soil profile from soil catena.

    3. Identify the factors influencing soil fertility

    – Thickness (depth): Thin soils are not good for agriculture. Thick soils are good

    for agriculture (above 1 m of depth).

    – Soil permeability: This is the ability of the soil to allow water to pass through it.

    Permeable soils are much more fertile than non-permeable soils.

    – Soil texture: This refers to the size of soil particles. Clay loam soil are much

    fertile than other soil.

    End unit assessment

    1. Conduct a field work study around your school and collect soil samples

    then study those samples to identify their constituents.

    2. Describe the soil catena using diagram.

    3. Distinguish:

    a. Soil structure and soil texture

    b. Soil colour and Soil PH

    – Soil acidity and alkalinity: The basic soils are much more fertile than acidic soils. 


    Key unit competence:

    By the end of this unit, I should be able to appreciate the importance of the

    atmosphere, weather and the impact of climate on the environment and human

    activities in the world.

    Introductory activity: 

    Read the following passage and answer the questions that follow: The climate is defined as the average weather conditions of an area in terms of temperature, atmospheric pressure, wind direction and wind speed, moisture, cloudiness, precipitation, and sunshine for a standard period of 30 years. Due to its high altitude, Rwanda enjoys a tropical temperate climate. The average annual temperature ranges between 16 and 20° C, without significant variations. Rainfall is abundant although it has some irregularities. Winds are blowing with a speed of 1-3 m/s. With an economy that is dominantly supported by agriculture.

    1. Referring to the passage above distinguish climate from weather.

    2. Identify the relationship between weather, climate and atmosphere on

    earth’s surface.

    3. Examine the influence of climate on the environment and socioeconomic development of Rwanda.

    8.1. The atmosphere

    8.1.1. Meaning of the atmosphere

     The atmosphere is the envelope of gases surrounding the earth. It contains the gases that support all forms of life on earth. This envelope of air is dense at the sea level. It thins out with increase in altitude.

    8.1.2. The structure of the atmosphere 

    The atmosphere is a mixture of various gases surrounding the earth. It provides all gases that are essential for sustaining all life forms on the earth. Based on temperature variation, the atmosphere is made up of four layers: the troposphere, the stratosphere, the mesosphere and the thermosphere. On the basis of chemical composition; the atmosphere is composed of two layers: the homosphere and the heterosphere.

    Thermal characteristics of atmosphere 

    The atmosphere is made up of four vertical layers. These are: troposphere, stratosphere, mesosphere and thermosphere.


    Climatically, the troposphere is the most important because all the elements of weather and weather phenomena occur in this layer (e.g. evaporation, condensation, and precipitation of different forms like fog, cloud, dew, frost, rainfall, snowfall, thunder, lightning, atmosphere storms, etc.). This layer contains about 75% of gaseous mass of the atmosphere, most of water vapour, aerosols and pollutants.

    It is characterized by the following:

    – It is the lowest layer of the atmosphere.

    – The upper limit of the troposphere is called the tropopause.

    – The temperature decreases with altitude at the rate of 6.5°C per km or


    – The atmospheric pressure decreases also with altitude and reaches to

    100 millibars and 250 millibars over the equator and poles respectively at


    – It lies between 10 km and 20 km above the sea level.

    – All weather phenomena occur in this layer. These are evaporation, condensation

    and precipitation. Condensation is the conversion of vapour or gas into a liquid

    of different forms. Precipitation is water that falls to the ground as rainfall,

    snow or hail.

    – The height of tropopause is 17 km over Equator and 9 to 10 km over the poles.


    The stratosphere is the layer which is extending from 16 to 50 km above the sea level.

    It is characterized by the following:

    – There is increase of temperature due to absorption of ultraviolet solar radiation

    by ozone layer and lesser density of air.

    – There is nearly absence of weather phenomena because of dry air and rare

    occurrence of clouds.

    – The lower portion of the stratosphere having maximum concentration of ozone

    is called ozonosphere, which is confined between the heights of 15 to 35 km

    from sea level. Depletion of ozone would result in the rise of temperature of the

    ground surface and lower atmosphere. The main causes of ozone destruction

    are halogenated gases called chlorofluorocarbons (CFCs: chlorine, fluorine

    and carbon). This results to global warming, acid rain, melting of continental

    glaciers and rise in sea level, skin cancer, poisonous smoke, decrease in

    photosynthesis, ecological disaster and ecosystem instability.

    – The upper limit of the stratosphere is known as stratopause.


    – The mesosphere extends between 50 km and 80 km.

    – Temperature decreases as height increases. In fact, the rise of temperature

    with increasing height in the stratosphere stops at the stratopause.

    – At the uppermost limit of the mesosphere (80 km), the temperature drops at

    -80° C and may go down as low as - 100° C to -133° Celsius at mesopause.

    – This layer is characterized by very low air pressure ranging between 1.0

    millibar at 50 km altitude representing stratopause, and 0.01 millibars at the

    mesopause (between 90 and 100 km).


     It extends from 80 km to 640 km. It is composed of Ionosphere and Exosphere.


    It extends from 80 km to 640 km. It is composed of Ionosphere and Exosphere.

    The ionosphere consists of the following ionized layers:

    – D layer: reflects low-frequency radio waves but absorbs medium and highfrequency waves. Being closely associated with solar radiation, it disappears

    as soon as the sun sets.

    – E layer: The E-layer is also called the Kennelly-Heaviside layer. It reflects the

    medium and high-frequency radio waves. It is much better defined than the D

    layer. It is produced by ultraviolet photons from the sun rays interacting with

    nitrogen molecular. This layer also does not exist at night.

    – Sporadic E-layer: This layer occurs ttunder special circumstances. It is believed

    that this sporadic layer is caused by meteors and by the same processes that

    cause aurora lights. This layer reflects very high frequency radio waves.

    – E2 layer is generally found at the height of 150 km and is produced due to the

    reaction of ultra-violet solar photons with oxygen molecules. This layer also

    disappears during the night times.

    – F layer consists of two sub-layers e.g. F1 and F2 layers (150 km-380 km) are

    collectively called the Appleton layer. These layers reflect medium and high

    frequency radio waves back to the earth.

    – G layer (400 km and above) most probably persists day and night but is not


    The exosphere represents the uppermost layer of the atmosphere. The density becomes extremely low. The temperature reaches 5568° C at its outer limit, but this temperature is entirely different from the air temperature of the earth’s surface as it is never felt. The atmosphere above the ionosphere is called the outer atmosphere and it is made of exosphere and the magnetosphere.

    8.1.3. Composition of atmosphere 

    Basically, the atmosphere is composed of three major constituents, namely: gases, water vapour, and aerosols.

    a. Gases

    The main gases are shown in the table below:

    The following are the most important gases: 

    1. Nitrogen, which is about 78.1% of the total gases. Nitrogen is for all life forms. It is an important part of amino acids which make up proteins. Nitrogen (N) is one of the building blocks of life: it is essential for all plants and animals to survive. Nitrogen (N2) makes up almost 78.1% of our atmosphere. Humans and most other species on earth require nitrogen in a “fixed,” reactive form.

    Life depends on nitrogen, which is a basic ingredient in amino acids that make up all proteins. While a substantial percentage of the atmosphere is comprised of nitrogen gas, it must be processed into a soluble form. This is done via a nitrogen cycle that occurs in the soil. Then plants and the animals that eat them can obtain dietary nitrogen.

    Plants with nitrogen deficiencies look weak. Their leaves, which should be healthy and green, may look wilted and yellow. Animals and people get dietary nitrogen by eating protein-rich foods like milk, eggs, fish, beef and legumes. Nitrogen, carbon, hydrogen and oxygen are found in amino acids, which are the main structures of every protein.

    The nitrogen cycle involves:

    – Transfer of atmospheric nitrogen into soils (known as nitrogen fixation);

    – Mineralization, nitrification and transfer of nitrogen from soils to plants.

    – Denitrification and return of nitrogen to the atmosphere. It helps the

    oxygen in combustion; it also helps indirectly in oxidation of some


    2. Oxygen: This makes up 20.9% of gases in the atmosphere. The molecular oxygen (O2) mostly occurs up to the height of 60 km in the lower atmosphere. It is produced through photosynthesis. This is the process by which green plants use sunlight, carbon dioxide and water to make their own food. It is one of the main elements that make up air, and it is necessary for the survival of all plants and animals e.g. animals breathe in oxygen.

    3. Carbon dioxide represents 0.03% of the total atmospheric gases. The gaseous carbon (CO2) plays two significant roles:

    – Carbon dioxide helps in the process of photosynthesis where carbon, hydrogen, and oxygen are combined by the autotrophic green plants of terrestrial and marine ecosystem. This is assisted by sunlight that enables the formation of the organic compounds.

    – Carbon dioxide is transparent to incoming shortwave solar radiation. It is opaque to outgoing long wave terrestrial radiation. It is evident that carbon dioxide is most significant greenhouse gas. The concentration carbon dioxide increases due to the anthropogenic activities, namely the burning of fossil fuels and wood and deforestation which lead to probable climate change through global warming.

    4. Ozone (O3): It is defined as “a three-atom isotope of oxygen (O3) or merely a triatomic form of oxygen (O3). It is formed with the separation of (O2). Then, these separated oxygen atoms (O) are combined with Oxygen molecules (O2) and thus ozone (O3) is formed. It is a life-saving gas because it filters the incoming shortwave solar radiation and absorbs ultraviolet rays. Therefore, the ozone layer protects the earth from becoming too hot. It is known as the earth’s umbrella to all organisms in the biospheric ecosystem against their exposure to ultraviolet solar radiation. The highest concentration of ozone is between the altitudes of 12 km and 35km in stratosphere; the said zone is also called ozonosphere.

    a. Water vapour: Liquid water turns into vapour through the process of evaporation. The content of water vapour decreases with the altitude in the atmosphere. More than 90% of the water vapour in the atmosphere is found up to a height of 5 km. The moisture content in the atmosphere creates clouds, fogs, rainfall, frost, snowfall and other forms of precipitation.

    b. Aerosols: These are suspended solid particles and liquid droplets. These particles are from various sources like volcanic eruptions, desert dust, spores and pollen. The concentration of these particles decreases with increasing altitude in the atmosphere.

    8.1.4. Importance of the atmosphere

    The atmosphere is important for the following reasons:

    – It protects living things from harmful ultraviolet rays of the sun. This role is

    played by the ozone layer. The atmosphere serves as a protective shield against

    radiation and cosmic rays.

    – Dense layers of molecular gases also absorb cosmic rays, gamma rays and

    x-rays, preventing these energetic particles from striking living things and

    causing mutations and other genetic damage.

    – Even during a solar flare, which can greatly increase the damaging output of

    the sun, the atmosphere is able to block most of its harmful effects.

    – It helps to regulate the heat during the day and night. On earth, however,

    molecules in the atmosphere absorb the sun’s energy as it arrives, spreading

    that warmth across the planet. The molecules also trap reflected energy from

    the surface, preventing the night side of the planet from becoming too cold.

    – The Earth’s atmosphere protects and sustains the planet’s inhabitants by

    providing warmth and absorbing harmful solar rays.

    – The atmosphere traps the sun’s energy and sends off many of the dangers of


    – It provides the various gases that are useful to living things. These include


    – It provides precipitation that helps to sustain life on earth.

    – The atmosphere is made of gases that are essential for photosynthesis and

    respiration, among other life activities.

    – The atmosphere is a crucial part of the water cycle. It is an important reservoir

    for water and the source of precipitation.

    – The atmosphere moderates Earth’s temperature because greenhouse gases

    absorb heat.

    – Atmosphere contains the oxygen and carbon dioxide, which living things

    need to survive.

    – The atmosphere also serves an important purpose as a medium for the

    movement of water. Vapour evaporates out of oceans, condenses as it cools

    and falls as rain. Hence, providing life-giving moisture to otherwise dry areas of

    the continents. Without an atmosphere, it would simply boil away into space,

    or remain frozen in pockets below the surface of the planet.

    Application activity 8.1 

    1. a. Explain why the atmosphere should be conserved. b. Describe how atmosphere should be conserved. 2. Identify the susceptible sources of aerosols in your area.

    8.2 Elements of weather and climate

    Definition of weather and climate 

    Weather can be defined as the state of the atmosphere at any given time. This state is about temperature, atmospheric pressure, wind speed and direction, moisture, cloud cover, precipitation, and sunshine. Weather keeps changing all the time. The change is from hour to hour and day to day.

    Climate is an average weather conditions over a long period of time (about 30 years). It is measured by assessing the patterns of variation in temperature, humidity, atmospheric pressure, precipitations, wind speed and direction.

    The elements of weather and climate: Weather and climate are made up of many elements. The main ones are temperature, precipitation, wind, atmospheric humidity, clouds, sunshine and atmospheric pressure.

    8.2.1. Temperature

    Learning activity 8.2.1

     Read the context below and answer the questions that follow: When people migrate from Bugesera to Musanze they are required to change clothing style. Some of them say that it is necessary to put on jackets due to climatic conditions of that area.

    1. Why is there constant change of clothing style from Bugesera to


    2. Identify the weather condition that led people to put on jackets in

    Musanze while in Bugesera they put on tee-shirts.

    3. What may be the possible causes/factors responsible for that constant


    Temperature is the degree of heating and cooling of the atmosphere at a given area and time. The sun is the source of heat to the atmosphere.

    Factors influencing temperature variation

     Temperatures change from place to place. These changes are influenced by the following factors:

    Latitude or distance from the Equator: The temperature of a place depends on the amount of sunshine reaching there. The amount of sunshine received decreases as one moves away from the Equator.


    Altitude: Temperatures decrease with increasing height from the earth’s surface. This is at the average rate of 6.5°C per 1km or 1000 metres ascent.


    Distance from the sea: Places on the coast are affected by land and sea breezes. Therefore, they have low temperature. Places that are far away from the coast have high temperature.

    Prevailing winds: Winds blowing from low latitudes to high latitudes raise the temperature of the regions where they reach. Winds blowing from high latitudes to low latitudes lower the temperature.

    Nature of land and water: The contrasting nature of land and water surfaces in relation to the incoming shortwave solar radiation largely affects the spatial and temporal distribution of temperature. It may be pointed out that land becomes warm and cold more quickly than the water body. The following reasons explain the differential rate of heating and cooling of land and water.

    – The sun’s rays penetrate to a depth of only one meter in land because it is

    opaque, but they penetrate to greater depth of several meters in water because

    it is transparent to solar radiation.

    – Heat is concentrated at a place where the process of redistribution of heat by

    conduction is very slow because the land surface is static.

    – There is more evaporation from the seas and the Oceans and hence more heat

    is spent in this process. This results in oceans getting less insolation than the

    land surface. On the other hand, there is less evaporation from the land surface

    because of very limited amount of water.

    – The reflection (albedo) of incoming solar radiation is more over oceanic water

    surface than over land surface and thus water receives less insolation than


    – Oceanic areas are generally clouded and hence they receive less insolation

    than land surface. But clouds absorb outgoing terrestrial radiation and

    counter-radiate heat back to the earth’s surface.

    Nature of ground surface: The nature of ground surface in terms of colour, vegetation, and land use practices affects distribution of temperature. Areas under vegetation absorb more heat from the sun than those without vegetation.

    Nature of ground slope: The slopes facing the sun receive more heat from the sun because its rays reach the surface more or less straight. These areas have higher temperature than those not facing the sun.

    Cloud cover: The amount of cloud cover affects temperature. It is observed that night time temperature during clear sky is much less than clouded nights. Cloudy nights and days are warmer than cloudless nights and days.

    Ocean currents: The warm Ocean currents flowing from tropical areas to temperate and cold zones raise the average temperature in the affected areas. For example, the Gulf Stream raises the average temperature of the coastal areas of north-western Europe while Kuroshio warm current raises the temperature of Japanese coasts.

    Mountain barriers: Mountains block the movements of air from one place to another. This has an influence on temperature on both sides of the mountain.


    Measurement and recording of temperature 

    The instrument used for recording temperature is called a thermometer. Temperature is measured in degrees:

    Terms related to temperature

    – Mean (average) temperature

    The following are types of average temperature:

    Diurnal average (the average temperature within 24 hours of the day)

    From the table above, the diurnal mean temperature is 18.4°C. This is got by adding the daily temperatures and dividing the total by 24 (hours).

    – The highest temperature recorded within 24 hours is called the maximum

    daily temperature. It is 22°C (from the above table).

    – The lowest temperature recorded within 24 hours is called the minimum daily

    temperature. It is 16°C (from the above table).

    Monthly mean temperature: This is the sum of mean daily temperatures for

    a given month divided by the number of days of that month.

    Annual mean temperature: This is the mean temperature of 12 months of the year.

    – Temperature range

    – The difference between the maximum and minimum temperatures of the day is called daily range of temperature. Therefore, it is 22°C - 16°C = 6°C (from the above table showing the daily temperature variations).

    – The difference between the maximum and minimum temperatures of the month is called monthly range of temperature.

    – The difference between maximum and minimum temperatures of the year is called annual range of temperature. Therefore, it is16°C - 10°C = 6°C (from the above table showing the annual temperature variations).

    – Impact of temperature on the environment

    – The following are the major impacts of temperature on the environment:

    – A rise in global temperatures could lead to an increase of evapotranspiration. This could eventually lead to the rise in amount of rainfall

    which impact on environment positively or negatively.

    – A rise of temperature leads to melting of glaciers in polar and mountainous

    regions or ice-capped highlands.

    – An increase in temperature leads to global warming which impacts

    negatively on the environment.

    – As the earth gets warmer, plants and animals that need to live in cold

    places like on mountain tops or in the Arctic, might not have a suitable

    place to live. If the Earth keeps getting warmer up to one fourth; all plants

    and animals could become extinct within 100 years.

    – Animals are changing migration patterns and plants are changing the

    dates of activity.

    – The sea level has been rising more quickly over the last century as a result

    of temperature change

    – Precipitation (rain and snowfall) has increased across the globe; on

    average as a result of temperature change.

    8.2.2. Precipitation

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    a. Meaning and forms of precipitation 

    Precipitation is any form of liquid or solid water particles that fall from the atmosphere and reach the earth surface. The following are main forms of precipitations:

    Rainfall is the most widespread and important form of precipitation. It occurs when there is enough moisture in the air that condenses above freezing point. Rain is liquid water, in form of raindrops with diameter of more than 5 mm with 8,000,000 cloud droplets. This is formed of droplets that have condensed from atmospheric water vapour and then becomes heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth.

    Snow: It is formed when condensation occurs below freezing point (less than 0°C). The fall of snowflakes is called snow-fall. Snowflakes are formed when air temperature is greater than -5°C but less than 0°C. A snowfall is ‘precipitation of white and opaque grains of ice’.

    Drizzle: It is defined as the fall of numerous uniform tiny droplets of water having diameter of less than 0.5 mm. Drizzles fall continuously from low status clouds, but the total amount of water received at the ground surface is significant.

    Fog: This is a cloud layer lying in contact and very close to the surface of the earth or sea.

    Frost: It is defined as transformation of gaseous form of water (water vapour) directly into solid form (the process being called sublimation) at the ground surface, in the soils and in the air, layer just lying over the ground surface due to condensation occurring below freezing point.

    Hail: It consists of large pellets or spheres (balls) of ice. Hail is a form of solid precipitation wherein small balls or pieces of ice, known as hailstones, have a diameter of 5 mm to 50 mm. In contrast, ice pellets (sleet; sometimes called small hail) have a diameter less than 5 mm.

    Sleet: It refers to mixture of snow and rain.

    Ice pellets are snow grains, transparent or translucent grains of ice which are formed when condensation takes place below freezing point. The diameter of ice pellets is up to 5 mm and shape of the pallets is highly irregular depending upon the conditions of condensation.

    Dew: These are droplets of water that are deposited on the ground. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at which it can evaporate, resulting in the formation of water droplets. When temperatures are low enough, dew takes the form of ice; this form is called frost.

    Rime: It is an opaque thin, white layer of ice that forms when the air temperature is below the freezing point of water, especially outside at night.

    b. The water cycle 

    The water cycle also known as the hydrological cycle or the hydrologic cycle describes the continuous movement of water on, above and below the surface of the Earth. The water moves from one reservoir to another, such as from river to ocean, or from the Ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, surface runoff and subsurface flow. In doing so, the water goes through different forms: liquid, solid (ice) and vapour. The following are the main components of water cycle:

    Evaporation of water from Ocean, sea, lakes, rivers, ponds and transpiration from the leaves of plants through insolation (solar energy).

    Conversion of water into water vapour or humidity (first and second phases are almost the same).

    Horizontal transport of atmospheric moisture over the Oceans and the continents by atmospheric circulation (advection).

    Condensation: The transformation of water vapour into solid form. This takes place when the dew point is below freezing point to form snowfall, and when dew point is above freezing point to turn the water vapour into liquid form (i.e. above 0°C temperature of the air), is called condensation.

    Precipitation (either in liquid form as rain water, or in solid form as snow and ice and other minor forms of dew, fogs etc.).

    Runoff and infiltration: Eventual transfer of water received at the earth’s surface to the Oceans via various routes and hydrological processes. Some portion of rainwater received at the ground surface enters the soil zone through infiltration and thus forms soil moisture storage. This portion of water reappears as seepage and springs through flow and interflow.

    Percolation: This is a portion of infiltrated water which percolates further downward to form groundwater storage. While other portion moves upward as capillary rise to reach ‘soil moisture storage. Other quantity is routed further downward through deep transfer and enters the underlying bedrocks.

    c. Types of rainfall

    Rainfall is precipitation in form of droplets. The following are the major forms of


    1. Convectional rainfall: It occurs when the earth’s surface is heated by the sun. The warm air rises, and it is replaced by the cold air. As the air rises, the pressure on it decreases, expands and cools. Further cooling makes the moisture in it to condense and form clouds. It later falls as rain. Two conditions are necessary to cause convectional precipitation:

    – Abundant supply of moisture through evaporation to the air. So that relative humidity becomes highly supersaturated, and

    – Intense heating of ground surface through incoming shortwave electromagnetic solar radiation (that is, insolation heating). After super saturation of the air, follows condensation and clouds formation (cumulonimbus clouds) and then rainfall is formed.

    Convectional rainfall has the following characteristics:

    – Convectional rainfall occurs daily in the afternoon in the equatorial regions.

    – It has a short duration but occurs in form of heavy showers (heavy downpour).

    – It occurs through thick dark and extensive cumulonimbus clouds.

    – It is accompanied by thunder and lightning.

    – It causes a lot of runoff.

    – Convectional rainfall supports luxurious evergreen rain forests in the equatorial


    – In temperate regions, it is slow and of longer duration so that most of rainwater

    infiltrates into soils.

    – Convectional rainfall in hot deserts is not regular, it only occurs suddenly

    2. Orographic rainfall: this type of rainfall occurs when a water body is heated by the sun. The heating causes evaporation. Moist air from the sea is blown over a hill or mountain side. The rising causes it to expand and cool. It condenses and forms clouds. Rainfall occurs on the wind ward side of the mountain. This is the side facing the moist wind. The opposite side known as leeward side is dry.

