- UNIT 1: CROSS SECTIONS AND SKETCH MAPSKey unit competence:By the end of this unit, I should be able to draw cross sections and sketch maps by
reduction or enlargement.Introductory activityUsing the previous knowledge and skills acquired in S4 and S.5, study the extract
of the topographic map of Rwanda provided below to answer the following
1. Use the above map to measure the distance from X to P
2. Describe the relief on the map above
1.1. Contour, contour interval and importance of contours
Learning activity 1.1
1. Explain the meaning of the term contour
2. Discuss the reasons why contours are important in map work
A contour is a line drawn on a map joining all the places with the same height above
sea level.Contours cannot cross each other because each has its own fixed height and they
can be close to one another in case of steep slopes. Contours are drawn at constant
intervals known as the contour interval (CI). This is also called Vertical Interval (VI).
Contour interval is the difference in height between two adjacent contour lines.
Contours are labeled from the lowest to the highest altitude as shown below.
In the study of geography, contours are important to analyze the landforms:
- They help to identify landforms like hills, plateaus, mountains and valleys on a
topographic map by analyzing the contour patterns formed by contour lines.
- Hills/mountains on a topographic map are shown as concentric rings of contours
with the highest values in the middle.
- In cases where contours are very close to each other, the relief of the area is steep.
When contours are separated by a wide space that shows a flat land.Application activity 1.1
Use the topographic map provided below to describe contours represented on
the map and their interval.MAP OF KARONGI
1.2. A cross-section
Learning activity 1.2
Use the topographic map provided below to identify flat slopes, gentle slopes
and steep slopes
A cross section is a topographical profile drawn between two points taken along a
straight line. It normally shows changes in relief of the area indicated by two points
on a topographic map.
When drawing a cross section, the following steps are followed:
• Determine the beginning and the end points of the section.
• Draw a straight line lightly in sharpened pencil from one point to another.
• Place a piece of paper with a straight edge along the pencil line.
• Mark the contour values and other important information like rivers,