    The following conditions are necessary for the orographic rainfall to occur:

    – There should be a mountain barrier across the wind direction, so that the moist

    air is forced on a barrier (an obstruction) to move upward.

    – If mountains are very close and parallel to the sea coasts, they become effective

    barriers. Such assists the moisture-laden winds coming from over the Oceans

    to rise upward and soon becoming saturated.

    – The height of mountains also affects the form and amount of orographic rainfall. Mountains or highlands near the seas or water bodies of reasonable height play a great role in formation of rainfall. Even low height coastal mountains are associated with rainfall formation since the moist air becomes saturated at very low height. On the other hand, the inland mountains should be of higher height because the air after covering long distances loses much of its moisture content.

    – There should be enough moisture content in the air. 

    3. Cyclonic or frontal rainfall Cyclonic or frontal rainfall occurs when air masses of different temperatures meet. One is warm and the other is cold. The warm air is forced over the cool air. As the warm air rises, the moisture in it is cooled. It condenses and falls as rain.

    Rainfall associated with ITCZ (Inter Tropical Convergence Zone) results from convergence of cold and warm air masses from the Southern and Northern hemispheres respectively in the equatorial zone. The following are the key conditions for frontal rainfall to occur:

    – The moisture content of the air is supposed to be positively correlated with


    – The convergent or divergent air circulation determines the ascent or descent

    of air and its adiabatic cooling or heating. This in turn determines the amount

    of precipitation.

    – Topographic conditions present both favourable and unfavourable conditions

    for precipitation (presence of mountain, near coastal land, etc.)

    – Distance from the source of moisture determines the amount of precipitation in different areas.

    d. Factors influencing rainfall formation

     The spatial and temporal distribution of precipitation over the globe is controlled by the following conditions:

    Moisture content of the air: The atmospheric moisture depends on evaporation of water or ice through the input of heat energy. The regions having high temperature and abundance of surface water, and wide-open Oceanic surfaces for evaporation, receive higher amount of annual rainfall. Equatorial regions are typical examples of such places.

    The convergent or divergent air circulation: The convergent or divergent air circulation determines the ascent or descent of air and its adiabatic cooling or heating which in turn determines the amount of precipitation. The convergence of two contrasting air masses creates frontal activity. In this case, warm air mass is pushed upward by underlying cold air mass resulting into uplifting of warm and moist air mass. The overlying warm and moist air is cooled, and precipitation occurs after condensation. On the other hand, divergent circulation allows descent of air from above and creates anticyclonic condition and atmospheric stability resulting into dry weather. Such situation develops in the subtropical high-pressure area. Therefore, this explain why most of the hot deserts of the world are found in this zone.

    Topographic conditions: Topographic conditions present both favourable and unfavourable conditions for precipitation. If the mountain barriers parallel to the coastal lands and there is onshore moist air, the moist air is forced by mountains to ascend and condense hence yielding precipitation. The leeward side would be dry while the windward side would experience rainfall.

    Distance from the source of moisture: Distance from the source of moisture determines the amount of rainfall in different areas. As the distance from the source of moisture (oceans, seas, rivers) increases, the moisture content reduces and hence the amount of precipitation decreases.

    e. Measurement and recording of rainfall 

    Rainfall is measured using a rain gauge. Many different types of rain-gauge have been designed and used. Most consist of a circular collector and a funnel that channels the collected rain into a measuring mechanism or into a cylinder where it may be measured later. The entrance to the gauge through the funnel is narrow to avoid debris clogging the mechanism and undesirable evaporation in hot weather. To make the rainfall measurement, the observer empties the collected rain into a graduated glass rain measure.


    f. Terms related to rainfall

    The data obtained from the weather station can be used to calculate the following:

    Daily rainfall total: This is the amount of rainfall recorded in a day.

    Monthly rainfall total: This is the total amount of rainfall received in a month.

    It is obtained by calculating the sum of daily rainfall totals in the month.

    The mean monthly totals: This is the average rainfall received per month in

    a year. It is obtained by adding the total monthly rainfall received over a given

    period (preferably 30 years) and dividing by the number of years.

    The annual rainfall total: This is the total amount of rainfall received in a year.

    It is obtained by finding the sum of all the monthly rainfall totals for 12 months.

    Mean annual rainfall: This is the average total annual rainfall received in an

    area. It is obtained by adding the total annual rainfall received over a given

    period (preferably 30 years) and dividing by the number of years.

    (i) Meaning of wind

     Wind is air in motion above the surface of the earth. It is caused by differences in atmospheric or barometric pressure. Air moves from areas of high barometric or atmospheric pressure to areas of low pressure.

    (ii) Measurement and recoding of wind direction

    The wind direction is measured with help of a wind vane and wind sock.

    Wind vane: It consists of a horizontal rotating arm pivoted on a vertical shaft. The rotating arm has a tail at one end and a pointer at the other. When the wind blows, the arm swings and points to the direction the wind is blowing from. The wind is named after this direction.


    Wind sock: It is common in airstrips. It consists of a cylindrical cloth bag tied to a mast. It always points towards the direction at which the wind is blowing.


    (iii) Measurement of wind speed 

    The speed of wind is measured using an anemometer. This consists of three or four metal cups fixed to metal arms that rotate freely on a vertical shaft. When there is wind, the cups rotate. The stronger the wind, the faster the rotation will be. The speed of rotation is recorded on a meter. Wind speed is measured in kilometres per hour. The regions with the same wind speed are called isotachs.


    (iv) Factors influencing the nature and movement of winds

    The following are the main factors that affect wind direction: 

    1. The pressure-gradient force: Air flows from areas of higher atmospheric/ barometric pressure to areas of lower pressure.  This is the pressure gradient force that sets the air in motion and causes it to move with increasing speed down the gradient.  The heating of the earth’s surface is uneven which causes the continual generation of these pressure differences. The greater the atmospheric/barometric pressure difference over a certain horizontal distance, the greater the force and therefore, the stronger the wind.


    2. The Coriolis force: Winds are deflected to the right in the Northern Hemisphere and to the left in the Southern. The Coriolis force is directed at right angles to the direction of air flow.  It does not affect the wind speed, only the wind direction.  However, the stronger the wind, the greater the deflecting force.  There is no deflection of winds at the equator, but it increases to its maximum at the poles.


    3. Frictional force: The irregularities of the earth’s surface offer resistance to the wind motion in form of friction. This force determines the angle at which air will flow across the isobars, as well as the speed at which it will move. It may also alter wind direction

    4. Centripetal Acceleration: Due to inward acceleration of air towards the centre of rotation on the rotating earth, it is possible for the air to maintain a curved path (parallel to the isobars), about a local axis of high or low pressure. It is known as centripetal acceleration.

    (v) Types of winds

    The following are the main types of winds:

    – Winds blowing almost in the same directions throughout the year are called

    permanent winds or planetary winds.

    – Winds which change their directions according to the season are called

    seasonal winds (e.g. monsoon winds).

    – Winds which change the directions according to the time of the day are called

    breezes (sea breezes and land breezes, mountain breezes and valley breezes).

    – Winds which blow in a particular locality are called local winds (e.g. Chinook,

    Sirocco, Harmattan, Mistral…).

    1. Permanent winds or planetary winds

    These winds blow from high pressure belts to low pressure belts.

    – Easterlies: There are winds which blow from East to West. These are found in both inter-tropical zone and polar zone. The winds blowing in intertropical zone are also called trade winds.

    Westerlies: These types of winds blow from West to East. They are found in the temperate zone between tropics and sub-polar zone.


    2. Seasonal winds or monsoon winds 

    Seasonal winds are large-scale surface winds which reverse their directions at least twice a year. Monsoons result from unequal distribution of land and water. They also result from the seasonal heating of the land and oceans. During winter, there is high pressure on the land due to low temperature. Therefore, the air blows from the land (Asia) to the Ocean (Indian Ocean).

    This situation is reversed during the summer. High temperature develops over the Ocean, with low pressure over land. Therefore, winds blow from high pressure areas (over the Ocean) to the low-pressure areas (over the land).

    Therefore, during summer, they blow from the Ocean (water) to the continent (land).

    These winds are found in the following areas:

    – In Asia: India Pakistan, Bangladesh, Myanmar (Burma), Thailand, Laos,

    Cambodia, North and South Vietnam, Southern China, Philippines.

    – Northern coastal areas of Australia.

    – South-West coast of Africa including the coasts of Guinea, Sierra Leone, Liberia

    and Ivory Coast; Eastern Africa and western Madagascar.

    – North-East coast of Latin America e.g. East Venezuela, Guyana, Surinam,

    French Guyana, and North-East Brazil, Puerto Rico, Dominican Republic in the

    Caribbean Island, parts of Central American and South East USA. 

    3. Diurnal local winds 

    These are winds that are caused by daily temperature and pressure variations. They occur because of the heating and cooling during the day and night, plus the difference in the pressures over land and water.

    Sea breezes: Land heats up faster than the sea during the day. Air over the land, therefore, becomes warmer than over the sea. The lighter and warmer air rises. The cooler and heavier air from the sea moves over the land to replace the rising air. This movement of air causes a sea breeze.


    Land breezes: At night, the land cools faster than the sea. The air over the land becomes cooler and heavier than the air above the sea. The warmer air over the sea is lighter. It rises while the cooler heavier air over the land moves to replace it. This creates an air current called a land breeze.

    Valley breezes: The slopes and floors of valleys are heated more at daytime. The warm air moves up the slope or upwards. This movement creates what is called a valley breeze or anabatic wind.


    Mountain breezes: During the night, the hill slopes lose heat faster. The valleys remain warmer. A low-pressure gradient is created in the valley. Air current moves down the valley to form what is known as a mountain breeze. It is also called katabatic wind. This explains why some valley floors have frost at night.


    4. Non-periodic/local winds

    These fall into two groups depending on their temperature (hot and cold winds).

    They are found in different areas of the world. 

    Hot local winds: Examples are Chinook, Sharmal, Foehn, Khamsin, Harmattan, Sirocco, Simoom, Norwester, Santa Anna, Brickfielder, and Loo.

    Cold local winds: Examples are Mistral, Purga, Bora, Bise, Blizzard, Laventer, Northers and Pampero.

    i. Chinook or Foehn winds: These occur when strong regional winds pass over a mountain range. They cool as they rise, dropping moisture on the windward side of the mountain. They descend on the leeward side as cold dry wind. As they go down, they are heated. They can absorb more moisture. Chinook is the name used in the United States of America over Rockies. In Switzerland over the Alps, they are called Foehn winds.


    These are other non-periodical local winds:

     – Hurricanes: These are revolving tropical storms of the Caribbean Sea and Gulf of Mexico. They occur in the Eastern parts of the Pacific Ocean along the coasts of Mexico, Guatemala, Hondulas, Nicaragua, Costa Rica and Panama. Hurricanes occur from August to October.

    Harmattan: These are warm and dry winds blowing from North-East and East to West in the Eastern parts of the Sahara desert. These winds are very dry because of their journey over the Sahara desert. They pick up sand as they blow.

    Sirocco: This is a warm, dry and dusty local wind. It blows Northwards from the Sahara Desert and crosses the Mediterranean Sea to Italy and Spain.

    Typhons: The tropical cyclones in the west Pacific and China sea are known as Typhoons. They occur from June to December. It is an increase low-pressure system which produces violent winds and heavy rains. The magnitude and intensity of typhoons generally exceed those of hurricanes.

    Mistral: This is a cold local wind which blows in Spain and France from NorthWest to South-East. It is more common and effective during winter.

    Tornadoes: Tornado is a rapidly rotating column of air developed around a very intense low- pressure centre. It is associated with a dark funnel-shaped cloud with extremely violent winds, i.e. more than 400 km per hour, accompanied by down pours/heavy rainfall.

    Bora: This is a very cold and dry North-Easterly wind which blows along the shores of the Adriatic Sea.

    Blizzard: This is a violent stormy cold wind that carries with it dry snow. It is common in Siberia, Canada and the USA.

    vi. Influence of winds on weather conditions and human activities

    Influence of winds on weather conditions

    – The way the air moves affects the weather of underlying places. Winds move

    heat and cold temperatures from one place to another, transporting conditions

    from one geographical zone to another.

    – Without wind, weather would not exist. Wind and ocean currents are the

    vehicles of water vapour leading to cloud formation which yield rainfall. Heat

    or/and cool air masses are moved from one area of the globe to another,

    creating weather variations within specific climate zones.

    – The wind direction will have an important influence on the expected weather.

    Wind direction changes often accompany changes in the weather.

    – The wind speed and direction can give the clues to the expected weather


    Influence of winds on human activities

    – The winds with high speed cause the destructions of physical and humanmade environment (e.g. Hurricanes, tornadoes, typhoon, etc.).

    – Most of time the tall trees are bent according to the wind directions.

    – Exposure of growing plants to hot wind results in dwarfing due to desiccation/

    dryness of plant tissue and reduced growth.

    – Wind increases crop water requirements by increasing evapotranspiration due

    to removal of accumulated humid air near the leaves.

    – Calm to moderate wind favour dew deposition needed under condition of soil

    moisture stress.

    – Moderate wind aids effective pollination. Heavy wind during flowering reduces

    pollination, causes flower shed, increases sterility and reduces fruit set in all


    – Wind speed more than 50 km per hour leads to destruction of crops leading

    to heavy loss.

    – In coastal areas, winds carrying salt sprays can have harmful effect on

    susceptible crops.

    – Soil and sand particles blown by wind strike the leaves and other plant parts

    making punctures, abrasions, scratches and tear the leaves into pieces and

    strips. It well marked in maize, sugarcane and banana.

    – The turbulence created by wind increase carbon dioxide supply and the

    increase in photosynthesis.

    – The hot and dry wind makes the cells expanding and early maturity. Therefore,

    this results into the dwarfing of plants.

    – The coastal area affected by strong winds faces the challenge of high salinity

    due to salt particles drawn and deposited on the mainland. This makes the soil

    unsuitable for growing plants.

    – The crops on the windward slopes yield more.

    vii. Air masses

    1. Meaning of air masses

    Air mass may be defined as a large body of air whose physical properties, especially temperature, moisture content, and lapse rate (of temperature) are uniform horizontally for hundreds of kilometres.

    2. Types of the air masses

    Based on geographical location of air masses, Trewartha classifies them into the following two broad categories:

    Polar air masses. This type is subdivided into continental air masses and Maritime Air masses.

    a. Continental polar air masses: These are characterized by cold temperatures and little moisture. They are found on the continent mostly in the polar zones such as Northern portions of the United States. These conditions usually result from the invasion of cold arctic air masses that originate from the snow-covered regions of northern Canada.

    b. Maritime polar air masses: These are cool, moist, and unstable. Some maritime polar air masses originate as continental polar air masses over Asia and move westward over the Pacific, collecting warmth and moisture from the Ocean.

    Tropical air masses: These are also subdivided into continental and maritime air masses.

     a. Continental Tropical Air Mass: The source region for this type is the desert Southwest, the high plains and Mexico with relation to the United States. The air has low dew points and warm to hot afternoon temperatures but with mild night time temperature. Skies are generally clear in Continental Tropical Air mass.

    b. Maritime Tropical Air Mass: This originates over the warm waters of the tropics and Gulf of Mexico, where heat and moisture are transferred to the overlying air from the waters below. The Northward movement of tropical air masses transports warm moist air into the United States, increasing the potential for precipitation.

    3. Characteristics of the air masses

    The basic characteristic features of air masses include the following:

    – Air masses can either be cold or warm depending on the source region.

    – They can be maritime (originating over oceans) or continental in nature

    depending on the trajectory taken.

    – Air masses have specific direction of movement from fixed source region.

    – Air masses move from one region to another following the pattern of

    barometric pressure. (From areas of high pressure to areas of Low pressure.)

    – Throughout the air body there is unvarying humidity content with the same


    – Air masses usually have uniform temperature characteristics over a long


    – Air masses are independent even when they come into contact with each

    other, do not merge into each other but retain their identity.

    – Air massess are directly connected or associated with the planetary wind

    systems. That is, every air mass is related with one or the other permanent

    wind belt.

    4. The formation of the air masses

    The air masses are formed from extensive and broadly uniform areas. The nature and properties of the originating areas largely determine the temperature and moisture characteristics of air masses. An ideal source region of air mass must possess the following essential conditions.

    – There must be extensive and homogeneous earth’s surface so that it may

    possess uniform temperature and moisture conditions. The source region

    should be either land surface such as a desert or Ocean surface.

    – There should not be convergence of air, rather there should be divergence of

    air flow. This is important for the air to stay over the region for longer period to

    have the ability of having uniform temperature and humidity.

    – Atmospheric conditions should be stable for considerable long period of time.

    This is necessary because the air must attain the characteristics of the surface.

    5. Effects of air masses on the environment

    – When air masses hover for a while over a surface area with uniform humidity

    and temperature, it takes on the characteristics of the area below, and

    influences the environment of that area.

    – When two air masses of different properties meet, the cold air pushes the hot

    air upwards. The uplifted moist air condenses to yield rainfall or precipitation. 

    – There is formation of a stationary front especially when neither air mass

    displaces the other. This leads to formation of clouds that yield varying kinds

    of precipitation.

    – Maritime air masses are associated with humid conditions. This is because as

    air travels over the Oceans it picks moisture that is later dropped in form of

    precipitation. Therefore, it produces the warm and humid conditions in the

    areas they bathe.

    – The continental air masses lead to formation of dry weather. This is because the

    continents just can’t compete with the Oceans when it comes to moisture. The

    continental air masses produce dry, cold weather in the winter and pleasant

    weather conditions in the summer.

    – When the air is lifted over an obstacle like a mountain, such lifting is known as

    orographic lifting. This leads to occurrence of varying weather conditions. The

    windward side receives rainfall while the leeward, becomes dry. 

    6. Cyclones:

    1. Meaning of cyclone

    A cyclone is a large-scale air mass that rotates around strong centres of low pressure. This is usually characterized by inward spiralling winds that rotate Anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth.


    2. Areas of cyclones in the world

     Most large-scale cyclonic circulations are centred on areas of low atmospheric pressure. The main cyclones are tropical cyclones which are the typhoon of the Pacific Ocean and hurricane of Atlantic; cold- polar cyclones and extra tropical cyclones.

    There are six general regions of occurrence:

    – The Caribbean Sea and Gulf of Mexico.

    – The Northwest Pacific from the Philippines to the China Sea.

    – The Pacific Ocean west of Mexico.

    – The South Indian Ocean east of Madagascar.

    – The North Indian Ocean in the Bay of Bengal and

    – The Arabian Sea.

    3. Characteristics of cyclones

    – A cyclone is simply an area of low pressure around which the winds blow

    counter clockwise in the Northern Hemisphere and clockwise in the Southern


    – Cyclones form and grow near the front.

    – Cyclones (lows) are cloudy, wet, and stormy.

    – The largest low-pressure systems are cold-core polar cyclones and extra

    tropical cyclones which lie on the synoptic scale.

    – Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie

    within the synoptic scale.

    – Upper level cyclones can exist without the presence of a surface low.

    – Tropical cyclones form due to latent heat driven by significant thunderstorm

    activity. In this case, humidity plays a great role in generation of this heat that

    drives the whole wind system.

    – Cyclones can transition between extra tropical, subtropical, and tropical

    phases under the right conditions. 

    4. Formation of cyclones (depressions)

    Tropical cyclones develop due to two factors as explained here under;

    – When two differing air masses of varying characteristics in terms of temperature

    and humidity meet over the surface of the Ocean. Or sometimes when there

    is the local heating from the surface especially of the Ocean water that creates

    the area of intense low pressure.

    – Winds of varying characteristics move towards this low pressure and yet hardly

    mix up. But instead, start circulating and spinning in a spiral nature around an

    area of low pressure.

    – As earier said, when two varying air masses meet, the warm air rises over

    the cold air and the moisture contained therein undergoes the process of

    condensetion and leading to terretial rainfall.ely

    – Through the process of condesetion latent heat is released and it is this energy

    that facilitates the rotation and spiral nature of the cyclone.

    – Over the tropical maritime/Oceans, the intense heating creates an area of low

    pressure and the cold air masses rush towards this area and setting the creation

    of a tropical cyclone as the humidity increases and latent heat generated.

    5. Effects of cyclones on the environment

    – There are several effects of cyclones on the environment. These are explained


    – Tropical cyclones are associated with heavy rain, strong wind, large storm

    surges and tornadoes.

    – They trigger landslides and mudslides. This is because of the heavy rainfall that

    comes along with the tropical cyclones.

    – They cause destruction of vegetation and wildlife in general. This is because

    of the stormy nature of the rainfall associated with cyclones. Therefore, trees

    break down and their canopies completely trimmed off.

    – They cause severe erosion especially along the coastal regions that are usually

    most hit by tropical cyclones. Especially the removal and reshaping of the sand



    Meaning of anticyclone

    An anticyclone is known as a large-scale circulation of winds around a central region of high atmospheric pressure. The winds blow clockwise in the northern hemisphere and counter clockwise in the southern hemisphere.

    Areas of Anticyclone 

    The classification of anticyclones is based on their location or areas where they occur. They are categorized into four classes which are described hereunder:

    The subtropical Highs: The subtropical highs are large, elongated, very deep

    anti- cyclones situated at tropical regions.

    The polar continental Highs: The polar continental highs (anticyclones) are

    prominent over Northern continents in winter. The conditions of Alaska, western

    Canada and Rocky Mountains are most favourable for their development.

    High within the cyclone Series: Anticyclones with small horizontal extent are

    sometimes present between the individual members of the cyclone family.

    The polar highs: These polar highs occur at the end of a temperate cyclone. 


    Occurrence of anticyclones

     The anticyclones occur in sub-tropical high-pressure, belt extending between the latitudes of 250-350 and in Polar Regions in both Hemispheres.

    Characteristics of anticyclones 

    The ‘highs’ or ‘anticyclones’ are characterized by divergent wind circulation. Whereby, winds blow from the centre outwardly in clockwise direction in the northern hemisphere and anti-clockwise in the southern hemisphere. The high-pressure systems are indicative of dry weather conditions. Therefore, anticyclones are called weather less phenomena.

    Effects of anticyclones on the environment

    The following are the major effects of Anticyclones on the environment:

    – Descending and divergence movements of the anticyclones result into

    cloudless skies leading to dry weather conditions. These dry weather conditions

    may result into drought in the affected areas.

    – The Anticyclones have been one of the causes of hot deserts of the world,

    especially in the western parts of the continents near the tropics.

    – The cloudless skies over the areas affected by anticyclones result into the

    reception of higher amount of solar radiation on the earth’s surface. This makes

    the soil to become drier and vegetation experiences the water shortage.

    – The anticyclone makes the temperature range to be higher because the days

    are hot, and nights are very cold. This has negative impact on the vegetation

    and other living things. 

    8.2.4. Atmospheric humidity

    i. Meaning of atmospheric humidity

     Atmospheric humidity is the amount of water vapour in the atmosphere. The water comes as a result of evaporation and transpiration by plants. It also comes from volcanic activity through hot springs. The water vapour determines the amount of precipitation in an area.

    ii. Measurement and recording of humidity

     Atmospheric humidity is measured using a hygrometer. It consists of a system of levers linked to a pen and record chart or graph paper. Within it, are threads of human hair. When humidity rises, the threads absorb moisture and become longer. When humidity falls, they dry and become shorter. The levers make these movements appear large. The pen records them on the graph paper.


    iii. Types of atmospheric humidity

    The following are the main types of the atmospheric humidity: 

    1. Absolute humidity: Absolute humidity is defined as amount of water vapour per unit volume of air at a given temperature. It is expressed in gram per cubic meter volume of air (gr/m3). Absolute humidity changes with changing temperature. For example, it is 1gr/m3 at - 200C in cold continental area during winter season while it will be more than 30 gr / m3 at 200 C in maritime equatorial region.

    2. Relative humidity: Relative humidity is defined as a ratio of the air’s actual water vapour content compared with the amount of water vapour air can hold at that temperature and pressure.

    3. Relative humidity is generally expressed as percentage. Relative humidity is calculated as follows:

    iv. Factors influencing atmospheric humidity

    The following factors affect humidity:

    Temperature: An increase in the temperature of the air increases its ability to

    hold moisture.

    Amount of water available: There is more evaporation over the Ocean than

    the land. Therefore, there is more moisture over the Ocean than the land.

    Wind-speed: Evaporation depends on the speed of wind. When the winds

    are light, a thin layer of air just above the surface gets almost full of moisture.

    When the wind speed is high, the air has less moisture. 

     Area of the evaporating surface: Larger areas where evaporation occurs

    increase the rate of evaporation.

    Air-pressure: Evaporation is also affected by the atmospheric pressure exerted

    on the exposed surface of water. Low pressure on open surfaces of the liquid

    results in higher rates of evaporation.

    Composition of water: The rate of evaporation is always greater over fresh

    water than over salty water.

    v. The significance of humidity to the environment

    – Humidity drives most of the observable weather phenomena starting with

    clouds, fog, rain, storms and finally to such dramatic weather conditions such

    as hurricanes.

    – It facilitates the weather fore casting. It is not possible to forecast the weather

    exactly without precise knowledge of humidity in all the layers of the


    – Correct relative humidity is important for our well-being and health.

    – It enables the hydrological cycle to operate normally. It enables much water

    in form of vapour to be kept or stored in the atmosphere. Such is condensed

    later to form precipitation.

    – Humidity plays a great role in stabilising climate of various areas. This is

    because of its regulating ability that prevents the occurrence of extreme levels

    of temperatures.

    – It affects many properties of air and of materials in contact with air.

    – Water vapour is a key agent in both weather and climate, and it is an important

    atmospheric greenhouse gas. This plays part in regulating the Earth’s


    – Humidity measurements contribute both to achieving correct environmental

    conditions that sustain various ecosystems.

    8.2.5. Cloud cover

    Learning activity 8.2.5

     Read critically the context below and answer the questions that follow: Daniella always wakes up and observes the horizon of the sky. One day, she observed the sky and found out that it was clear with white colour. She continues to take note of the daily occurrences of the nature of the sky. The next day she observed black colour in the sky, another day she found sky approaching the tops of hills and the last day of her last observation; she observed the sky being dark and reaching the ground. Now Daniella is asking herself what is happening in the sky.

    Help Daniella to be satisfied with clear explanations to her question through

    answering the following questions:

    1. Explain what causes the different colours that arise in the horizon of the


    2. Describe the effects of the last observation of Daniella to the


    i. Meaning of cloud

    A cloud is an aggregation or grouping of moisture droplets and ice crystals that are suspended in the air. A cloud is made up of water droplets or ice particles suspended in the air. These particles have a diameter ranging between 20 mm and 50 mm.

    ii. Types of clouds and their characteristics

    Clouds are classified according to altitude and form. With regard to form, there


    a. Stratified clouds: These are layered clouds. They look like blankets and cover

    large areas. They can give large amounts of rain or snow.

    b. Cumuliform clouds: These have bubble-like bodies. They give rain over a small


    With regard to altitude, clouds are classified as high clouds, middle clouds and

    low clouds.

    a. High clouds (form above 6,000 m above the sea level). They look like feathers.

    They appear in separate groups in fair weather. In bad weather, they are joined

    together. The following are examples:

    – Cirrus

    – Cirrostratus

    – Cirro-cumulus

    b. Middle clouds (form between 4,000 and 6,000 metres). They are thick clouds.

    The following are examples:

    – Alto-stratus

    – Alto-cumulus

     They are mostly distributed over the whole sky. They appear white or grey.

    c. Low clouds (Form below 2,000 metres). They are usually shallow. The following

    are examples:

    – Stratus: They are dense, low-lying fog-like clouds of dark grey colour. They are

    composed of several uniform layers.

    – Strato-cumulus: They are generally associated with fair or clear weather but

    occasional rain or snow.

    – Nimbo-stratus (Ns): They are middle and low clouds of dark colour. They are

    associated with rain.

    d. Clouds with great vertical extent: They are found between 2,000 and 10,000

    metres. They are white but may appear grey or black. They give heavy rainfall.

    The following are examples:

    – Cumulus: They are very dense, widespread and dome-shaped. They also

    have flat bases and are associated with fair weather. These sometimes are

    characterised by thunder.

    – Cumulo-nimbus: They are thunder-storm clouds. They show great vertical

    development/extent and produce heavy rains, snow or hailstorm accompanied

    by lightning, thunder and gusty winds. They appear like mountains or huge



    iii. Factors influencing the formation and the shape of clouds

     The sun plays a major role in formation of clouds. The first step of cloud formation is related to the rising of the air into the atmosphere. Such air must be moist or carrying water vapour that is later condensed to form clouds. The following are the major factors influencing the rise of air into the atmosphere:

    Bottom of Form

    Bottom of Form

    – Sun: The sun heats the earth and thus the air rises, expands, and cools.

    – Topography: Clouds are also formed when air encounters mountains or other

    topography. The air rises and cools, condensing to form clouds.

    – Low pressure: This influences air to move towards the areas of low pressure.

    Therefore, different kinds of air of varying characteristics meet but not

    necessarily mixing up. The cold air forces the warm and moist air to rise and

    hence, cool and condensing to form clouds.

    – Clouds can be formed when air rises along the slope of a mountain. This aids

    the moist air to reach the atmosphere where it cools down, condensing and as

    a result, clouds are formed.

    – Warm and cold fronts: Weather fronts cause the rise of the air. Specifically,

    warm fronts make the air to condense because the warm air rises above the

    cold air. This results into the formation of clouds. 


    iv. Effects of clouds on weather

    The clouds are much important on weather conditions prevailing in a given area as

    it is explained below:

    – During the day, the earth is heated by the sun. If skies are clear, more heat

    reaches the earth’s surface (as in the diagram below). This leads to warmer



    If skies are cloudy, some of the sun’s rays are reflected off the cloud droplets back into space. Therefore, less of the sun’s energy is able to reach the earth’s surface. This causes the earth to heat up more slowly. This leads to cooler temperatures.


    Cloudy skies lead to existence of lower temperatures since much of the sun’s isolation will be blocked from reaching the earth’s surface. The clear skies during day, leads to increase in temperature. At night, cloud cover has the opposite effect. If skies are clear, heat emitted from the earth’s surface freely escapes into space, resulting in colder temperatures.


    If clouds are present, some of the heat emitted from the earth’s surface is trapped by the clouds. This heat is reemitted back towards the earth. As a result, temperatures decrease more slowly than if the skies were clear.


    i. Meaning of sunshine 

    The sunshine is composed of two words: sun and shine; it means the period of shining of the sun. The sunshine is observed and recorded during the day time.

    ii. Measurement and recording of sunshine

     This is done using a Campbell-stokes sunshine recorder. The instrument records the duration and intensity of sunshine.

    This is a glass sphere which is partially surrounded by a metal frame on which there is a sensitized or calibrated card. The card is graduated in hours and minutes. When the sun shines, the glass sphere focus the sun’s rays on the card and as the sun moves across the sky, the rays burn a trace on the card. This only happens when the sun is shining. At the end of the day, the card is taken out and the length of trace is turned into hours and minutes, which represents the total amount of sunshine for the day. A line on the topographic map joining places of equal sunshine is called “Isohel”


    iii. Factors influencing the amount of solar radiation

    All parts of the world do not receive the same amount of solar radiation because of the following factors:

    Angle of the sun’s rays: The angle between the rays of the sun and the tangent to the surface of the earth at a given place, largely determines the amount of insolation to be received at the place. The angle of the sun’s rays decreases as one moves towards the poles. Vertical rays bring more solar radiation.

    Length of the day: The shorter the duration of sunshine and longer the period of night. This results into lesser amount of solar radiation received at the earth’s surface and vice versa. This happens due to spatial variations of the length of the day from the equator to the polar region, due to the inclination of the Earth’s axis (Reference to the unit 4 on the consequences of revolution of the earth).

    Distance between the Earth and the Sun: The distance between the sun and the earth changes during the course of a year. This is because the Earth revolves around the sun in an elliptical orbit. The average distance between the sun and the earth is about 149 million km. At the time of perihelion (on January 3) the earth is nearest to the sun (147 million km) while at the time of aphelion (on July 4) it is farthest from the sun, that is, (152 million Km) away. At the time of perihelion, the earth should receive maximum insolation while at the time of aphelion it should receive minimum insolation.

    Sunspots: Sunspots are defined as dark areas within the photosphere of the sun. They are created in the solar surface (photosphere) due to periodic disturbances and explosions. The increase and decrease of the number of sunspots is completed in a cycle of 11 to 24 years. It is believed that the energy radiated from the sun, increases when the number of sunspots rises and consequently the amount of solar radiation received at the earth’s surface also upsurges. On the other hand, the amount of solar radiation received at the earth’s surface decreases with decrease in the number of sunspots due to less emission of radiation from the sun.

    Effects of the atmosphere: As solar radiation travels a long distance from the sun to the earth’s surface; there are some portions of the solar energy which are lost through the processes of reflection, diffusion, absorption and scattering.

    iv. Influence of sunshine on the environment

    The sunshine has both positive and negative effects on the environment as it is

    explained below:

    Positive effects

    Health benefits: Humans require between 1,000 and 2,000 units of vitamin D

    daily for optimum health. The skin creates vitamin D naturally when exposed

    to solar radiation, and spending 10 or 15 minutes a day outside can give all

    body to stay healthy

    Needed for photosynthesis: Sunshine is needed in photosynthesis processes

    for autotrophy green plants and algae to produce the compounds necessary

    for their survival.

    Disinfection: Exposing bottled water to sunlight for six hours or more can kill

    many harmful pathogens, and developing nations often use this technique

    as a low-cost method of treating water supplies against common bacterial


    Production of energy: Sunshine can produce the energy which may be used

    for different purposes.

    Negative effects

    Skin Cancer: The ultraviolet radiation present in sunlight can also cause

    damage to the human and animal skins. Short exposures to intense sunlight

    during the summer months can produce painful sunburns, while longerterm exposure to ultra violet can damage cells, altering the DNA and possibly

    leading to skin cancer.

    Eye Damage: Solar radiation can also prove harmful to the human and animal


    – Damaging the crops and other vegetation: The excess of daily sunshine without rain for long-term, becomes harmful to crops and other vegetation because all water which would support the crops/vegetation evaporates.

    8.2.7. Atmospheric pressure

    Meaning of atmospheric pressure

     Atmospheric pressure is the force per unit area that is exerted against the Earth’s surface by the weight of air above it. The unit area could be one square centimetre or one square meter. Atmospheric pressure is measured in terms of the height of mercury in the glass tube in a mercury barometer. The standard air pressure at sea level is 1013.25 mb or 29.92 inches or 760 mm at temperature of 15 °C at the latitude of 45°.

    Measurement and recording of atmospheric pressure 

    The barometer is the instrument used to measure atmospheric pressure. There are two types of barometers: Mercury barometer and Aneroid barometer.

    a. Mercury barometer 

    It is made of a one-meter-long glass tube. It is closed at one end and filled with mercury. The open end of the tube is then opened below the surface of mercury in a bowl. A vacuum is left above the mercury and the column is supported by the air pressure outside. Air pressure is obtained by observing the length of the mercury column.


    b. Aneroid barometer 

    Aneroid barometer is made of a small metal box which contains a very little amount of air. It expands and collapses under the influence of any change in atmospheric pressure. See the figure below.


    Factors influencing the distribution of atmospheric pressure

    The following factors influence atmospheric pressure:

    Altitude: Air pressure at sea level is higher than at the top of a mountain. This means that pressure decreases with increase in altitude. The pressure at the ground level is higher than that at the top of high mountains. This is because air at the ground level has to support the weight of the air above it, and the molecules in the bottom air must push outwards with a force equal to that exerted by the air above it.

    Temperature: When air is heated, it expands. When this happens, the outward pressure of its molecules is spread over a large area. This means the pressure of the air decreases. The pressure of the air therefore rises when its temperature falls.

    Latitude: The earth is not a perfect sphere and therefore force of gravity varies according to latitude. This is at maximum at the poles and a minimum at the equator. Atmospheric pressure is therefore lower at the equator and higher at the poles. Therefore, Air pressure increases with latitude

    Season of the year: Atmospheric pressure changes with seasons of the year being high over the cold continental interiors in winter and conversely low over the heated continents in summer.

    The nature of earth’s surface: During the day, land heats up more than the water and hence air pressure is lower over land than the sea. Air blows from the sea to the land as a sea breeze. On the other hand, during the night, the land cools more quickly than the sea and hence air pressure is lower over the sea than land. Wind blows in from the land to the sea as a land breeze.

    Pressure types 

    Air pressure is generally divided into two types. These are high pressure (HP: above 1013.25 mb), and low pressure (LP: below 1013.25 mb).

    High pressure systems are also called highs or anticyclones. They are characterized by highest air pressure in the center of almost closed isobars where pressure decreases from the center outwardly. The lowest pressure is found at the outer margin of the high-pressure system.

    Low pressure systems are also called low or simply L or cyclones or depressions. These are centres of low pressure, having increasing pressure outwardly. This has closed air circulation from outside towards the central low pressure in such a way that air blows inwardly in anti- clockwise in the northern hemisphere and clockwise in the southern hemisphere.


    Location of different world pressure belts and their characteristics

     There are seven pressure belts on the globe: 

    Equatorial low-pressure belt: It is located on either side of the geographical equator in a zone extending between 5° N and 5° S latitudes. But this zone is not stationary because there is seasonal shift of this belt in relation to the northward (summer solstice) and southward (winter solstice) migration of the sun. The equatorial low-pressure belt represents the zone of convergence of North-East and South-East trade winds.

    Sub-tropical high-pressure belt: It extends between the latitudes of 25°- 35° in both the hemispheres. The divergence movement is prevailing over the surface on that belt. The descending movement of winds results into the contraction of their volume, increases in density, and ultimately causes high pressure. Therefore, this explains why this zone is characterized by anticyclonic conditions which cause atmospheric stability and aridity.

    Sub-polar low-pressure belt: It is located between 60°-65° latitudes in both hemispheres. It may be noted that due to the great contrast of temperature of the continents and Oceans, during Northern summer, the low-pressure belt becomes discontinuous and is found in a few low-pressure cells. While in winter season the pressure increases, and the low-pressure belt becomes less regular.

    Polar high-pressure belt: Temperature remains below freezing point during most part of the year. This results into the high-pressure systems throughout the year. Winds blow from the polar high pressure to sub-polar low-pressure cells. These are called polar winds which are North-Easterly in the Northern hemisphere and South-Easterly in the Southern hemisphere.


    8.3. Factors that influence world climate

    Learning activity 8.3

    Explain how the following influence climate of any region:

    a. Latitude

    b. Altitude

    c. Water bodies

    The following are the main factors influencing world climates:

    Latitude: The climate of a place is influenced by latitude. Temperature and

    precipitation are high near the equator while they are low at the polar zone.

    This is because of the amount of sunshine received at these places. The amount

    of sunshine received by the ground surface decreases away from the equator. 

    Altitude: Temperature decreases with increasing in height from the earth’s

    surface. Precipitation on the other hand increases with altitude. Temperatures

    at the top of mountains are very low. On the other hand, precipitation and

    humidity are very high.

    Presence or absence of water bodies: Places near a lake or sea have low

    temperatures. This is because of the cooling effect of air currents or breeze

    from the water bodies. Areas far away from water bodies have extreme weather


    Vegetation: Vegetation attracts precipitation and moderate temperatures.

    Through transpiration, clouds form near the forests and precipitation occurs.

    Transpiration is the evaporation of water from leaves and stems of plants. It

    rains a lot in the equatorial dense forests. In the deserts where there are few

    plants, it is dry and hot.

    Human activities: Pollution from industries affects the climate. Chemicals and

    gases that are released into the atmosphere cause acid rain. Cutting down of

    forests also affects the climate. Where forests are cut down in large numbers,

    reduced precipitation is recorded.

    Ocean currents: The warm ocean currents from tropical areas to cold zones

    raise the temperature in these areas. For example, the Gulf Stream increases

    the temperature of the coastal areas of North-Western Europe. While Kuroshio

    warm currents raise the temperature of the coasts of Japan.

    Primary wind circulation also called the primary atmospheric circulation is the main factor controlling the spatial distribution of climates in the world. This circulation involves flow patterns of permanent wind systems in latitudinal zones from the equator towards the poles. This primary wind circulation of Easterlies (trade winds) and Westerlies divide the world into three major zones. These include Intertropical zone, mid-latitude zone and sub-polar zones. In these areas winds blow from high pressure belts to low pressure belts.

    The description the three primary wind circulation major zones: 

    Intertropical zone: This is the zone where winds from subtropical highpressure areas blow towards. It is an equatorial low-pressure area. Therefore, marks the zone of convergence that creates Intertropical fronts (ITF) or Intertropical Convergence (ITC). Then, the air near the equator is heated due to solar radiation, rises upward to yield enough rainfall in this zone.

    Mid-Latitude zone: Mid-latitude zonal circulation extends between 30° and 60° latitudes in the northern and the southern hemispheres. This zone is under the influence of subtropical high-pressure belt (300-350 latitudes). This belt separates two wind systems. Trade winds (Easterlies) and Westerlies. It is also apparent that the subtropical high-pressure belt is the source of the origin of trade winds. These blow towards equatorial low-pressure belt. On the other hand, the westerlies blow towards sub-polar low-pressure belt. This is because winds always blow from high pressure to low pressure. This movement of winds makes the zone to be drier.

    Sub-polar zone: This zone is confined mostly between 600-900 latitudes in both hemispheres and is characterized by surface polar Easterly winds. Winds blow from the polar high pressure to sub-polar low-pressure cells. These are called polar winds which are North-Easterly in the northern hemisphere and South-Easterly in the Southern Hemisphere. Therefore, sub-polar low-pressure belt becomes convergence zone; where the front is formed to allow the air to rise upward to yield rainfall around this belt.

    Monsoons winds: These are seasonal winds which reverse their direction at least twice a year. This results into the air blowing from the land to the Ocean in winter and from Ocean (water) to the land in summer. This situation makes the summer to be hot and wet and winter to be cold and dry in the affected areas.

    Application activity 8.3

    Read the following context and give feedback to the questions that follow: 

    Generally, climate of any region or area is influenced by both physical factors (Latitude, altitude, water bodies, vegetation, ocean currents and aspect in relation to location) and human factors (pollution from industries, Chemicals released into the atmosphere, cutting down of forests /deforestation, over cultivation, overgrazing, land reclamation and construction). Illustrate these geographical factors in Rwandan context.

    8.4. Types of climate and their characteristics

    The classification of climatic zones is based on temperature and rainfall. There are three world climatic zones. These are: Tropical, Temperate and Cold zones.

    8.4.1. Tropical zones 

     i. Equatorial climate

    Equatorial climate is also called tropical wet climate or tropical rainforest climate. It is found along the equator extending from 5° to 10° South and North latitudes. Along the Eastern margin of continents, it spreads to 15° - 25° of latitudes. This type of climate is found specifically in the following regions:

    – The Amazon River Basin in South America

    – The Congo Basin and Guinea coast in Africa

    – Malaysia, Indonesia and the Philippines Islands in South-Eastern Asia

    – Eastern and Central America (parts of Panama, Costa Rica, Nicaragua, Honduras,

    British Honduras and Guatemala), some islands in the Western Colombia

    – Coastal lowlands of Eastern Brazil

    – Eastern Madagascar


    ii. Climatic characteristics of equatorial region

    – This climatic region is located within 5° North to 10° South of the Equator.

    – The average monthly temperatures are over 18°c. However, many places

    record average monthly temperatures of 24° to 27°c.

    – The Equatorial regions lie in a belt where the winds are light. It is a low-pressure


    – The annual range of temperature is very small (the difference between the

    highest and the lowest temperatures) varies from 5°to 8°c.

    – The equatorial regions get heavy precipitation throughout the year. Many

    areas receive 2000 mm of the rain per year. The annual average rainfall in the

    equatorial climate is nearly 2500 mm.

    – There is a large amount of cloudiness.

    – Conventional rainfall is received, and it is usually accompanied by


    – High temperatures and heavy rainfall encourage growth of natural vegetation.

    There are evergreen dense forests.

    Below there is an example of Singapore weather station:

    iii. Tropical marine climate 

    It is found on the East coasts of regions lying between 10° N and 25° N and 10° S and 25° S of the equator. These areas come under the influence of on-shore Trade Winds. Examples are East coast of Brazil, the Eastern coastlands of Madagascar, Mexico, Nicaragua, Guatemala, Venezuela, the lowlands of Central America, West Indies, the coast of Queensland (Australia) and the southern islands of the Philippines.

    Climatic characteristics of Tropical maritime/maritime climate

    The annual temperature range is about 8°c.

    Temperatures reach 29°c during the hot season while it is about 21°c during the cold


    Annual rainfall varies from 1000 mm to 2000 mm.

    Rainfall received is both convection and orographic brought by onshore Trade


    Humidity is high throughout the year.

    Sea breezes lessen the effects of the heat.

    Tropical maritime climate is good for tree growth. The lowlands have tall and

    evergreen trees. 

    iv. Tropical continental climate

     This climate occurs between 5° N and 15° N and 5° S and 15° S. It is in West, East and Central Africa, South America, parts of the Deccan plateau (India) and the areas to the north and east of the Australian Desert.

    Climatic characteristics of Tropical continental climate

    – Heavy convectional rainfall is mainly in the summer.

    – Annual rainfall is about 765 mm.

    – In some regions, the offshore winds are strong and hot. An example is the

    Harmattan of West Africa.

    – Humidity is high during the hot, wet season.

    – Summers are hot (32° C) and winters are cool (21° C).

    – The annual temperature range is about 11°C. The highest temperatures occur

    just before the rainy season begins. This is in April in the northern hemisphere

    and October in the southern hemisphere.

    v. Savannah climate (Tropical wet-and-dry climate) 

    This is located between 5° and 20° latitudes on either side of the Equator. It is found in Latin America. These are the Llanos of the Orinoco Valley including Colombia and Venezuela, the Guyana Highlands and the Campos of Brazil.

    In Africa, it is found in the South of the Congo basin, the Southern part of Democratic of Republic of Congo, Angola, Zambia, Mozambique, Tanzania, Uganda, Botswana, South-Western Madagascar, Central Nigeria, Southern Kenya, Togo, Ghana and Ivory Coast.

    Climatic characteristics of Savannah Tropical climate

    – High temperature of around 20°c.

    – The annual range of temperature is greater than in the equatorial regions. It is

    over 3° but not more than 8°c.

    – Total annual average precipitation varies from 1000 mm to 1500 mm. Much of

    the rain falls during the summer.

    – The vegetation is grasslands with scattered trees and bushes. 

    vi. Tropical desert climate 

    Most of these deserts lie between 15° to 35° N and S. The hot desert climate is found in the following deserts: Atacama (the coastal deserts of Peru and Chile in South America), the Namib and Kalahari deserts of coastal Angola and South West Africa, interior part of Botswana and South Africa, the great Australian desert, the Sahara and the Arabian deserts, the Iranian desert, the Thar desert of Pakistan and India, California (USA) and the deserts of Northern Mexico.

    Climatic characteristics of Tropical desert climate

    – Hot deserts have the highest temperatures recorded.

    – They have cloudless skies and little or no water vapour.

    – The daily ranges of temperature vary between 22° to 28° C. In rare cases, the

    diurnal range may be as high as 41.7° C.

    – The annual average precipitation is less than 250 mm.

    – Relative humidity is high.

    – There is little plant cover.

    – Below there is an example of Khartoum (Sudan) weather station:

    vii. Tropical Monsoon Climate

    This is found in areas with seasonal land and sea winds. On-shore summer winds blowing from over tropical warm Oceans bring about heavy precipitation. Offshore winds from over the land make the weather dry during winter. Monsoon climate is found in the coastal areas of Eastern and Southern Asia. These places include India, Burma, Bangladesh, Indo-china, Southern China, and Philippines, Taiwan, Japan and Korea.

    In tropical Africa, it is found along the South-West coast of West Africa. These areas include the coasts of Guinea, Sierra Leone, Liberia, and Ivory Coast. Other areas are the Northeast coast of Latin America from the mouth of Orinoco River in Eastern Venezuela through Guyana, Surinam and French Guyana to the North-Eastern part of Brazil. The North coasts of Puerto Rico and the Dominican Republic in the Caribbean Islands which have a mild monsoon climate.

    Characteristics of Tropical monsoon climate

    – High temperatures (32°c) in the hot season (summer) and low temperatures

    (15°c) in the cold season (winter).

    – High annual range of temperatures of about 17°c.

    – Summers receive high rainfall of up to 2500 mm.

    – The winters are dry.

    – There is a reversal of winds. In one season they blow from sea to land (onshore).

    These bring heavy rainfall. In the other season, they blow away from the sea

    (offshore), such come along with little rain. 

    8.4.2. Temperate zone

    Mediterranean Climate

    Mediterranean climate is found between 30°and 40°N and S of the equator. This is

    on the western sides of the continents. This climate is found in five regions of the


    – North of the Mediterranean Sea from Portugal to Turkey and beyond in the

    Iranian Highlands, Morocco, Northern Algeria, and Tunisia, and North of

    Bengasi in Libya.

    – The central and Southern California coast in the United States of America.

    – Central Chile.

    – The Cape Town area of South Africa, and

    – Southern Western coasts of Australia.


    Characteristics of Mediterranean climate

    – The average temperature of the coldest month is between 4.4°C and 10°c. That

    of the hottest month is between 21° C and 27°c.

    – The mean annual temperature ranges are between 11°and 17°c.

    – The average annual precipitation is between 350 and 750 mm.

    – There is rain in winter while the summers are dry.

    – Below there is an example of Algiers (Algeria) weather station:

    – Algiers (Algeria) weather station

    Temperate Maritime Climate

    This type of climate is found between 40° and 65° N and S of the equator. These are

    regions to the West of continents.

    It is mainly found in Western Europe including Great Britain, North Western France,

    Germany, Denmark, Holland, Belgium and Norway.

    In North America, it is found along the West coast up to 60° N of the equator. It

    borders the Sub-Arctic climate of Canada and Alaska. In Europe, it extends along the

    west coast of Norway to 68° N.

    In the Southern Hemisphere, it is found to the Southwest coast of Chile, Southeast

    coast of Australia, the islands of Tasmania and New Zealand.

    Below there is an example of Brest (France) weather station:

    Characteristics of Temperate Maritime climate

    – Temperatures are influenced by the warm Ocean currents.

    – This climate has cool summers and mild winters.

    – The annual range of temperature is about 7° C.

    – Average temperatures in summer are between 15° C and 18° C. Winter

    temperatures range between 11° and 17° C.

    – In Europe, the lowlands receive an average precipitation of 500 mm to 850

    mm. On the windward side, it is between 2500 mm and 3750 mm.

    Continental Temperate climate 

    This type of climate occupies a large part of the United States of America. In Europe, it is found in Romania and Bulgaria. It occupies the lower Danube Valley. In Eastern Asia, it is found in North China bordering the yellow Sea, North and South Korea, and Northern Honshu in Japan.

    – Temperate continental climates are found on continents in the Northern

    Hemisphere between 40o-70o

    – Climate in these areas is controlled by the fact that they are not located near

    Oceans where temperatures are moderate.

    – Temperate continental climates are also called micro thermal climates, because

    they are located away from the Oceans. These climatic zones experience the

    extremes of temperatures.

    – Summers are warm and can be very humid while winters are cold with

    snowstorms and blustery winds.

    – The annual average temperatures are around 100 C.

    8.4.3. Cold zone

    Polar and Tundra climate 

    The Tundra climate is found in the northern hemisphere beyond 60° N of the equator. These are areas to the North of Asia and Canada. It also occurs on the coast lands of Greenland. Polar climate is found in Greenland, interior of Iceland and in the Antarctica.

    Characteristics of Tundra climate

    – The average annual rainfall is 250 mm.

    – Precipitation is in the form of snow in winter and rainfall in summer.

    – Humidity is low because of low temperatures.

    – Winter temperatures are low. They range from - 29°c to 4°c. Summer

    temperatures average about 10°c.

    – Vegetation consists of mosses, lichens and dwarf trees and shrubs. It is called

    tundra vegetation.

    Characteristics of Polar climate

    – Temperatures are always low. They are below 0°c, which leads to snow.

    – Precipitation mainly occurs in summer. It averages between 100 mm and 250


    – Winters are associated with one continuous night. Summers are one continuous


    – Blizzards are common. These are snowstorms with high winds. Visibility is low.

    – There is hardly any vegetation. This is because of snow and ice cover. 

    Mountain climate 

    This type of climate is found in the mountain ranges of the world. These include Mt. Kenya, the Ethiopian Highlands and the Alpine ranges of Europe. Others are the North Western part of Rwanda, especially over volcanic region, the Andes of South America and the Rockies of North America.

    Characteristics of Mountain climate

    – High rainfall on the windward slopes. It is less on the leeward slopes.

    – Orographic rainfall is received.

    – Pressure and temperature decrease with altitude.

    – But if the mountains are high enough, there is a height at which maximum

    precipitation occurs and above which it decreases.

    – It is also characterized by strong local winds (mountain and valley breezes).

    Application activity 8.4

    a. Draw a world sketch map and on it mark and label the world climatic


    b. Describe the characteristics of each climatic zone shown on that world

    sketched map.

    8.5. Influence of climate on human activities

    Learning activity 8.5

    “People in North Western part of Rwanda grow Irish potatoes, while people in

    Southern Rwanda grow cassava and those of Eastern Rwanda grow bananas.

    Again, in some regions of Rwanda tea is grown as a cash crop while other does


    Hence explain how those crops grown are influenced by climatic conditions in

    each part.

    Climate influences the distribution of population. This is because of temperature

    conditions, amount of precipitation and length of crop growing season.

    The relationship between climate and human activities is summarized below:

    Human activities in equatorial regions: Heavy rainfall and high temperatures

    support growth of forests. The main human activities are lumbering and

    agriculture. Lumbering is the cutting down of trees and making them into

    timber. Crops such as coffee do well in this type of climate.

    Human activities in savanna climate: This type of climate is good for

    agriculture and dairy farming. There is enough grass for the animals. Growing

    of vegetables is done in this type of climate.

    Human activities in desert climate: The high temperatures and low rainfall

    are not supportive for agriculture. Animal keeping is also not well developed

    only camels, goats and sheep are kept in such type of climate. Furthermore, it

    is only around the oases that some farming is done. An oasis is a fertile spot

    in the desert where water is found. Growing of crops is done by irrigation.

    However, mining activities are done in some deserts for instance gold is

    extracted in West Australia; diamonds in the Kalahari and petroleum in Algeria,

    Libya, Saudi Arabia, Kuwait, Iran and Iraq.

    Human activities in temperate climate: This type of climate is suitable for

    agriculture and livestock keeping. Most developed countries are in this type of

    climate. These zones have high population.

    Human activities in polar climate: The main activities in this zone are the

    fishing and hunting. Few people are found here.

    Application activity 8.5

    “Most of entrepreneurs look for climatologists and meteorologists to advise them

    when locating their firms”, Discuss the statement.


    1. With aid of diagram describe the structure of atmosphere.

    2. To what extent atmosphere plays a considerable role in regulating/

    control negative effects of solar radiation?

    3. Visit a weather station near your school to identify instruments used to

    measure and record weather conditions.

    4. (a) Briefly describe the characteristics of Rwandan climate,

    (b) Explain the factors influencing the climate of Rwanda.

    5. “Human activities depend upon climate and weather conditions of an

    area” with relevant examples in Africa, support this statement.


    Key unit competence

    By the end of this unit, I should be able to appreciate the distribution of different

    types of vegetation in the World.

    Vegetation refers to a community of plants which grow in an area and which gives it distinct character. Vegetation in biological terms is known as “flora”, that is, all vegetation types growing on land and in water.

    The world vegetation communities are into categories: forests and grasslands. They are distributed according to the vegetation zones and they include tropical, temperate, tundra, desert, mountain and aquatic/marsh or mangrove vegetation.

    9.1. Tropical forests and their characteristics

    9.1.1. Equatorial forests

     The equatorial rainforests grow within the tropics between 10 °N and 10 °S of the equator. Equatorial rainforests are sometimes called “rainforests”. Equatorial rain forests cover only a small part of the earth’s surface which is about 6%. They are situated in the Amazon basin in South and Central America, Congo basin in Central Africa, Malaysia, Burma, and West African coastal belt (Nigeria, Ghana, Ivory Coast, Liberia and Central Africa Republic).

    The conditions necessary for the growth of equatorial rainforests:

    – Equatorial rainforests receive rainfall throughout the year about 2000 mm per


    – The equatorial rainforests grow in areas with good fertile soils and well-drained


    – The equatorial rainforests require much sunshine to support their dense


    – The equatorial rainforests require about 23-240c temperature.

    Equatorial rainforests are characterized by the following:

    – They are evergreen, since the forests receive heavy rainfall throughout the

    year with no clear distinct seasons.

    – They grow buttress roots in order to support their enormous sizes.

    – The major tree species in equatorial are characterized by a long gestation


    – Equatorial rainforests receive rainfall throughout the year with no distinctive


    – The vegetation in equatorial regions is comprised of four vertical layers starting

    from the canopy of the tree to the ferns on the ground.

    – Equatorial rainforests are dominated with valuable tree species such as

    mahogany, ebony, green heart and redwood.

    – The forest floor receives only 2% of the sunlight. This has led to the existence

    of little undergrowth.

    – The trees are very tall (35 m - 40 m) with a very dense and thick canopy. The

    result is that the canopy blocks most of the sunlight falling on them depriving

    the plants under them of sunlight.

    – Most of the trees in Equatorial rainforests become tall in the search of light.

    – There are broad-leaved evergreen forests of dense and prolific growth of flora

    as well as fauna.

    – The major tree species do not grow in pure stands. Trees of pure stands are

    scattered all over the forest.

    – Beneath the tree canopy exists a well-developed layering of understory

    vegetation, which is so dense and this limits light to reach the floor of the

    forested area.

    The Equatorial rainforests are associated with various economic activities. These include lumbering as a major activity, provision of local materials which are used in craft industry, provision of fuel, research and study, herbal medicine, agriculture support, etc.

    9.1.2. Tropical Monsoon forests 

    The Tropical Monsoon Forests are found beyond the equatorial region, between 10 0 and 250 North and South of the equator. This type of vegetation is found in areas such as; Burma, Thailand, the Indo-China region, parts of India, East Java, parts of Northern Australia, small parts along the South Western coastal areas of West Africa.

    The conditions necessary for the growth of tropical monsoon forests:

    – The tropical monsoon forests receive heavy rainfall, which is around 2000 mm

    per year. This is received mostly in summer.

    – In cooler seasons such as winter, very little rainfall is received. This is because

    these regions lie under the offshore trade winds.

    – The tropical monsoon forest requires temperatures of about 270

    c. This

    temperature is moderately enough to support the growth of various plants.

    Tropical monsoon forests are characterized by the following:

    – The trees possess trunks that they use to store water during the dry seasons.

    – The tropical monsoon forests can grow up to 30 metres in height.

    – Trees possess long tap roots that penetrate into the ground to access

    groundwater. Inorder to sustain plants’ growth, especially during the dry

    winter season when there is unreliable rainfall.

    – Tropical monsoon forests shade off their leaves during the dry seasons in order

    to minimize water loss.

    – Trees have broad leaves due to sufficient rainfall (2000 mm) received during

    the wet season.

    – Tropical Monsoon forests experience temperatures that reach 280c especially

    in Summer.

    – Tropical monsoon forests contain valuable hard wood tree species such as the

    teak and sandalwood.

    Tropical monsoon forests are associated with various economic activities such as lumbering on the wider area, agriculture, craft industry, settlement pattern and provide herbal medicine.

    9.1.3. Mountain tropical forests 

    The tropical mountain forests are mainly located at a high altitude in the mountains. These usually vary largely along the slopes of Himalayas Mountain ranges and East African Mountain peaks of Rwenzori, Mt. Kenya and other good examples include; Cameroon Mountains, and Ethiopia Highlands. The trees grow in plenty between the altitude 1,500 and 3,500 meters. This explains why such areas are dominated by mountain forests. They have evergreen trees like Teak, Bamboo, and other tree species such as Pine, Fir, Oak, Maple, Deodar, Laurel Spruce, Cedar, cedar pod carp and camphor. All these grow abundantly and dominate the natural vegetation in the area.

    The conditions necessary for the growth of mountain forests:

    – Mountain forests require much and reliable rainfall;

    – Mountain forests require adequate temperature for the growth of tress;

    – Mountain forests need deep fertile soil for the growth of forests;

    Mountain forests have the following characteristics:

    – Mountain forests grow broad leaves and they are evergreen;

    – Mountain forests grow thick under growth;

    – The forests contain giant evergreen trees that grow on the windward slopes of

    the mountain;

    – The main tree species are characterized by long gestation period;

    The mountain forests are associated with various economic activities such as lumbering on the wider area, provision of local materials, hunting of animals and provide herbal medicine.

    Application activity 9.1

    1. Briefly explain the geographical conditions that determine the location

    of equatorial forests.

    2. It has been noted that the environment is composed of varieties of

    natural resources that support the socio-economic development of any

    country. Forests fall under such natural resources. Explain the influence

    of different categories of tropical forests on the development of Africa.

    9.2 Temperate forests and their characteristic

    Learning activity 9.2

    Read the passage provided below and give feedback to the questions that


    Temperate forests occur in Eastern North America, North Eastern Asia, Western and

    central Europe. Temperate forests are those which grow outside the tropics. They are

    divided into three categories namely deciduous forests, Mediterranean forests and

    coniferous forests. Temperate forests are located between 300 and 700 North and 30

    0 and 70o South of the equator.

    1. Identify the types of forest mentioned in the passage above.

    2. Compare and contrast the types of forest indicated in the passage above.

    The temperate forests are categorized into 3 categories that include: deciduous forests, Mediterranean forests and coniferous forests.

    The conditions necessary for the growth of temperate forests:

    – Temperate forests need little supply of sunshine for the successful growth.

    – Temperate forests require enough fertile soil and availability of water for


    – Temperate forests require amount about to 750 mm of rainfall in dry periods.

    The characteristics of temperate forests

    – With high levels of precipitation, humidity, and a variety of deciduous trees.

    – Temperate trees are trees that lose their leaves in Winter.

    – Trees shed their leaves in fall and bud new leaves in spring when warmer

    temperatures and longer hours of daylight return.

    – Temperate forests have tall evergreen trees dominating the regions.

    – They have Redwood trees which are the tallest in the world, about 360 feet


    – The most prominent tree type in temperate forests is the Douglas fir, that is

    growing 280 feet tall.

    – Temperate forest mature species of cedar and spruce trees typically exceed

    200 feet in height.

    – Temperate forests have epiphytes species such as mosses and ferns that live on the branches and trunks of the trees, especially the broad-leafed maples.

    – Temperate forests have many species of large ferns which occupy the shady

    forest floors.

    – Temperate forests can get from 60 to 200 inches of precipitation annually.

    The temperate forests are associated with various economic activities. These include tourism which is supported by the presence of many different species of birds such as broad-winged hawks, cardinals, snowy owls, and pileated wood peckers that attract very many people from different parts of the world. There is also hunting due to different types of animals such as white-tailed deer, raccoons, opossums, porcupines and red foxes.

    9.2.1. Mediterranean forests 

    The Mediterranean forests are located mainly in South West America, Spain, Italy, France, Australia, Algeria, Morocco, Tunisia, and Central Chile. Mediterranean forests grow also well on the tip of South Africa near Cape Town.


    The characteristics of Mediterranean forests are:

    – Tree species in the Mediterranean are deciduous.

    – The mediterranean vegetation is characterized by open and evergreen


    – The mediterranean vegetation has thicket with thin and waxy leaves.

    – There is inadequate undergrowth.

    – They are composed of broadleaf trees, such as the oak and mixed sclerophyll


    – Mediterranean vegetation has dense foliage composed of broad-leaved

    evergreen shrubs, bushes, and small trees usually of less than 2.5 m (about 8


    – Tall trees grow in regions lying between 30° and 40° North and South latitudes.

    – Trees even fully grown are often stunted.

    – There are woody, evergreen shrubs or small trees that have developed various

    strategies of growth and usage of available water during the dry period.

    – Mediterranean deciduous tree species have a long gestation period.

    – Mediterranean plants have long taproots to reach underground water, called

    “Xerophytic Plants”.

    – Mediterranean trees adapt themselves to dry summers with the help of their

    thick barks and wax coated leaves. These reduce the rate of transpiration.

    The Mediterranean forests are associated with various economic activities such as mining of oil, tourism, and very limited population settlement. There is also rearing of sheep and growing of crops such as wheat, oats and cultivation of chestnuts.

    9.2.2. Coniferous forests / Taiga 

    Forest The coniferous forests are located across North America, Europe, and Asia. These forests are found within the extent of 50˚ to 60˚N. The coniferous forest is the largest terrestrial vegetation covering about 17% of Earth’s land area. Countries such as Canada, Russia, and Scandinavia are almost entirely covered by these coniferous forests. The vegetation is identified by its climate, which occurs almost exclusively in the high latitudes of the Northern hemisphere.  


    Coniferous forests are favoured by the following environmental conditions:

    – The coniferous forests require inadequate supply of sunlight.

    – The coniferous forests need medium fertile soil with availability of water.

    – The coniferous forests grow well with an average temperature of below 430 F

    during the winter season.

     The characteristics of coniferous forests are:

    – The coniferous forests consist of tall and softwood evergreen trees.

    – There are limited species of trees. The existing trees are evergreen and grow


    – The coniferous tree species grow tall, straight and contain few branches.

    – The type of trees in these coniferous forests grow in pure stands;

    – Trees are conical shaped with needle-like leaves. These include firs, pine and

    cedar which are important variety of trees in these forests;

    – The tree species often grow to a height of over 30m (100ft) tall;

    – Coniferous tree species grow shallow roots and can collect enough water from

    top soil;

    – They have shallow roots used to absorb the nutrients and water from the top


    – The coniferous vegetation has adapted to harsh conditions associated with

    winter season.

    – Leaves are small, narrow and often needle-like with the capacity of reducing


    The coniferous forests are associated with various economic activities which include lumbering, tourism, apiculture (bee keeping), hunting, herbal medicines, fruit gathering and Research and study.

    9. 2.3. Deciduous forests 

    Deciduous forests grow well within the latitude of 40° N and 60°N and 30° S and 50oS of the equator.They can be found in the Eastern half of North America, and the middle of Europe. There are many deciduous forests in Asia. Some of the major areas having deciduous forests include Southwest Russia, Japan, and Eastern China. South America has two big areas of deciduous forests in Southern Chile and in the Middle Eastern coast of Paraguay. These are also located in New Zealand and South Eastern Australia also.


    The conditions necessary for the growth of deciduous forests:

    – Deciduous forests require moderately distributed rainfall.

    – Deciduous forests need low sunshine supply.

    Characteristics of deciduous forests:

    – Trees shed their leaves in the dry season to conserve water.

    – Deciduous forests are characterized by the existence of epiphytes which

    include mosses.

    – Trees like sandalwood, teak, ebony, bamboo, etc. are the common trees found


    – Deciduous forests have a short growing gestation.

    – Deciduous forests grow in pure stands.

    – Deciduous forests require low sunshine supply.

    – They contain hard wood tree species such as maple, oak, beech and hazel.

    The deciduous forests are associated with various economic activities which include lumbering, tourism activity, hunting, herbal medicines and fruit gathering.

    Application activity 9.2

    Describe briefly different types of temperate forests.

    9.3. Grasslands in the tropical zone and their characteristics

    Tropical grasslands are commonly known as savannah vegetation. Tropical grasslands grow well within altitude of 50 N and 15o N and 50 S and 15o S of the equator. Tropical grasslands grow well in Africa, South America specifically in Campos in Brazil”. They can also be found in Guyana, Australia, Eastern Madagascar and India. Tropical grassland is divided into two groups: Savannah and steppe.

    9.3.1. Savannah humid vegetation

     Savannah humid forests grow well in regions experiencing the average total rainfall of 1000 mm per year. Mambo woodlands of Central Tanzania is one of the examples of savannah woodlands in East Africa. Other examples of savannah humid forests are found in Madagascar, Indian subcontinent, South East Asia and New Guinea.

     The conditions necessary for the growth of the Savannah humid vegetation:

    – The savannah humid can grow well in regions experiencing temperatures

    ranging between 250 c to 320 c.

    – The savannah humid vegetation grow well in areas which experience rainfall

    about 750 mm to 1000 mm per annum.

    – The savannah humid needs maximum sunshine and light necessary for the

    plants to make chlorophyll.

    Its characteristics include the following:

    – The tree species are deciduous, and shade leaves during the dry periods.

    – The vegetation is mostly composed of shrub and short grass.

    – The species of trees such as baobab and acacia are mostly common in the area.

    – The grass can grow very tall (about 3 to 4 meters high). The common type of

    grass is known as “elephant Grass”.

    – Near riverbanks and water holes, deciduous trees can grow, e.g. Acacia,

    baobab, etc.

    – The Savannah vegetation is characterized by undergrowth dominated by

    shrubs and short grasses.

    – Most tree species in the savannah woodlands form small umbrella –like tops

    such as acacia.

    – The tree species are deciduous and shade –off leaves during the dry season.

    – The tree species such as the acacia and baobab are more dominant in savannah

    humid forests.

    – Shrubs growing in this area have yellow or white flowers and can grow over

    six feet tall.

    – The non-thorny trees such as baobab, candelabra, and the Jackal berry are

    found in savannah grasslands.

    The Savannah humid vegetation is associated with various economic activities such as hunting, herbal medicine collection, fruit gathering, rearing of animals and subsistence farming, settlement, mining and gazetting of national park and game reserves.

    9.3.2. Steppe/ Savannah dry vegetation 

    Savannah dry covers almost half the surface of central Africa and large areas of Australia, South America, and India. The climate is the most important factor in creating a savannah dry vegetation. Savannahs are always found in warm or hot climates where the annual rainfall is from about 508 to 1270 mm per year. These regions receive rainfall for about 6 to 8 months. This is followed by prolonged dry period that usually affects fire out-breaks.


    The conditions necessary for growth of Savannah dry/steppe vegetation:

    – The soil which is dry and porous, with rapid infiltration of water.

    – Dry climatic conditions that support the growth of different grasses due to the

    disparities in rainfall and soil conditions.

    – Availability of the average annual rainfall of 762-1016 mm.

    – The presence soils that are too thin. Trees require the existence of termite

    mounds where they grow.

    The main characteristics of Savannah dry vegetation are:

    – The trees and grass grow through direct competition for water, light and


    – The open canopy allows sufficient light to reach the ground to support an

    unbroken herbaceous layer consisting primarily of grasses.

    – Annual herbaceous plants die completely at the end of the growing season or

    when they have flowered and fruited. These grow again from seed when the

    wet season sets in.

    – The vegetation consists of tall grasses and scattered trees.

    – The grasses are usually two meters high or more.

    – The trees are mainly found near watercourses. The main types of tree species

    are acacia;

    – Some trees lose their leaves in the dry season.

    – Some plants have thick barks and thorny leaves to reduce water loss.

    The Savannah dry /steppe vegetation is associated with various economic activities

    including hunting, fruit gathering, rearing of animals, settlement, agriculture and

    gazetting of the national park and game reserves.

    Application activity 9.3

    1. Describe savannah grasslands with reference to South Africa.

    2. With reference to the Rwandan context, explain the importance of savannah

    grasslands in the economic development.

    9.4 Grasslands in temperate zones

    Temperate grasslands are known by different names in various regions. “Prairies” in North America; “Pampas” in South America (Argentina), “Downs” in Australia “Velds” in South Africa and “Steppes” in Europe. These are found in the mid- latitudinal zones and in the interior part of the continents. The Temperate grasslands are found in Central Asia from Black Sea to Central Russia, North Central USA and Southern Canada, South-East Australia, Southern Africa and Argentina.


     The conditions necessary for the growth of temperate grasslands:

    – Temperate grasslands require minimum light for the plants to make their own


    – The temperate grassland requires moderate fertile, fine drained and humid


    – Temperate grasslands need average sunshine in a dry and cool winter time.

    – Temperate grasslands require moderate rainfall of about 500 mm - 750 mm for

    the best growth of grasslands.

     The characteristics of temperate grasslands are:

    – Grasslands found here are short and juicy, which is suitable for cattle feed.

    – The region is known for the growth of maize and wheat in large amounts. This

    explains why the area is known as “the bread basket” of the world.

    – The perennial temperate grasses mostly belong to the family of “Gramineae”.

    – The steppes form the largest segment of the temperate grassland biome.

    Steppes are divided into: Forest steppes, Meadow steppes and grass steppes.

    – America Prairies are divided into three sub regions: tall grass prairie, mixed

    grass prairie and short grass prairie.

    Brief description of Temperate grassland areas:

    – Based on the rainfall, the Pampas in South America (Argentina) are divided

    into two types: humid pampas in the Eastern part and Sub-humid pampas in

    the Western part of Argentina.

    – Velds in South Africa are sub-divided into three types: Themed veld (altitude

    varies between 1500-2000m), Sour veld and Alpine veld (2000-2500) of the

    Drakensberg mountain.

    – Dows grown in Australia are divided into three types:- Temperate tall grasslands

    found in the Eastern coast of New South Wales to Victoria and Tasmania. -

    Temperate short grasslands found in the North of the Temperate tall grassland

    region. - Xerophytic grasslands developed in the interior lands of New South

    Wales and Queensland where semi-arid climate prevails.

    – Canterbury grasslands are extended especially over the Eastern and the

    Central part of New Zealand.

    The temperate grasslands are associated with various economic activities which

    include: hunting, fruit gathering, rearing of animals, settlement, agriculture and

    gazetting of the national park and game reserves.

    Application activity 9.4

     Assess the contribution of Temperate grasslands to the economy of the countries where they are found.

    9.5. Desert Vegetation

    Desert vegetation grows in the Western margins of the continents between 15° – 30° North and South of Equator. The iggest deserts are: Sahara and Kalahari in Africa, Thar in India, Arabia desert covering the countries of Saudi Arabia, Iraq, Iran, Syria and Israel, Atacama Desert (Peru and Chile), Southern California in USA, Sonora in Mexico and Victoria in Australia. Desert Vegetation is divided into two types, namely hot desert and cold desert vegetation or Tundra.

    9.5.1. Hot desert vegetation 

    Hot deserts are located between the latitudinal belts of 15°-30° North and South of the equator. These deserts can be found in North America, South Asia, South and Central America, Africa and Australia. Hot desert vegetation experiences hot climatic conditions throughout the year. The rainfall is unreliable. This is caused by the dry winds that blow over the area, leading to arid conditions. Such conditions leave behind very poor vegetation in the desert region.

    The conditions necessary for growth of hot desert vegetation:

    – The presence of poor quality and infertile soils.

    – Availability of about 250mm or less as total rainfall per year.

    – The prevailing of high temperatures ranging between 290c and 310c to

    support the growth of plants associated with arid areas such s deserts.

    Hot desert vegetation is characterized by the following:

    – Plants in Hot deserts have small leaves, with sunken or restricted openings,

    pale and reflective leaves.

    – There are a few plants with succulent stems, long roots and leaves.

    – The desert trees shed off their leaves occasionally primarily to minimize on the

    water loss from the excessive temperature.

    – Desert vegetation types especially the tree species grow long taproots to have

    access to water that is found deep in the underground water table.

    – The main vegetation growing here is mainly thorny acacia, bushes, euphorbia

    and turfed coarse grasses.

    – Some desert vegetation types grow no leaves in order to avoid excessive water loss through evapotranspiration.

    9.5.2. Cold desert vegetation 

    The cold vegetation is located in high flat areas called plateaus. It is also common in mountainous areas in temperate regions of the world. Temperate regions lie between the Polar Regions and the tropics. Like other types of deserts, cold deserts get very little rain or snow and are mainly in the Northern part of Canada, North Russia, North Sweden and Finland islands in Arctic Ocean.


    The conditions necessary for growth of cold desert vegetation:

    – Cold desert vegetation needs low sunshine for its successful growth.

    – It requires a combination of freezing temperatures, poor soil quality, lack of

    moisture and sunlight.

    – This vegetation requires soils associated with relatively high soil salinity.

    – The cold desert vegetation needs very little rain fall (of about 250 mm) during

    the summer season and snowfall during the winter.

    – It requires areas characterized by frost conditions.

    The major characteristics of cold desert vegetation include the following:

    – Plants are widely scattered.

    – The main plants are deciduous, mostly having spiny leaves.

    – The cold desert vegetation grows in areas with large amount of snowfall in

    winter (and sometimes in summer).

    – The cold desert vegetation experiences short and wet moderately warm


    – It experiences the mean average winter temperature that ranges from - 2 to

    4º c.

    – This vegetation receives a mean annual precipitation that ranges from 90 mm to 260 mm. – The cold desert vegetation does well in areas with good drainage that facilitates the leaching of most of the salts.

    The cold desert vegetation is associated with various economic activities that include tourism, mining and agriculture on small scale and establishment of the national park and game reserves.

    Application activity 9.5

     Referring to Sahara Desert, describe the impact of the desert vegetation and climatic conditions on the population settlement in any country around it.

    The tundra vegetation is found in coldest regions of the world. The term tundra is derived from a Finnish word “tunturi” which means “treeless plain”. It is among vegetation types that strive in the harshest conditions. This vegetation is found in the arctic region on top of mountains where climate is cold, windy and with limited rainfall. The tundra vegetation is found in regions that cover the areas of Alaska, parts of Russia, Northern Scandinavian countries and in some parts of Canada.

    The following are the conditions necessary for the growth of tundra


    – The Tundra vegetation requires winters that are cold, long and dark.

    – It does well in regions that experience about 6 to 10 months with monthly

    temperatures below 32° F or 0° c.

    – This vegetation needs limited precipitation and the existence of strong and

    dry winds. 

    – It requires snowfall conditions that support the survival of plants and animal

    life. It sometimes acts as a protection layer on the surface of the ground.  

     The characteristics of the tundra vegetation are as follows:

    – There is a limited variety of trees.

    – The vegetation arrangement is simple. 

    – The season of growth and reproduction is short. 

    – The drainage system is nearly limited. 

    – The nutrients and energy here is in form of dead and organic material.

    The tundra vegetation is associated with various economic activities that include tourism which is associated with the following tourist attractions: Birds like ravens, falcons, snowy owls and snow geese and animals such as foxes, wolves and some smaller mammals like the lemmings and snowshoe rabbits. These areas also support hunting, oil exploitation and research and study.

    Application activity 9.6 

    For either Russia or Canada, describe the necessary geographical conditions for the growth of the tundra vegetation in that country.

    9.7. Mountain vegetation

    Learning activity 9.7

    Using your previous knowledge and geographical sources, answer the

    following questions:

    3. Describe the mountain vegetation in the tropical zone.

    4. Describe the characteristics of mountainous vegetation with reference

    to East Africa.

    Mountain vegetation is categorized into two types; namely, tropical mountain vegetation and temperate mountain vegetation.

    9.7.1. Tropical Mountain vegetation

     the mountain vegetation has a variety of vegetation ranging from the tropical to the temperate types. On a typical Mountain slope, various vegetation types show clear demarcation zones. The savannah vegetation grows from the foothills, followed by the layer of tropical rainforests, bamboo forests, mountain heath and moorland.The rest is bare rock. This type of vegetation is traced in areas such as: Mt. Kenya, Mt. Aberdares, Mt. Kilimanjaro, Mt. Meru, Mt. Elgon, Mt. Cameroon, Mt. Ruwenzori, Mt. Virunga, Mt. Simien and Mt. Bale.


    The conditions necessary for the growth of the mountain vegetation

    – This type of vegetation requires the steep gradient.

    – It needs well distributed rainfall characterised by humid conditions.

    – It grows well in areas which generally are associated with strong seasonal


    – It requires temperature ranges of about 20° C at 900 m and 4°c in the summit


    – It requires the annual precipitation of around 900 mm on the foothills, around

    2000 mm at 1500 m and well above 3000 mm between 2000 and 2300 m on a

    windward side.

    – It requires fertile soils that are well-developed with moderately acidic soil pH

    values, such as Andosol.

    Characteristics of the mountain vegetation

    – The vegetation on the mountain slope grows in clearly demarcated zones

    from the foothills to the summit.

    – The mountain vegetation is dominated by tussock grasses and stands of giant


    – The mountain heath and moorlands grow between the bamboo forests and

    the snow-line or bare rocks.

    – The tree species, mainly of the lower canopy are the wild olive.

    – Soils in the mountains are mostly young and fertile which favours the growth

    of trees.

    – Above snow-line, plant life is always impossible. This is attributed to low

    temperature and the presence of eroded bare rocks that makes it hard for

    plant growth.

    – In mountainous areas, the decrease in temperature with increasing altitude

    leads to the corresponding change in natural vegetation.

    – The wet temperate forests are suitable between 1000 and 2000 mm.

    – Temperate forests containing coniferous trees like pine, deodar, silver fir,

    spruce and cedar are found between 1500 and 3000 mm.

    The mountain vegetation is associated with various economic activities. These include gazetting of the national park and game reserves, tourism etc.

    9.7.2 Temperate mountain vegetation

     The temperate mountain vegetation grows well in High Mountains of temperate regions. This grows best between 350 N and 600 N of the equator. The temperate mountain vegetation is in the Alps, in Western Europe, in Norway, Sweden and Finland. Other traces of the temperate mountain vegetation can be found in California on the Rocky Mountain slopes, in British Columbia and Andes in South America.


    Characteristics of the Temperate mountain vegetation

    – Temperate mountain vegetation contains both deciduous and coniferous

    plant species such as poplar, birch, oak and elm.

    – The vegetation arranges itself on the slope in clear demarcated zones.

    – Some trees species grow a thick bark in order to store enough water to be

    used in the winter season.

    – Tree species shed-off their leaves in winter when the temperature drops below

    6° c.

    – Tree species especially the coniferous forests adapt themselves to the climatic

    conditions by growing tiny needle-like leaves.

    – Tree species become shorter and more dispersed towards the zones containing


    Application activity 9.7

    1. Explain the use of the mountainous vegetation with reference to North


    2. Critically examine the role of the temperate vegetation to the economy of


    (1) Mangrove vegetation 

    This is the type of vegetation that grows in marshy and swampy areas along the coast of East Africa. It is found in areas such as the coastal regions of Kenya and Tanzania, Deltas of the Ganga, Mahanadi, and Ganga Brahmaputra delta in India. The Mangrove vegetation is evergreen and grows along the coastal margins between 5o N and 5oS of the equator.

    The conditions necessary for the growth of mangrove forests:

    – They need average temperatures of the coldest month higher than 20°C.

    The seasonal temperature range should not exceed 5°C. They can tolerate

    temperatures of 5°C, but the development will be affected because they are

    not resistant to freezing.

    – They need a large tidal range. This causes limited erosion and deposition of


    – They need a fine-grained substrate. However, there could be some exceptions.

    This is the case in Papua New Guinea and Kenya, where the mangroves grow

    on corals.

    – The shores must be free from strong wave action and tidal current.

    – The mangrove vegetation requires swampy and marshy areas with deep soils

    which must be salty in nature.

    – The mangrove vegetation requires a high temperature necessary for

    chlorophyll making.

    Characteristics of mangrove forests:

    – Mangrove vegetation have broad branches and leaves and they are evergreen.

    – They are associated with saline soils with poor drainage.

    – The dominating plants are trees.

    – The mangrove vegetation grows butters roots.

    – This type of vegetation has a long gestation period.

    – They exist in areas with poor drainage and sufficient water supply. Such areas

    are waterlogged.

    – They grow in areas that are hypoxic (oxygen deficient) waterlogged soil strata,

    with limited tidal pressures, strong winds and sea waves.

    – Mangrove forest species survive under temperatures above 66° F (19° C). They

    do not tolerate temperatures below 18° F (10° C). However, temperatures

    below freezing should not occur for a long time.

    (2) Aquatic plants:

     The aquatic plants or vegetation is referred to as hydrophytes or macrophytes. These plants require special adaptation for living submerged in water, or at the water’s surface. Aquatic plants can only grow in water or in soil that is saturated with water.

    The following are the Characteristics of aquatic plants:

    – They have reduced and shallow roots. The primary function of these roots is to

    anchor the plant to the ground.

    – Plants that normally are submersed, typically form their flowers raised above

    the water surface.

    – Some of the aquatic plants float on the surface of water with no attachment

    to the mud or bottom. These have inflated portions of leaves, stems, or special

    hairs that enable the plant to remain floating.

    – Plants rooted in the mud have immersed leaves with photosynthetic stems.

    They also have relatively small leaves similar to those of typical leaves of

    terrestrial plants living nearby.

    – They have real roots that link with underground roots. Such have numerous

    pores over their surfaces that allow gaseous exchange.

    – The aquatic plants have structures that anchor as seaweeds to the substratum,

    such as the bottom layer or submerged bedrock.

    Marsh vegetation: 

    A marsh is a wetland that is dominated by herbaceous rather than woody plant species. Marshes can often be found at the edges of lakes and streams. In such places they form a transition between the aquatic and terrestrial ecosystems. The marsh vegetation is dominated by grasses, rushes or reeds. Familiar examples of marsh vegetation include cattails, sedges, papyrus and sawgrass.

    The following are the characteristics of a marsh:

    – The mash vegetation grows in poorly drained water.

    – The mash vegetation is a common characteristic of wetlands areas.

    – The mash vegetation grows in both fresh and salty waters.

    – The mash vegetation is found along the rivers and lakes.

    Swamp Vegetation 

    The swamp vegetation occurs along large rivers where they are critically dependent upon natural water level fluctuations. When a swamp vegetation is dominated by forest, it is called a wetland. Some swamps have hammocks or dry-land protrusions, covered by aquatic vegetation or the vegetation that tolerates periodic inundation.

    Characteristics of the swamp vegetation are:

    – They are characterized by poorly drained soils and different plant life

    dominated by trees.

    – The latter characteristic distinguishes a swamp from a marsh, in which plant

    life consists largely of grasses.

    – They grow in waterlogged areas where there is sufficient supply of water which

    allows or stimulates the decay of organisms and prevents the accumulation of

    organic materials.

    – They are often found in lowlands associated with rivers that supply the water

    to some lakes.

    – The number of plant species in swamps is small. While the one found in

    areas associated with well-watered conditions and no waterlogged land,is

    sifignicantly great.

    All swamp vegetation such as mangrove, mash, wetlands and aquatic forests, are associated with various economic activities. These include tourism, the hunting of large invertebrate sand waterfowl,the fishing of crayfish and mudfish. They are also used for research and study purposes. These support art and craft making.

    Application activity 9.8

     Clarify conditions that prove that the East African coast is dominated by mangrove vegetation.

    9.9 The factors which influence natural vegetation

    Learning activity 9.9 

    Make a field trip in your home area and observe types of vegetation. Use the results of your observation to explain the factors influencing their distribution.

    There are various factors that influence the growth and distribution of natural vegetation. There is no single factor that plays a key role alone, but rather a combination of two or more factors. These factors include:

    – Rainfall: the growth of vegetation depends on the amount of the rainfall. For example, Equatorial rainforests have evergreen and dense vegetation. On the other hand, places with low rainfall have scattered vegetation. This explains why there is little vegetation in deserts.

    – Temperature: Forests found in cool areas have fewer tree species. Those in hot

    areas have more species. The cold mountain tops have heath and moorland.

    – Relief and altitude: It has been noticed that with a rise in the altitude, the

    plants in the region show a stunted growth. Trees such as pine, silver fir, birch,

    and juniper fall in this category of vegetation. These contribute to variations in

    the vegetative zonation along the slope.

    – Slopes: Areas on the opposite sides of mountains have different vegetation.

    Steep slopes have more runoff. Gentle slopes allow water to sink into the soil.

    Plants use this water.

    – Soil types: This factor provides basis for different types of vegetation. The

    sandy soils in the desert support cactus and thorny bushes. Wet, marshy or

    delta soils support mangroves and other deltaic vegetation.

    – Human activities: These include the settlement, mining, farming and livestock

    keeping. For example, vegetation is cleared to create space for building houses.

    Trees are cut for firewood and timber. New or artificial vegetation is planted.

    – Drainage also determines the vegetation of a place. There are plants that grow

    best in areas of good drainage while others grow well in swampy conditions;

    for example the papyrus which only grows in swampy areas.

    Application activity 9.9

     Describe the influence of human activities on the vegetation distribution in Rwanda.

    9.10. Importance of the natural vegetation

    Learning activity 9.10

     Make a field trip in your home area, observe the nature of the vegetation and explain its significance to man.

    The following are the significance / importance of vegetation to man:

    – Plants that form thevegetation are the main source of food for humans.

    Thisfoodisin theform of vegetables, fruits, grains, cereals, leaves, seeds and it

    consistsof carbohydrates, oils, proteins, vitamins and minerals.

    – It provides construction materials; for example: trees are used for the

    construction of houses, bridges and poles.

    – The vegetation helps regulate the flow of numerous biogeochemical cycles

    in the atmosphere, most critically those of water, carbon, and nitrogen. It also

    contributes in the local and global energy balances.

    – The natural vegetation plays an important role in our ecosystem. Whereby,

    plants are known as the primary producers since they can manufacture their

    own food through the process of photosynthesis using sunlight.

    – The natural vegetation provides man with a variety of products which include

    flowers, stems, roots, oil and many others. These are used to meet man’s needs

    such as themaking of perfumes, cosmetics and aesthetic purposes.

    – The natural vegetation provides food for some domestic and wild animals.

    Human beings also get food from some plants.

    – The natural vegetation has contributed hugely to the world´s economy,

    particularly in the use of fossil fuels as an energy source. It provides thebiomass

    and some vegetation residuals are used to produce biogas.

    – The natural vegetation provides timber for furniture. Items as beds, chairs and

    tables are made from timber. Timber is also used in construction activities.

    – The natural vegetation plays a key role in soil formation. Their roots facilitate


    – Dead vegetation becomes humus, which makes the soil fertile.

    – The natural vegetation is also a natural resource thatprovides anumber of

    uses to man;i.e. the products like ropes, rubber, gum, papers, and wood

    used in themanufacturing of books, rope, tyres, and seatscome from the

    natural vegetation. Some plants have medicinal contents. Herbs are used in

    thetreatment of various diseases that threaten human lives as well as those of

    domestic animals.

    – The natural vegetation is thesource of materials such as cotton,used in textiles

    and fabric materials to makefor humans.

    – The natural vegetation such as forests and grasslands attract tourists. These

    pay (money) when they visit to see the animals and a variety of flora. The

    money is used to develop the social facilities like schools, hospitals etc.

    – The natural vegetation helps clean or purify air through harvesting carbon

    dioxide from the atmosphere. Again, trees produce oxygen that human beings

    and animals use for theirsurvival.

    – Places with forests receive more rainfall. This is through the process of


    – Areas with forests act as asource of rivers. These are called water catchment


    – Many people get jobs. They are employed as researchers, forest guards and

    forest officers.

    – The natural vegetation makes the landscape beautiful.

    – Tree and plant roots hold the soil together. Therefore, forests protect the

    ground (soils) against soil erosion, mass wasting and the general impact of

    heavy rainfall. 

    The natural vegetation has also the followingnegative influences on man:

    – The natural vegetation associated with some pests such as tsetse flies and

    ticks which put the lives of people and animals at great risks, since they cause


    – Some plants are thorny-leaved and they are harmful to human beings and


    – The natural vegetation is a homeplace for dangerous animals which may

    attack or harm human beings.

    – Some plants are poisonous and may kill human beings and animals when


    Application activity 9.10

    Examine the value of the natural vegetation for thesustainable development of


    End unit assessment

    1. Discuss the distribution of the natural vegetation in the world.

    2. Describe the relationship between vegetation and land use.

    3. Draw a map of the world and on it, show the following vegetation types:

    – Savannah humid

    – Mediterranean vegetation

    – Desert vegetation

    – Mountain vegetation

     4. How do the following factors influence the distribution of vegetation in Africa?

    – Variation in temperature.

    – Variation in relief


    Key unit competence:

    By the end of this unit, I should be able to discuss the problems of the population

    growth in the world and the ways of controlling it; explain the impacts of early sex,

    HIV/ Aids, health risks and STDS on the world’s population.

    Introductory activity

    Population is one of the main complex issues in geography; its study is essential for

    proper national planning in relation to the provision of social services to the people. In

    general, today there is fear that the rate at which the population is increasing presents

    great challenges to the world resources.

    1. Explain the term “population” and its related concepts.

    2. Discuss the factors influencing the population distribution in any area.

    3. Compare and contrast the population problems in developed and developing


    4. Describe the population policies that should be taken by the world countries

    to control such a rapid population growth.

    10.1. Human diversities

    Learning activity 10.1

    The world population is composed of billions of people from different countries,

    speaking different languages, praying from different churches and having different

    cultures. This makes what geographers call “human diversity” in the World.

    1. Referring to the statement above explain the following concepts:

    (i) Human diversity

    (ii) Race

    (iii) Languages

    (iv) Religion

    (v) Culture


    The term refers to the number of people living in an area at a given period. The

    study of population growth, density, distribution and movement is referred to as


    Human diversity 

    This is a term used to mean the existence of a wide kind of human beings with distinctive or distinguishing characteristics such as race, languageand political divisionwhich form different ethnic groups of people.This means that individuals are unique, and it is very important to recognize our individual differences. These can be along the dimension of race, religion, language and state.

    10.1.1. Race 

    The term race refers to the similarities of genetic patterns among the aggregates of individuals of human populations. Race clearly expresses the varying genetic patterns expressed in the physical traits of human species. Races can be differentiated based on the following aspects:

    – Hair types: People’s hairs also differ. Some have black hair such as the Africans,

    others long and reddish hair suchas the Europeans and yet others have white,

    short or brown hair.

    – Skin colour: People of the world have different skin colours. For instance, in

    Africa the majority of the people are black while others may be brown. In other

    continents, people are white and others are red like the red Indians.

    – Climatic factors: Races are also differentiated basing on climatic location. For

    example, most people of the tropics are black and others though brown have

    black hairs.

    Generally, two types of classification of races have been identified. These include the


    a. Phenotypes: This is the classification criteria used while grouping or

    classifying human population into groups basing on their physical


    b. Genotypes: This is also another classification technique used to classify

    population into groups basing on genetic origin of their physical traits.

    10.1.2. Religion

    Religion refers to a unified system of beliefs and practices that join all those who

    adhere to them into a single moral community. Indeed, religion is a unifying factor

    for the people with the same religious beliefs. For instance, some religions encourage

    people to join their religious affiliations or denominations. 

    Religion greatly sets grouping of population into varying classifications entirely based on their set of beliefs. There are various religions of the world and they include the following: Christianity, Islami, Hinduism, Buddhism, and many others. The following figure 10.260 shows the portions of the main religions in the world.


    10.1.3. Languages

     This refers to any systematic method of communicating ideas, attitudes, or intent through the use of mutually understood signs, sounds, or gestures. The term language can also mean an organized system of spoken words by which people communicate with each other with mutual comprehension.

    Various groups of people speak different languages. For instance, people in Rwanda speak Kinyarwanda, those of Burundi speak Kirundi, Kenyans speak Swahili, Ugandans speak Luganda and other languages etc. There are local languages spoken by a group of people within a country as well as national and international languages. Some people speak more than one language. The language acts as unifying factor that plays an important role inbringing people together. The languages can be classified according to the number of speakers as it is presented below:

    10.1.4. States

     A state refers to a nation or territory considered as an organized political community under one government. In this case, a state is taken as a country. Nevertheless, in some cases a state is different from a country in political organizational structures based on federalism. A good example is the United States of America where a country is made up of union of 50 states. State expresses merely the actual organization of the legislative or judicial powers, i.e. people permanently occupying a fixed territory bound together by common habits and custom into one body.States have independent governments that administer them and control the people who are the registered citizens of them. In 2017, the independent countries in the world were evaluated at 195, including South Sudan, which is the youngest country that got its autonomy recently. However, there are countries made of many states such asthe USA, India, and the United states of Arabs, the United Republic of Tanzania etc. The African continent is composed of 55 countries with South Sudan inclusive.

    Application activity 10.1 

    1. With typical examples from Africa, identify and describe the religions operating on the continent. 

    2. Examine the main aspects considered in differentiating the types of races.

    10.2. World population distribution

    10.2.1. Population density

    The world population distribution describes how people are spread out across the globe. The population is not spread evenly. Some locations are sparsely populated while others are densely populated. Arid regions or areas associated with rugged reliefs have low population while others are completely unsettled. The world locations with flatter land, good soils and mild climate (for example the Ethiopian highlands and the Nile valley in Egypt) are more densely populated.

    Population distribution is the spread of the people across the world. It describes the pattern of where people live. There are places which are sparsely, moderately and denselypopulated.

    Population density is a measurement of the number of the people in an area. It is an average number. Population density is calculated by dividing the number of people by area. Population density is usually shown as the number of the people per square kilometer. The World population distribution includes the following categories:

    (i) The densely populated areas:

    These are the regions with more than 100 people per square kilometer:

    – East and south East Asia: This region includes countries like Singapore, China,

    India, Bangladesh, Japan, South Korea, the Philippines and Taiwan.

    – Central and Western Europe: This includes countries like Germany, the United

    Kingdom, France, Italy, Belgium and the Netherlands.

    – The Caribbean countries: These include Jamaica, Trinidad and Tobago, Cuba,

    West Indies and El -Salvador, Eastern USA and south eastern Canada.

    – Nile valley and delta which is located in Egypt.

    (ii) The moderately populated areas:

    These are the regions which have between 25 and 100 persons per square km. They are usually dominated by agricultural occupations and typical examples include Australia, Brazil, USA, Argentina and Chile etc.

    (iii) The sparsely populated areas: 

    The sparsely populated areas are the regions with few people per square kilometre. They include Sahara, Atacama, Kalahari and Australian deserts. There is also sparse population in the high mountain ranges; for example: the Himalayas, the Rockies, the Andes, the Alps and the dense forests of equatorial regions in the Amazon, Congo and the outlying islands of Indonesia.

    10.2.2. Factors responsible for the distribution of population in the World 

    The population of the world in general is unevenly distributed. This is attributed to a number of factors. These factors include the following:

    – Reliable and adequate Rainfall: There is a close relationship between the areas

    of high rainfall and the population distribution. Areas that receive high and

    reliable rainfall over 1500 mm attract high population densities because they

    are suitable for the agriculture.

    – Soils: Areas blessed with fertile soils encourage the growth of a variety of

    crops; hence food and employment. This explains why such areas attract many


    – Pests and diseases: Areas associated with pests such as tsetse flies and ticks

    discourage the settlement. Many people fear to live in areas known for their

    pests since they cause diseases like trypanasomiasis and East coast fever that

    discourage farming and settlement in some areas. Such places are therefore

    sparsely populated. Good examplesareBunyoro and Miombo woodland areas

    (in Uganda and Tanzania respectively).

    – Altitude: This refers to the height of the land above the sea level. Altitude has

    an influence on the population distribution. Example from 2700 m above the

    sea level, temperatures are extremely cold, which discourages the settlement.

    There is also severe soil erosion at this altitude.

    – Vegetation: The natural vegetation of an area is also an important influencing

    factor on the population distribution. Dense forests, bush land, deserts and

    swamps are unfavourable areas for the settlement; while grasslands are easy

    to settle, thus capable of attracting high population densities.

    – Relief: The rugged areas or mountainous landscapes discourage the settlement.

    For instance, the rift valley and lowlands, especially along the rivers, are

    unfavourable for human settlement; while some gentle sloping areas are easy

    to work and build and hence attract more people.

    – Slave trade that led to the depopulation of various areas of origin and led to

    increased population to the areas where slaves were taken.

    – Migrations: The internal migration such as rural-urban migration leads to

    an increase in population in urban areas, and the rural-rural migration may

    influence the population distribution too. The influx of refugees from other

    countries results in increased population in the receiving countries. For

    example, the Northern part of Uganda has had its population increased as a

    result of the refugees from SouthSudan.

    – Civil wars: This is one of the serious factors that have influenced the population

    distribution on the African continent and the world at large. Countries such

    as South Sudan, Somalia and the Democratic Republic of Congo have had

    their population reduced because of civil upheavals. In some countries tribal

    clashes push people away from their homelands to other places. For example

    in some parts of Kenya´s Rift Valley Region.

    – Government policies: Some areas may have reduced population because of

    the government policies prevailing. Land is set aside for the establishment of

    national parks or the construction of dams, whereas some policies such as the

    establishment of irrigation schemes in dry lands or settlement schemes lead

    to the population increase.

    – Industrialization: Industrial towns like Nairobi, Kigali, Kampala and Kinshasa

    have large industrial establishments producing chemicals, foodstuff, plastics

    and textiles which attract people for paid employment, hence a high

    population. However, their demarcated areas for theindustrial establishment

    may result in thedisplacement of people.

    – Energy resources and minerals:Energy resources and minerals attract people

    to settle in a given area. Minerals like coal in their prime age have greatly

    influenced the pattern of population distribution in countries like Great Britain,

    France, and Germany. This is because of the employment opportunities that

    are created and other social benefits or advantages.

    – Historical factors:Areas that were occupied by kingdoms, especially thosenear

    the King’s palaces, attracted a high population due to security. For example,

    the central part of Uganda where the population is high, the large population

    is attributed to the presence of the king’s palace that marked a centre of

    attraction to many people.

    – Economic factors. The economic viability of an area lies in its carrying capacity

    and its ability to provide employment opportunities. This in turn affects not

    only the size of the population but also its spacing. A specific economic system

    tends to arrange people in a specific distributional pattern.

    – Transport and communication:The establishment of roads, railways and

    communication lines attract dense population due to theaccessibility of the


    – Political factors:Political factors have a great influence on thepopulation

    distribution and density in different areas. For example, countries that are

    politically stable attract a high population, whereas political instability in some

    countries can cause thepopulation displacement and migrations.

    – Demographic factors:The changes in the distribution and density of

    thepopulation in the world take place through variations in the rate of natural

    increase (fertility and mortality rates). Developing countries that have a

    high birth rate will always have a high population growth leading to a high

    population density.

    – Natural hazards and disasters:Areas known to be prone to earthquakes,

    landslides, volcanic eruptions, floods, glacial advances, storms, epidemics, fire,

    and severe droughts discourage thesettlementand will therefore have sparse

    population than theregions that are free from such challenges. 

    Application activity 10.2

    1. With the aid of a drawn sketch population map of the world, identify the

    densely and sparsely populated areas.

    2. Explain factors controlling population distribution in Rwanda.

    10.3. Population concepts and related effects 

    10.3.1. Optimum population

    Learning activity 10.3

     Worldwide, areas with favourable climatic conditions, abundant natural and water sources, fertile soils, social amenities, security and political stability tend to pull people to settle there and, therefore, being over crowded areas. On the other hand, areas with unfavourable climatic conditions, infertile soils, poor drainageand limited social facilities tend to limit population settlement, hence being less populated areas.

    1. Why is under population undesirable in a country?

    2. Referring to the above passage, explain the following population concepts:

    – Optimum Population

    – Under population

    – Overpopulation

    The term optimum population refers to a situation where the resources available can satisfactorily support the existing population in a state of equal balance and sustainability. This takes place where the population and the resources are equal. Optimum population is considered as one that lies within the limits, large enough to realize the potentialities of human creativity to achieve a life of high quality for all the inhabitants, without affecting the existing ecosystems.

    In other words, the optimum population can be regarded as that state of equilibrium between the population and the resources, which satisfies the well-defined needs of all the members of a community and which varies both in time and space.

    The following are the major characteristics of the optimum population:

    – The optimum level is that size of the population which yields the highest

    quality of life. A life described by adequate food supplies, energy, water and

    other social facilities that are enough to satisfy the needs of people.

    – The presence of the highest per capita output characterised by the marginal

    productivity of the labour exceeding the average productivity and where the

    rates of growth to the total production are the highest.

    – The availability of enough employment opportunities. That is, where every

    person fit to work is able to find a job.

    – Having a very low dependency ration that is not over-burdening the working

    class or straining the national budget. 

    The basic criteria for assessing the optimum population include: per capita production, average standard of living, degree of employment, longevity of life, dependency ratio, social harmony, family stability, attainment of knowledge, growth of purely intellectual functions, per capita food consumption, proportion of expenditure on food, rationality of land use, balanced demographic structure and rational development of resources.

    10.3.2. Under population 

    Under population is a situation whereby the size of the population is small in relation to the available resources of the country. It is a situation where the size of the population is below the available resources, hence they under-utilize the available resources.

    (i) Positive effects of under population

    The following are positive effects of under population:

    – No Congestion: A country with less population experiences little or no


    – Employment opportunities as a result of the small size of the population.

    There will be enough job opportunities for the people.

    – Increase in social and infrastructural facilities: an under populated country

    experiences higher production per capita (in terms of social and infrastructural

    facilities available to the people in the country).

    – Availability of idle resources: The fact that a country is less populated means

    that the resources available in that country are higher than the number of

    people. Hence, many idle resources would abound everywhere.

    – Low pressure on social amenities: Owing to the low population, there is also

    low pressure on the social amenities in the area.

     (ii) Negative effects of under population

    The following are negative effects of under population:

    – Underutilization of resources: Resources are highly underutilized in a country

    with low population. This means that the resources will not be economically


    – Shortage of enough people to defend the country: At times of war and

    emergency, a country might find it difficult to mobilize enough people to

    defend itself.

    – Shortage of labour force: This is one of the effects of under population. The

    shortage of labour force results in failure to exploit the available natural

    resources, capital stock and technology.

    – Lack of specialization: Under population makes specialization impossible.

    As the supply of labour is limited, specialization and rationalization schemes

    cannot be undertaken.

    – Low per capita income: The underutilization of resources in the under

    populated countries leads to a low per capita income.

    10.3.3. Overpopulation 

    This refers to a situation where the number of existing human population exceeds the carrying capacity of the country or an area.The economic development of a country depends largely on the quantity and quality of the population. Therefore, when there is over population, resources become scarce and other negative effects set in. Such a situation reduces the quality of life and the government spends much addressing the social challenges instead of developing the economy. The following are the effects of overpopulation:

    – Food shortage: Overpopulation results in food shortage. This is one of the

    serious effects associated with overpopulation. It becomes too costly to

    purchase the food stuffs because of high demand.

    – Unemployment: The excessive population leads to massive unemployment.

    This results from the lack of proper economic development, since much is

    spent on solving the social challenges such as birth control and the purchase

    of medicine among others.

    – Fall in the efficiency of labour: When the population increases after a particular

    stage, the number of labour also increases. As a result, each labour gets fewer

    amounts of capital and this leads tothe fall in the efficiency of labour.

    – Increase in dependents: Overpopulation implies the increase in population

    beyond a particular level. Birth rate increases due to overpopulation. The

    size of the working population becomes less when compared to the size of

    the children and old-aged persons. As the dependence ratio increases,it

    undermines the future investments since the working class spend every

    income they get to sustain people that depend on them. Therefore, little is


    – Pressure on the land increases: The excessive population brings pressure on

    land. As a result, the marginal productivity of the additional labourers is zero.

    Eventually, the average productivity of labour and land remains less.

    – Decline in standards of living: Over population leads to the shortage of the

    necessary or essentials of life. There occurs the shortage of foodstuffs, scarcity

    of houses, spread diseases, uncontrolled migrations and high cost of living. All

    result in a decline in the standards of living of the people.

    – Inflation: The overpopulation results in the growth of inflationary conditions in

    the country. As the output and productivity in different sectors fall, and as the

    demand for various goods and services increases, prices of goods and service

    rise up.

    – Limited transport: This is increased pressure on transport and communication

    facilities or services. It leads to the wastage of time and in some cases business

    becomes stagnantly developed. The funds that would be used to establish

    new roads, railway lines and other related services are used to cater for urgent

    challenges such as the control of diseases, crime cases etc.

    – Limited educational facilities: Over population results in the lack of financial

    resources to invest in education due to the ever-growing population.

    – Over exploitation of natural resources. There is over utilization of the resources

    of all kinds. This results in the depletion of some resources and forests are

    cleared too. The swamp and wet lands are reclaimed. All these put together

    result in environmental degradation.

    – Housing facilities: The overcrowding results in the development of slummy

    areas and their associated evils such as immorality and drug use.

    – Diseases: The overpopulation leads to the congestion and easy spread of

    disease such as dysentery, cough, cholera and others.

    Application activity 10.3

    1. Basing on the examples of Rwanda; discuss the effects of over

    population to the economic development of a country.

    2. With clear examples, identify thenegative effects of under population. 

    10.4. Population problems of developed and developing countries

    Learning activity 10.4 

    Read and analyse the passage below:

     The developed countries are characterized by high levels of industrialization and urbanization, high per capita income, dependence of a major part of the workforce on secondary and tertiary activities, and an efficient and productive agricultural sector. Unlike developing countries which are characterized by low level of technology, low level of skilled labour, poor infrastructures, low level of industrialization, low income per capital etc.

    Referring to the above passage:

     (a) Explain the characteristics of developed countries.

     (b) Explain the challenges faced by population in developing countries.

    10.4.1. Population problems of developed countries and their solutions

    The developed countries experience various challenges in relation to their

    population as indicated below:

    – High proportion of old age population: As the birth rate is low, the proportion

    of younger people in the population is relatively small. The low death rate and

    high life-expectancy mean that there is an ever-increasing proportion of older

    people in the population. Many retire from active work in their sixties and then

    become dependent on the working population. The provision of pensions

    and other facilities, e.g. extra health services, for elderly people pose financial


    – Shortage of labour: The improved standards of education has resulted

    in children remaining longer at school and joining the workforce later. This

    implies that there will be slow expansion of the workforce. In addition, the

    low population growth leads to the shortage of labour force. As a result, many

    countries of Europe face the shortage of labour. High wage rates disrupt the

    overall economic pattern.

    – Congestion in towns: The increased expansion of urbanization has caused

    high pressure on the civic amenities, transport, housing etc. The effects of this

    (pollution and social tensions) lead to heart diseases, breathing problems,

    lung and skin problems etc.

    – Development of slum areas: The unregulated growth of urbancentres leads to

    the growth of slums, which creates many social and environmental problems.

    The highly productive agricultural land is encroached by urban houses, roads

    and industries. 

    – Rural depopulation: Towns provide amenities such as shops, entertainment

    and better social serviceswhich cannot be matched in the country districts;

    and the employment is usually easier to find in urban areas. This has influenced

    the rural youth to migrate to urban centres, yet these would be the ones to

    develop the rural economy. There is stagnant rural economy.

    – Congestion in towns: There increased expansion of urbanization which has

    caused high pressure on the civic amenities, transport, housing etc. the effects

    of this such as pollution and social tensions lead to heart diseases, breathing

    problems, lung and skin problems etc.

    – Development of slums areas: The unregulated growth of urban - centres leads

    to growth of slums, which creates many social and environmental problems.

    The highly productive agricultural land is encroached by urban houses, roads

    and industries.

    – Rural depopulation: Towns provide amenities such as shops, entertainment

    and better social services, which cannot be matched in country districts,

    and employment is usually easier to find in urban areas. This has influenced

    the rural youth to migrate to urban centers, yet these would be the ones to

    develop the rural economy. There is stagnant rural economy.

    The following are some of thesolutions to the problems of thepopulation in developed countries:

    developed countries:

    – Most developed countries are now encouraging and promoting vertical

    expansion through setting up skyscrapers.

    – Federal and state governments continue to invest many capital and skilled

    resources in thedevelopment of social infrastructures and housing facilities.

    – Most developed countries like USA continue to strengthen theirimmigration

    operations worldwide as a means of controlling illegal immigrants entering

    the country.

    10.4.2. Population problems in the developing countries and their solutions

    The developing countries experience various challenges in relation to

    population as indicated below: 

    – Low levels of technological development: This is directly linked to low

    productivity levels in the countries of Sub-Saharan Africa, South East Asia and

    Latin America. Low productivity refers to the slow economic growth, which is

    the root cause of rapid population growth in these countries.

    – Low standard of living: This is the strange case of many countries having

    abundant natural resources but lie untapped economically. In such countries

    poverty remains an active descriptor of their economies. 

    – Rapid population growth: This is because of improved health facilities and

    reduced mortality rates. This younger section puts tremendous pressure on a

    comparatively small working population.

    – Lack of diversification of economy: The lack of development of secondary and

    tertiary sectors leaves limited employment opportunities for the skilled and

    educated people. These most commonly move to more developed towns or

    to foreign countries in search for better job opportunities.

    – Under-nourishment and lack of hygiene: Due to low standards of living, the

    incidence of diseases is high, leading to high rates of mortality especially

    among the children and pregnant mothers.

    – Inefficient agricultural sector: The developing countries are characterized

    by the subsistence agriculture with traditional and backward methods of

    production leading low productivity.

    – Weak industrial base: The lack of capital, out-dated technology and inadequate

    skilled work force have resulted in a weak industrial base in most of the

    developing countries. This has prevented any substantial improvement in the

    living standards of the populations of these countries.

    – Tradition-bound societies: The inward looking attitudes restrict the flow of

    awareness as regards birth control, family planning etc. Standing systems

    inhibit the social mobility in different societies.

    – Unfavourable physical conditions: Many of the under populated countries

    have hostile climatic or topographical conditions. Such conditions obstruct

    the development and it is both difficult and expensive to overcome these


    The following are some of the solutions to the problems of the population in the developing countries:

    – Rehabilitation: An alternative to this scheme is to provide the residents of

    shantytowns with the materials to improve their existing shelters. Residents

    are also encouraged to set up community schemes to improve education and

    medical service. For instance, Bolivia and Pakistan.

    – Sewage rehabilitation: Several cities have been repairing water and sewerage

    pipes and this improves the safety and quality of water in the city and reduces

    mortality rates.

    – Encouraging the population migration from urban areas to rural areas of the

    same country. This is done through the beautification of rural areas.

    – Setting up social - economic infrastructures such as education, health and

    transport networks. 

    – Through the modernization of agricultural sectors as a means of increasing

    their productivity and output. That is the use of the best seeds and fertilizers

    to boost agricultural output and check on the issue of food shortage.

    – Family planning and education has helped in reducing the rapid population


    – Government policy that aims at establishing house facilities, especially in town

    areas, to improve on the housing facilities. 

    Application activity 10.4

    3. Compare and contrast the population problems associated with

    developed and developing countries.

    4. Referring to Rwanda, propose possible remedies to curb the problems

    identified in (1) above. 

    10.5. Population growth

    Learning activity 10.5

    Read the passage and provide answers to the questions that follow:

    Population growth can be explained as the average annual percent change in the population, resulting from a surplus (or deficit) of births over deaths and the balance of migrants entering and leaving a country. The rate may be positive or negative. The growth rate is a factor in determining how great a burden would be imposed on a country by the changing needs of its people for infrastructures (e.g., schools, hospitals, housing, roads), resources (e.g., food, water, electricity), and jobs. Identify the factors that affect the rate of the population growth and explain why this is an important aspect in population studies of a given area.

    Population growth rate: Population growth rate refers to the change in population over a unit time period, often expressed as a percentage of the number of individuals in the population at the beginning of that period. This can be calculated using the following formula:

    A positive growth rate indicates that the population is increasing while a negative growth rate indicates that the population is decreasing.

    10.5.1. Factors influencing birth rate

    Birth rate is the total number of live births per 1000 in a population in a year or period of time.

    Birth rate is influenced by different factors that include:

    – Social and religious beliefs: for example, Islam allows polygamy and Catholics

    do not support abortion.

    – Economic prosperity: (although in theory when the economy is doing well

    families can afford to have more children; in practice, the higher the economic

    prosperity the lower the birth rate).

    – Poverty levels: Children can be seen as an economic resource in developing

    countries as they can earn money through dowry.

    – High mortality rate: A family may have more children if a country’s mortality is

    high. Families produce more since some are expected to die.

    – Limited health facilities which limit easy accessibility to the use of family

    planning techniques.

    10.5.2. Factors influencing death rate

    Death rate is defined as the ratio of deaths to the population of a particular area or during a particular period of time, usually calculated as the number of deaths per 1000 people in a given year.


     Death rate is influenced by different factors as follows:

    – Limited medical facilities and health care increases death rate.

    – Low nutrition levels and poor feeding lead to malnutrition which results in

    high death rate.

    – Poor Living standards due to high poverty levels reduces the life span and

    results in high death rate.

    – Lack of clean drinking water that leads to easy contamination of contagious


    – Low level of hygiene caused by poor sanitation conditions, thus enhancing

    death rate.

    – Social factors such as conflicts and levels of violent crime. This leads to death

    of many people unselectively.

    – Wars in different countries have led to high rate of death. For instance,

    Somalia, Southern Sudan, Nigeria, Syria and Afghanistan.

    10.5.3. Factors influencing the rapid population growth

    Below are some of the factors that could influence the rapid population growth rate:

    – Improved health care which has reduced mortality rates by diagnosing health

    problems in a timely manner. This includes also the use of vaccines which has

    helped to prevent illnesses that used to kill many people in the past. As a result,

    this has helped in saving the lives of many people.

    – Introduction of better farming techniques. This has boosted the production

    of food. In addition, efforts toward food security have been successfully

    implemented in many countries. Areas that were associated with deaths

    caused by drought and famine can now get enough food supply, thus saving


    – Increase in the fight against poverty has also contributed to population

    increase. In the past, only a few families had enough money to support

    themselves while many faced various problems including health issues due to

    poverty. Many homesteads are able to meet medical charges, which has saved

    many lives.

    – Immigration has also contributed in the population growth in developed

    countries and urban centres. Many people immigrate to other countries in

    search for better living standards. This has led to an increase of population in

    many developed countries.

    – Poor family planning in many families is one of the major causes of the high

    population growth. Many families tend to have many children, and yet they

    cannot support them.

    – Children are regarded as a religious duty: Many people have the religious belief

    that having children is regarded as a holy and religious duty of the married


    – Malnutrition which caused many people to die every day. Parents are not sure

    that all of the children can survive. Therefore, they want to give birth to as

    many children as possible.

    – Polygamy is also one of the causes of the population increase. A man gets

    married to multiple women, and these women give birth to more babies. This

    is associated with the cultural and traditional beliefs in developing countries.

    – Cultural factors also may cause high birth rates. Many people do not want to

    apply the measures of family planning. They think it is unholy to use family

    planning measures to prevent conception. They believe that the birth of a child is a gift from God. Therefore, they do not want to stop births.

    – The social and religious values and customs are mainly based on the traditional

    attitudes of people. Parents do not get respect in the society if they have no

    children, especially a son. It is the conservative belief that the son is necessary

    to inherit the parental property, continue the family line and perform the

    funeral rites.

    – Artificial population increase. This arises from incoming migration of refugees.

    In Africa, Latin America and Asia, the natural phenomena such as war, flooding,

    famine and landslides, force many people to move to other places. Therefore,

    these massively displace people causing a rapid increase in the population of

    the receiving country.

    – Level of education: Families that are educated prefer to have fewer children.

    In addition, people take long at school and come out at about 25 years; this

    lowers the reproductive and fertility rate than a person who dropped out of

    school and married at 16 years.

    10.5.4. Effects associated with rapid population growth

    The following are some of the effects of the rapid population growth:

    – Available facilities become insufficient for the growing population. It leads to

    the falling of living standards.

    – It is difficult to provide suitable employment opportunities for all. The result is

    large-scale unemployment which also causes the living standards to decline

    – Natural resources are over-utilized and their quality degrades. Eventually, they

    are depleted.

    – The environment is polluted and environmental problems occur.

    – Crimes such as robbery, theft, murder and abduction increase and this leads to

    the killing of people. In the struggle to search for better life, some people may

    resort to stealing and breaking commercial banks.

    – It will be more difficult to meet the basic needs such as food, shelter, clothing,

    education and health services.

    – If the population increases rapidly, there is unemployment problem in the


    – Due to the rapid population growth, there is deforestation for agriculture and

    settlement since more land will be needed.

    – Land shortage which results into land fragmentation, conflicts, food shortage

    and famine.

    – Heavy strain on government expenditure in the provision of social services and

    others such as the importation of drugs and food to sustain the population.

    – High population growth rate results in great dependency burden. This leads to the low saving of capital, low capital accumulation and low investments; which results in the slow economic development of a country.

    10.5.5. Ways of controlling population growth

    The following are some ways of controlling population growth:

    – Family planning: Family planning is to ensure that a woman gets a child when

    she actually wants one. It reduces accidental pregnancies and ensures a

    controlled family size.

    – Raising the level of education: Education particularly that of women is very

    important in population control. Education changes the cultural beliefs and

    attitudes of people.

    – Increasing employment opportunities for women: As more women join career

    jobs, they tend to be occupied by work. This limits the chances of having more


    – Increasing income: Increased income results in greater demand for durable

    goods such as electronic goods, houses and furniture (instead of children). This

    increases the expenditure for families, which may influence them to consider

    the importance of having manageable family sizes.

    – Increased security in old age: In some communities where children are regarded

    as security in old age, the introduction of pension schemes, insurance schemes

    and income for old people, can substitute as security for the old age. This can

    be more effective if more efforts are made to ensure that more people join the

    National Social Security Fund.

    – Enacting child labour laws: Where children are regarded as suppliers of

    labour, strict laws should be enforced to restrict the minimum age for child

    employment. For instance, laws should be enacted to make it illegal for

    anybody to employ a child below the age of 16.

    – Balanced economic growth: This should be through the decentralization of

    industries and other economic activities. It ensures better regional income

    distribution and it removes pressure from a few urban centres. When people get

    employed in such industries, they have limited time to spend at home,which

    could assist in the creation of manageable family sizes. 

    Application activity 10.5

    1. With reference to specific examples, discuss the impacts of the

    population growth on the environment.

    2. Explain the appropriate population control measures that can be used

    in Rwanda.

    10 .6 Population structure and the composition

    Population structure refers to the composition of the population of a region. It relates to the age and sex of the population. It shows the population structure of a given region. Population Pyramid: A population pyramid shows the age and sex structure of a country. In addition, it is a type of graph that is divided into males and females and then age groups in correspondence to their totals.

    (i) Sex composition

    Communities differ in sex composition,i.e. the composition of male and female. Sex ratio is an index linked with the socio-economic conditions of an area. It is an important tool for regional analysis. It has a profound effect on the demographic structure of a region. It is a function of three basic factors: sex ratio at birth, sex ratio at death and sex-selectivity among migrants.

    The proportions of children and older persons have much to do with the balance of national expenditures on schools, childcare, immunization and reproductive health, as against the expenditure on old-age, social security systems and health care for chronic and degeneration diseases.

    – Measurement of age structure:The age structure of a population can be

    analysed in a number of ways. The most commonly used method is the one in

    which the percentage of the population distribution in various age groups is

    worked out.

    – Age groups:Generally, the population is categorized into three broad age

    groups; the young, the adults, and the old. Thus, the three broad age groups

    that emerge are 0 to 14 (young), 15 to 59 (adults) and 60 and above (old).

    – Age pyramids:One of the most commonly used methods of analysing age

    composition is the age pyramid, which is commonly known as age and sex

    pyramids. The age pyramids are constructed for portraying the age structure

    of a population along their vertical axis. In such pyramids, age groups at a

    regular interval ‘say 5 years’, starting from 0 to 4 and ending according to the

    age structure of the population under review.

    – The horizontal axis of such pyramids represents the total population or the

    populations of males or females that are expressed in percentages. The

    horizontal axis represents males and females separately. The pyramid is divided

    vertically into two halves, the right side representing the females and the left

    side catering for males.


    (iii) Active and inactive population

    A distinction has often been made between the total population and the work force. While the total population refers to the entire population inhabiting the area, the work force consists of only those persons who could participate in economically gainful activities in the event of need.

    Different countries classify the work force further into two-subcategories: The economically active population and the economically non-active population. The economically active population is that part of manpower which is actually engaged in the production of goods and services. It consists of both males and females.

    The economically non-active population is that part of work force which is engaged in activities like household duties in their own homes or at the place of their relatives, retired personnel, inmates of institutions, students and those living on royalties, rents, dividends, pensions, etc.

    (iv) Standard of living 

    Standard of living is a grade or level of subsistence and comfort in everyday life enjoyed by a community, class, or individual. Thus, it is the degree of wealth and material comfort available to a person or community. Therefore, the developed countries have the higher standard of living compared to the developing countries.

    (v) Education 

    Education managers in developing countries are fighting for the young people to gain access to quality education and they further advocate for the acquisition of skills that provide the foundation for lifelong learning. More efforts are invested in programs that aim at ensuring that every child receives quality primary and secondary education. Many countries work hard to reduce the barriers or constraints that had existed for centuries between the education for boys and girls. The reduction of such gap targets has enabled the girl-child education.

    A quality basic education gives children and youth the knowledge and skills they need to face their daily life challenges; whereby, empowering them to take advantage of the economic and lifelong learning opportunities. It is also a key driver for reducing poverty, fostering the economic growth, achieving gender equality and social development in developing countries.

    Application activity 10.6

    1. Explain why life expectancy is short in developing countries.

    2. Compare and describe population pyramid of East Africa countries.

    10.7 Population Policies in the World

    Learning activity 10.7

    Worldwide, each country comes up with its own policies that help in

    controlling the rapid population explosion. One of those policies implemented

    in developing countries (such as China)is regulating the number of children

    per family.

    a. Suggest any other policies that could be put in place to control the

    rapid population growth in Africa.

    b. Explain the effects of “one child per family” policy in a country where

    it is applied. 

    A population policy is defined as procedures taken by a country to adjust the way its population is changing, either by promoting large families or immigration to increase its size, or by encouraging the limitation of births to decrease the number of people living in the country.

    To be effective, the population policies should address all the sources of the continuing population growth to a moderated rate, purposely to sustain the development of each country whether a developed or developing one.

    10.7.1. Population policies in developed countries

     Provision of incentives and favourable conditions for big families. Several social

    and economic measures have substantial effects on a desired family size.

    – To encourage families to increase birth rates in order to get enough active


    – Provision of pensions and other facilities, e.g. extra- health services, for elderly


    – Increase the number of population to improve the utilization of facilities and

    resources available in the country.

    – Delayed marriage and child-bearing period as a way of addressing the needs

    of young women.

    – Continuing improving thegirl child education and educational attainment

    for all, specifically among girls. The availability of mass education changes the

    value placed on large families and encourages parents to invest in fewer but

    “higher-quality” children

    10.7.2. Population policies in developing countries

    The developing countries face numerous challenges related to big families due to the persistence of high fertility and mortality, no access to contraceptive tools, poverty, early marriage and lack of education on the impact of population growth. Below is a detailed explanation of the population policies witnessed in developing countries:

     Improvement in population health through access to child health care

    services, contraceptive measures and sterilization.

    – Eradication of mass epidemic diseases through improving the living standards

    of the population.

    – Family planning as a dominant component of the population policies and

    integrating in schools’ curriculum the avoidance of early marriage.

    – Investing in women and providing them with economic prospects and social

    identities apart from motherhood. Improvement in the socio-economic and

    legal status of girls and women in general are likely to increase their bargaining

    power in the decision making. This gives them a stronger voice in family

    reproductive and productive decisions.

    – Assisting mothers to become economically productive by enabling them to

    have enough time in their professional work.

    10.7.3. Effects of population policies on population growth

    The effects of population policies on the population growth will result in the following:

    – The decline of both birth and death rates will lead to the occurrence of

    an ageing and (economically) non-active population. This has undesired

    economic implications.

    – Some of the population policies create ground for the resources to be in the

    hands of the few, who are rich.

    – There will be an increase in literacy and educational levels and high levels of


    – There will be stagnation in the population growth and therefore, there will be

    less young people in the society to supply the required labour.

    Application activity 10.7

     Referring to population problems in Rwanda, describe the population policies that have been adopted by the government to deal with such problems.

    10.8. Impact of early sex, health risks, HIV/Aids, STDs in the world

    Learning activity 10.8

    Read the passage below and use it to answer the questions that follow:

    World AIDS day takes place on the 1st December each year. It’s an opportunity for people worldwide to be united in the fight against HIV, to show support for people living with HIV, and to remember those who have died from AIDSrelated illnesses. Founded in 1988, World AIDS day was the first ever-global health day. Having sexual intercourse at a very early age, especially during adolescence can affect your health. It can have a negative impact on your physical as well as psychological health. Sexual intercourse at an early age can have a longlasting effect on your body.

    1. What do you learn from the above passage?

    2. dentify the diseases mentioned in the above passage and discuss their

    impacts on the society.

    3. Describe the ways in which the mentioned disease is spread?

    4. Referring to the passage above, explain the term “early sex”.

    5. Explain the effects of having sexual intercourse at an early age.

    10.8.1. Causes of Early Sex

    1. Peer pressure

    Many young people feel great pressure from other teenagers to become sexually active. They do not want to be different or be rejected by their friends.

    2. Pressure from a partner

    For many young people it is difficult to say “NO” especially to someone they care about and this leads to early engagement in sexual activity.

     3. Sexual attraction 

    Hormones in the teenagers tend to be more active. This increases their urge and desire for sexual intercourse.

     4. Social and mass media pressure 

    There is too much and a wide range of media and social communication ways that have become the perfect means of exchanging pornographic data. The youth begin to learn and witness sexual messages in movies, TV shows, magazine and in some countries billboards. All these put together arouse the sexual desire especially in young people.

    5. Parental example of permissiveness

    There are some parents who have a problem with the abuse of alcohol or drugs, and sex. They fail to control their sexual desires even when the children are present. This ends up planting bad seeds in the minds of the teenagers at an early age. To them, having sex becomes a normal practice regardless of age.

    6. Improper execution of sex education programs or practices

    Sex education programs are sometimes extended to the teenagers in a wrong manner with no professionalism at all. Sometimes when the teenagers are taught on how condoms are used; if the facilitator does not handle it well, he or she leaves the majority curious and wishing to practically exploit the new experience. On the other hand, in many homes and societies, sex education is a sensitive aspect. It could leave the children or teenagers to discover things on their own.

    7. Alcohol and Drugs

     There is increasing misuse of drugs and alcohol by the teens and adults. The use of alcohol and drugs increases the drive for pleasure and increases the willingness to take risks by decreasingthe inhibitions and impairing good judgment. This has always resulted in making improper decisions.

    Impact of early sex and possible prevention measures

    The negative early sex affects the livelihood of the teens in the world as follows:

    – Unwanted pregnancy at early age resulting in school dropouts and poor

    school performance.

    – Increase of infected people (by HIV/AIDS and other transmissible diseases).

    – Increase of mortality rate which results in a smaller skilled population and

    labour force.

    – Poor cognitive development, social isolation and mental problems like anxiety

    and depression.

    – Increase of the population growth resulting in poor feeding, malnutrition and

    the existence of street children.

    Possible solutions to prevent early sex and related consequences:

    – Sexual education should be incorporated in school curriculum to avoid early

    age sex.

    – Peer group awareness between the governmental and non-governmental

    agencies, which should organise training on how to prevent unwanted and

    early pregnancies.

    – Promoting cultural clubs and discussions about sexual abstinence.

    – Sensitize about sex abstinence by educating about the negative effects of sex intercourse 

    – Use of condoms in times of lack of abstinence.

    – Abstain from sexual activities or be in a long-term mutually monogamous relationship with an uninfected partner.

    10.8.2. The impact of HIV/Aids and STDs on social economy development of country and possible prevention measures

    HIV/AIDS is made of two abbreviations (HIV and AIDS). They stand for: HIV-Human Immune deficiency Virus and AIDS is Acquired Immune Deficiency Syndrome. HIV spreads primarily by having unprotected sex (including anal and oral sex), contaminated blood transfusion and hypodermic needles and from mother to child during pregnancy, delivery, or breastfeeding; whereas STDs are sexually transmitted diseases. The methods of prevention include safe sex, needle discharge programs (disposing used needles/syringes), treatment of those who are infected, and male circumcision.

    The impact HIV/Aids and STDs on social economy development of country

    The following are the main impacts of HIV/Aids and STDs on social economic development:

    – On the level of the household, AIDS causes the family members to spend all their income on healthcare. The people who would be working and supporting the family members become bed-laden, yet the medical expenses accumulate and the end result is terrible poverty. Children fail to go to school since much of the money is used for healthcare and the funerals.

    – Reducing the resources available for public expenditures such as education and health services. That is, there is increased pressure on the state’s fund since such diseases must be eradicated or contained to moderate or controllable levels. Such affects the economic development of the country.

    – The tax base is reduced since there are increased expenditures by the homesteads. Much money is spent on treating the sick and looking after AIDS and STDs orphans.

    – The increased mortality in the regions affected by HIV/Aids results in the occurrence of a smaller skilled population and labour force.

    – If the economic conditions are not good, a person with HIV/AIDS or STDs may decide to become a sex trade worker to earn more money. As a result, more people become infected with HIV/AIDS or other STDs.

    – Poverty increases as a result of HIV/AIDS. This has undesired impact on the productivity and it is worsened by the loss of the lives of the economically productive members of the society. In general, HIV/AIDS adversely affectsthe production and the productivity in all the sectors of the economy, most notably agriculture, manufacturing and service industry.

    – Considering the impact of HIV/AIDS on women and their roleespecially in food production, food security and export, it could create the risks of having malnutrition and undernourishment. 

    – HIV/AIDS morbidity and mortality have affected negatively the efficiency and effectiveness of the labour force in various work places. It is a result of increased absenteeism, increased staff turnover, loss of skills and experienced staff, and the declining of staff morale. Thus, it lowers the productivity of the labour.

    Possible HIV/AIDS and STDs prevention measures

    – Get tested and treated. It is very important that your partner is also tested and treated.

    – Use a new condom for every act of sexual intercourse throughout the entire sex act (from start to end). – Wrap the condom in a tissue and throw it in the trash where others won’t handle it.

    – Have less risky sex. HIV is mainly spread by having sex without a condom.

    – Limit your number of sexual partners. The more partners one has, the more he is likely to have a partner with HIV or whose HIV is not well controlled or a partner with sexually transmitted diseases.

    – Do not engage in unprotected sex unless you know your partner is not infected with HIV or STD.

    – Do not have sex with people who use intravenous (IV) drugs.

    Application activity 10.8

    (1) Discuss the impact of HIV/AIDS and STDs on the economic development of a country

    (2) Assume that you are appointed as a specialist in charge of epidemic diseases and prevention in the Ministry of Health.

    Propose the possible prevention measures that should be taken by the Ministry of Health to control HIV/AIDS and STDs in the country.

    10.9. Migration

    Migration is the movement of people from one place to another with the intentions

       Learning activity 10.9

    When her husband left her alone with four children, Amina decided to leave her home country of Nigeria to seek a better life for her family. Unfortunately, Amina and her children felt victims to one of the smuggling networks that operate between the Horn of Africa and the Arabian Gulf. They were held captive for three months by a gang which tried to get money from Amina’s family. Eventually, after a dramatic rescue, Amina and her children were referred to IOM Yemen’s Migrant Assistance and Protection team. They are now receiving food, water and shelter. Thanks to IOM’s assistance.

    1. Identify the type of migration explained in the above passage.

    2. With reference to the passage above, explain the causes of migration.

    Migration is the movement of people from one place to another with the intentions of settling permanently or temporarily in a new location.

    10.9.1. Types of migration

    There are various types of migration. It is very important to note that migration has often been classified into various categories or types. Below is a detailed description of the main types of migration:

    Internal migration: This is the movement of people within the country. It is further subdivided into rural-urban, urban-rural, rural-rural, urban-urban migrations.

    International migration: This is the movement of people from one country to another. For example, if a person leaves Rwanda and settles in the USA, this type of migration will be called international migration or external migration. In this case, the Rwandan society will refer to him or her as an emigrant while in the USA he or she will be classified as an immigrant.  Immigration is the coming of people from one country to another or leaving one place such as a village to another.

    On the other hand, emigration occurs when people leave their mother countries or a place of residence to another. In the example used above, the act of leaving Rwanda and settling in USA, within the country of origin the situation will be known as emigration. It is very important to note that, an emigrant is the out-going person that has migrated to another place or country.

    Permanent migration: This is the type of migration that involves the movement of people from one place to another without theintention of coming back to the source area.

    Temporary migration: This type of migration involves the movement of people or person from one place to another but with the hope of returning back to the source area.

    Voluntary migration: This is when people move from one place to another out of their own will without being forced. It is their choice to move.

    Involuntary migration: This is when people are forced to move from the areas of origin. The good example is the refugees who are forced to leave their homeland because of the war.

    10.9.2. Causes of migration

    The following are some main causes/factors of migration:

    – Technology factors:The people may move to places where there is advanced and more sophisticated technology. This is one of the common causes of migration in developing countries, where the influx of people go to the developed world.  

    – Economic reason: The lust (desire) for virgin lands for cultivation, the depressed economic resources from the motherlands, force people to migrate. This is because people want to settle in the area where the conditions reflecting the economic prosperity offer greater employment potential or opportunities.

    – Underemployment and unemployment: These force people mostly the youth to leave their homes to the places and countries where the employment opportunities are abundant. 

    – Overpopulation: An excess of the population in an area in relation to the resources and technology available is known as overpopulation, and it can force people to migrate to another area in their search for virgin places.

    Social and religious causes: The human desire to stay, work and enjoy life with the people of his or her ethnic, social and religious groups, is also an important cause of migration.

    – Political policies: There are policies established by the government such as the construction of a given infrastructural facility in a given area,which may result in forcing people to leave and resettle somewhere else for the erection of this large-scale public system. Example: the establishment of the New Airport in Bugesera, in Rwanda´s Eastern Province, where homesteads were paid and resettled in other places.

    – Peer influence: Some people may move from one place to another as a result of influence friends or relatives. This is the main cause of migrations amongst the youth.

    – General rise in the aspiration: People move from one place to another in search for employment opportunities. This is usually concerned with the working class or professionals who are always in search for better opportunities, jobs with high salaries and working conditions.

       – Wars: This factor has caused many people to move from one place to another in search for peace and security. People are migrating to Europe from Libya, Syria and Southern Sudan among others. War has been an important cause of human migration. For instance, the two world wars I&II. World War I (19141919) displaced six million people and World War 2 (1939-1945) displaced 16 million people.

      – Love for adventure: There are people who are naturally interested inadventuring new places. Having holidays or recreation in distant and new places; for instanceEuropeans coming to Africa (Rwanda and East Africa).

    10.9.3. The effects of migration in the world

    There are both positive and negative to areas of origin (source are) and destination (Receiving area) of migrants. They include the following:

    Positive effects to areas of destination

    – Simplifies easy exchange of ideas among people of different countries such as religious beliefs, technology, culturalvalues etc. which are essential for the development of the country.

    – Provision of cheap labour force to the receiving areas. For example, immigrants in the UK and Japan provide Low-priced labour, thus adding to the growth of industry, agriculture and service sectors.

    – Migrants provide security, for instance UK immigrants serve in security firms. They look after banks, shopping centres, suburban areas and in National security services; therefore, defending the country’s interests.

    – Immigrants are a source of revenue to central governments through the payment of Visa fees, entry fees and work permits in the receiving countries. Such revenues are used to develop the economies of the receiving countries.

    – Immigrants provide ready market for produced goods such as electronics, textiles and food stuffs.

    Negative effects to areas of destination/recipient area

    – Migration has led to the spread of diseases from the origin place to the receiving areas. Such has often led to the loss of lives and poor standards of living by the people who are infected with diseases.

    – High crime rate, for instance robbery and terrorism in Nigeria, Somalia and Southern Sudan, which has compromised the standards of living, security and sometimes resulting in the loss of lives.

    – Development of Slummy areas with undesirable results; for instance in Kigali, Bujumbura, Kampala and Nairobi. Such a case boosts poor sanitation conditions which result in ill-effects.

    – The large influx of immigrants in a given area or country results in the congestion on roads, delayed provision of services, and jam-packed markets. For example:This is what is happening in Kampala, Nairobi and Kigali cities. 

    – High unemployment cases are being created. The immigrants flock into the country and begin to offer cheap labour. Usually, these are highly skilled and professional than the local natives. It therefore lowers the standards of the natives who eventually may find it hard to make a living.

    Positive effects on the areas of origin

    – Reduced conflicts, for instance political emigrants from Iraq, Southern Sudan, Syria, Nigeria and Burundi etc. It creates some peace in the source areas which results inpolitical stability.

    – Decreases pressure on land and other resources, for example in China and India people are encouraged to migrate to other countries and export human resources,which leaves behind less pressure on the resources. This is economically important since they always send money back home.

    – Vanishing of criminal rate, for instance thieves, thus boosting the people’s standards of living. That is to say, criminals migrate to other areas or countries leaving peace and security in the places of origin.

    Negative effects on the areas of origin

    – Insufficient market for goods, which affects various economic sectors engaged in the production of goods and services. The mostly affected ones are the industrial and agricultural sectors. It thus leads to the under development of rural areas.

    – There are limited investments established in the source areas. For instance, lack of standard hospitals, advanced research centres and modern markets.

    – Low agricultural productivity in the rural areas due to the active youth that run away for urban centres, in search for employment opportunities, leavingbehind old people whose efforts cannot yield a lot. Such a low output may cause the shortage of food.

    10.9.4. The measures to be taken to control migration            

    The following are some of measures to be taken to control migration:

    – Governments should encourage urban-rural migration in order to develop the rural areas.

    – Resettlement schemes should be established in the rural areas to cater for the homeless and landless people. Where resources allow, affordable housing systemsshould be put in place in the suburbs of urban centres. 

    – Improvement of the security to control high crime rate especially in the rural areas and slummy places. Such a situation will enhance investments.

    – Creation of markets for the locally produced goods in the rural areas as a way of improving the income of people. This can create or provide employment to local people.

    – Setting up rural electrification to develop the industries. Thus, improving the standards of living in the rural areas by getting people access to television, milling maize industries, and appropriate lighting systems during the night.

    – Improvement and provision of clean water and sanitation facilities in the rural areas. This can be done through the development of piped water, so that water based diseases like cholera outbreaks are minimized.

    – Improvement of communication and transport services in the source areas to boost the transportation of goods to the market centres.  

    – Setting up micro-finance projects in the source areas so that the provision of loans to farmers, businesses, and homesteads can improve the trading capacity and small-scale industries. When this is done,it can boost the standards of living of the population.

    Application activity 10.9

    1. With reference to examples, discuss the reasons for and the results of internal/ international migration. 2. Migration has occurred in Rwanda in the past; briefly discuss the causes and consequences of this migration to Rwandans

    10.10. Case Studies 

    10.10.1 The population of Nigeria, Gabon and Bangladesh

    Learning activity 10.10.1

    1. Draw sketch map of Nigeria, Gabon, and Bangladesh; and on each, mark and label the areas that are densely and sparely populated.

    2. Explain the factors responsible for the population growth in Nigeria, Gabon and, Bangladesh.

    Population of Nigeria

    Nigeria is located in Western Africa. It neighbors are the Gulf of Guinea, Benin on the West and Cameroon on the East. The country’s landmass extends from the Gulf of Guinea in the South to the Sahel in the North. Nigeria’s population is now estimated at 195,809,553 over a total area of 923,768 km² (in 2018), with the population density of 212.0 persons per square kilometre.


    (i) Nigeria’s population distribution

    The Nigerian population is not evenly distributed. In some areas, dense population is found. Such areas include  the Coast region, South East, South West, Abuja and Lagos; the moderately populated regions are Kano, Kaduna, Rivers state, Katsina, Oyo state, Kano and Sokoto; while the sparsely (low) populated regions include the middle belt, semi-arid region of North and the central parts of the country.

     (ii) Factors influencing population distribution (density) in Nigeria

    – The presence of favourable climate: This is experienced by the regions along the coast, in the South-West and South-East of the country. This explains why such areas are densely populated. On the other hand, areas with unreliable rainfall discourage the settlement; they are thus sparsely populated. 

    –  Fertile and well-drained soils in the South-East and South-West parts support the growing of crops. This has encouragedhigh population densities.

    – Favourable relief.  The relief in the South-East and South-Western is largely hilly and therefore favourable to the rapid population settlement.

    – The Southern parts of Nigeria are gifted with mineral resources especially oil or petroleum deposits. Minerals exploitation has attracted a dense population settlement due to employment opportunities in the area.

    – Inadequate water resources particularly in Northern Nigeria has led to sparse population while areas with enough water resources have encouraged a dense population.     

    – The powerful tribal groupingsuch as the Yoruba, Ibos and Hausa Moslems have glorified and preserved their traditional and cultural attitudes. The traditional attitudes preserved have forced these people not to move away from their traditional and ancestral areas; thereby causing a dense population.

    – Slave trade and internal slave raids by powerful tribes like Ibo and the Yoruba led to a low population in the middle belt.

    – Government policy has encouraged a wider spread of settlement by developing transport, mineral resource exploitation, power supplies and urbanization leading to a high population in some places.

    – The process of large-scale urbanization, especially in the South-Western (Lagos, Ibadan), has also contributed to the high rate of population growth in Nigeria.

    (iii)  Nigeria’s population problems

    – Unemployment challenges in Nigeria due to the densely populated SouthEast and South-Western agricultural regions. Many people in the rural areas of Nigeria still live under the poverty line.

    – High rate of rural-urban migration from the densely populated regions to the country’s large urban centres such as Logos, Ibadan, Kano, Sokoto. This has resulted in numerous urban population evils.

    – Growth of slummy areas and the general absence of housing facilities.

    – Congestion and lack of space for the expansion of large Nigerian cities.

    – Moral decay in the form of prostitution, high crime rate and robberies. 

    – Challenges of the population explosion on land specifically in the South-East and South-West. This issue has resulted in a number of ecological problems such as severe soil erosion and deforestation. Severe soil erosion has led to land degradationand the decline in crop yield. Hence, famine in the country is a common experience.

    – There are generally inadequate social services such as hospitals, schools, clean water, and energy resources in most of the rural areas. Where such facilities exist, they are highly overstrained, thereby causing great shortage. 

    – Nigeria still experiences a challenge of a high dependency ratio and low life expectancy.

    (iv)  Solutions to the problems facing population growth in Nigeria

    – Family planning techniques and education are the main tools to check the problem of high population growth rates.

    – The Government should continue to develop other sectors such as industry, mining, trade and commerce in such a way that employment opportunities are created.

    – The Nigerian Government should continue investing in the establishment of social infrastructures. This shall help in making the upcountry self-reliant in the industrial and agricultural sectors.

    – The government of Nigeria should encourage and promote vertical expansion through setting up skyscrapers in Abuja and Lagos. Therefore, horizontal expansion of large cities should be discouraged.

    – The local government should constantly develop the under populated regions. Thus being engaged in the establishment of irrigation farming and mineral exploration as well as encouraging migration into such states which are sparsely populated.

    Population of Bangladesh

    Bangladesh is one of the world’s most densely populated countries. The total population is 166,325,373 (2018). The total area of Bangladesh is 144,000 km2 and the population density is 1,155.0 per km2. The most populated areas are found along river Bengal.

    (i)  Factors influencing population distribution in Bangladesh 

    – Lowland plains: the flat river valleys, deltas, and volcanic areas with fertile soil tend to have high population densities. Mountainous areas with steep slopes and poor quality soil tend to have low population densities.

    – Climate:Areaswhich are very dry, very cold or very wet, tend to have sparse populations whereas, areas which have a moderate climate with evenly distributed rainfall or with monsoon type of climate have denser population.

    – Soil: Areas with fertile soils encourage successful agriculture. This tends to attract high population densities. However, areas with poor quality soils discourage agriculture which, in turn,causes the latter to be sparsely populated.

    – Water supply: Water is essential for the human survival and development. For this reason, the areas which have sufficient water tend to have dense population compared to those having irregular water supply.

    – Natural resources: Areas which are endowed with natural resources such as oil and coaloften have higher population densities than the ones which don’t have. 

    – Pests and diseases: These may affect the population density as people try to avoid the areas where pests and life threatening diseases prevail. 

    – Industrialisation: Areas in which the manufacturing has developed tend to be densely populated. This is because of the availability of employment opportunities associated with such areas.

    – Availability of transport and communication networks: Areas with welldeveloped transport infrastructures and links through road, rail, shipping, canals and air tend to be densely populated. On the hand, areas with poorly developed forms of transport discourage the settlement, and thus become sparsely populated.

    (ii) Population problems in Bangladesh

    – Traffic jam: This is one of the most challenging problems in Bangladesh. It comes as a result of many people living in the cities and urban centres of Bangladesh. This is at worst levels during the rushing hours.

    – Environmental degradation: Numerous people are landless and forced to depend on cultivating flood-prone lands. With the use of poor farming practices, the soils have been degraded, soil erosion reaching its undesired levels and forests cleared in favour of farming. All these are attributed to the population explosion in Bangladesh.

    – Health problems:Manypeople in Bangladesh suffer from various diseases. For example, several children in this country are blind because of poor nutrition. 

    – Limited social amenities:   The large number of population has resulted in increased pressure on the social facilities. Bangladesh faces a challenge of social amenities like enough hospitals. In relation to this, the patient to doctor ratio is very high. For instance, one qualified doctor for every 25,000 people on average.

    – Housing problems: The rapid increase in population in Bangladesh has resulted in a serious shortage of residential houses. This has led to the growth and development of slums with poor housing conditions mostly in the outskirts of urban areas. 

    – Poverty: Bangladesh is a poor country with a rapid growing population. This implies that an increasing population in a country puts pressure on the national funds and limited resources. The case in Bangladesh has resulted in persistent poverty among the country´s citizens.

    – Overcrowding especially in urban areas like Dhaka, has caused serious problems like noise, pollution, traffic jam, easy spread of contagious diseases and increased crime rates in these areas.

    – Scarcity of food:Every year Bangladesh imports huge quantity of food from foreign countries to cater for the increasing food demand. This has increased government expenditures.

    (iii) Ways of controlling population in Bangladesh

    – Discouraging early marriage: This can be done through encouraging formal education.  Education may prove to be more successful in preventing child marriages. This is so because people spend more time in educational institutions. In addition to this, educated people are well informed about the dangers of high population.

    – Use of family planning methods: The use of contraceptives and other family planning methods can control the birth rates which eventually reduces the population growth rate in Bangladesh.

    – Mass education:  People can be sensitized about the threats of having big families. Hence encouraging them to have manageable families.

    – Introduction of sex education in schools. This is meant to provide awareness to the youth about the dangers of early marriages and uncontrolled population growth.

    – Providing incentives to families that have small and manageable families. For example, over taxing the families with large members. This can be an efficient measure in combating the challenge of rapid population growth in the country.

    – Paying some money to the people with few children or free and discounted education for the people with a single child.

    The population of Gabon

    The population of Gabon is estimated to be 2,069,885 people (2018). Its total area is estimated to be 257, 670 km2. This makes its population density to be 8 people per km2.

    (i) Factors influencing population distribution in Gabon

    – Drainage: Low-lying areas that are prone to periodic flooding and water logging have sparse population. Such areas are unsuitable for agriculture and other economic activities, therefore, push away people who would otherwise settle there. On the other hand, well drained areas like gentle sloping zones encourage high population densities because they are suitable for the settlement and agriculture.

    – Vegetation:Forested areas have wild animals, disease vectors and discourage the human settlement and other economic activities. However, areas which are not densely forested encourage the population distribution because there are less incidences of pests and diseases attacks. 

    – Climate: The cool and wet areas are densely populated because they are suitable for farming. Hot dry areas have sparse population because they are unsuitable for farming.

    – Relief: Mountainous and hilly areas have low temperatures and rugged terrain which discourage the settlement or development of infrastructures.  Plains and gently sloping areas are usually densely populated because they are suitable for the settlement and other economic activities. Flat depressions, flood plains and low-lying places are sparsely populated because of poor drainage which causes flooding in the wet season.

    – Soil fertility: Fertile soils are suitable for agriculture thus attracting large population. On the other hand, areas that have infertile soils do not enhance economic activities like farming, hence a sparse population prevails.

    – Pests and diseases:Areas that are infested with pests and disease-carrying vectors discourage the settlement since the conditions are unhealthy for both livestock and human beings, leading to sparse population. However, areas which are disease free, are favourable for the human settlement and agricultural activities, hence high population densities.

    – Historical factors: Gabon,like other countries in West Africa, faced slave trade which led to some people to be taken away by the slave masters and colonial rulers of the time. Some Gabonese had to run away from their cradle land in search for peace and safety. These resettled far away from homeland and never came back. Such a situation resulted ina low population density in the source areas and increased population growth in the receiving regions.

    – Tribal conflictsand wars have led some places to be sparsely populated. This is sobecause many people move to other places where they are assured of peace and safety, especially in the neighbouring countries.

    – Economic factors like the exploitation of natural resources (Forestry and mining) attract a large population of job seekers for paid employment. In areas where such a situation exists, the population becomes higher than the regions that have no such activities.

    – Infrastructures and industries in Libreville pulled many people from the rural areas to settle in industrial and urban areas.  

    – Political factors and government policies have led to the resettlement of people from the gazetted conservation areas like national parks and forest reserves. Therefore, such areas have a sparse population.

    (ii) Problems associated with the rapid population growth in Gabon

    The population of Gabon faces various problems as follows:

    – Poverty has become a characteristic associated with the rapid population growth of Gabon. This has led to under development of economic activities in the rural areas. Farmers in this case cannot afford to purchase better farming tools.

    – Rural -urban migration: Many young people (the Youth) are running away from the rural areas to Libreville in search for jobs and leaving old people (whose productivity is low) in the rural areas. 

    – Poor government policies that are not supporting the development of rural areas through establishing social services such as schools, industries, health centres and other infrastructures. 

    – Pollution of the land is a problem in Gabon’s growing urban centers due to the industrial and domestic garbage.

    – There is likely to be a problem of dependency on theimportation of manufactured goods, foreign aid and social services.

    – Rampant deforestation: Most Gabonese use charcoal and fire wood, which leads to thecutting of trees in large numbers.

    – Poaching: There is illegal procurement of protected wildlife such as fish, game, logging or plant collecting.

    (iii) Possible solutions to the problem of high population growth in Gabon 

    – Family planning techniques and education are the main tools that the Government can use to check on the problem of high population growth rates.

    – The Government should continue to develop other sectors such as industry, trade and commerce as a way of increasing employment opportunities to the increasing local population.

    – The Government should allocate huge or reasonable amount of funds to the development and establishment of social infrastructures. Such an act would assist the masses through having accessibility to a better life style.

    – The government of Gabon is encouraging and promoting the vertical expansion through setting up skyscrapers in Libreville, discouraging horizontal expansion of other large cities.

    – The Local government constantly develops her under populated regions in the form of establishing irrigation farming, mineral exploration and migrations into such states that are meagrelypopulated.

    Application activity 10.10.1

    3. Analyze the factors leading to thepopulation explosion and suggest possible ways of reducing population problems in Nigeria or Gabon.

    4. Describe the impact of rapid population growth in Bangladesh.

    10.10.2.The population of Germany, USA and China

    Learning activity 10.10.2

    Despite a drop in the country’s growth rate, Germany‘s population is now estimated at 82.29 million (in 2018).  The country has a population density that stands at 227.9 persons per square Km as per March 2018. This makes Germany the 17th most populous country in the world. It is also the largest country in the European Union. The total area of Germany is 357,021 km2.

    5. Referring to the above text, identify the ranking position of Germany at world level in relation to the Population density.

    6. Using Geographical resources, draw a sketch map of the USA and on it mark and label the densely, moderately and sparsely populated regions.

    7. Using Geographical resources, explain the factors responsible for the population growth in Germany, China and USA.

    Population of Germany

    Germany is located in Western-Central Europe. The country is bordered by Poland, Denmark, the Czech Republic, Austria, France, Switzerland, Luxembourg, Belgium, and the Netherlands. Germany is the most populous country in the European Union with an estimated population of 82.29 million people in 2018. It ranks 17th populated country in the world.


    The population structure represented by the above pyramid is divided into three main age groups. These arethe population under 15, between 15 and 64 and aged population-over 65-year-old. Germany’s population pyramid displays a contracting structure. This type of pyramid is more common or associated with highly developed countries, known for having low birth and death rates. Usually,the countries with such kind of population age groups have long life expectancy due to improved standards of living they enjoy. Such people have access to high level education and assured of better and affordable health care.

    (i)   Factors influencing population distribution in Germany

    The factors that influence the population distribution in Germany are briefly short listed hereunder:

    – The availability of enough and sufficient arable land that has supported large numbers of people to settle where they wish to be.

    – The country has vast forested areas which have supported the growth and development of Lumbering and other associated industries such as ship building, pulp and paper. The areas where these economic activities exist have become the centres of attraction for large numbers of people.

    – The availability of transport and communication facilities. Areas that are assured of having such facilities attract many people to settle there. While, on the other hand, where they do not exist, such areas become sparsely populated.

    – Historical factors. After World war II, many countries sent their technicians to rebuild the country of Germany. These increased the population of the country. At the same time, those who had taken refuge elsewhere started coming back.

    – The country has a well-developed sector of urbanization. There are many cities and towns in the country. These have attracted many young and old people from the rural areas to urban centres. 

    – Drainage has also played a great role in influencing the distribution of the population in Germany. Areas with better drainage have been settled by many while poorly drained areas have discouraged the settlement. 

    – Relief and landforms: Lowland plains, flat river valleys, deltas, and volcanic areas with fertile soil have played a role in attracting many people from all over the country of Germany to settle there.Mountainous areas with steep slopes and poor-quality soil tend to have low population densities.

    (ii)   Population problems in Germany

    – There is increased congestion as a result of several cars, buses, etc. on the road.

    – There is an increase in prices of the commodities which has resulted ina high cost of living. The housing services are costly.

    – Increase in air, noise and water pollution; for instance open sewage points, fires, etc.

    – A high rate of environmental degradation in the form of pollution or poor disposal of wastes.

    – Development of Slum areas particularly in large cities like Ruhr and Berlin.

    (iii)  Germany can address population problems in the following ways:

     – Improve the work-life-balance for women and men (including flexible work schedules and high quality day care).

    – Implement an immigration policy that meets the requirement of the German labour market.

    – Adapt its structures and institutions to a declining and ageing population.

    The population of the USA

    The Population of theUnited States of America is estimates 326,903,782 people. The population density in the United States is 36 persons per Km2 and the total land area is 9,147,420 Km2.

    The USA’s population is unevenly distributed. Some areas are densely populated where the average densities is over 100 persons per km2.  For instance, New York, the Great lakes region, much of the Eastern part, Los Angeles and San Francisco. The moderately populated areas are Chicago and Birmingham where the average density is between 10 -100 persons per km2 and the central. Much of the Western parts of USA are sparsely populated where the average density is less than 10 persons per km2.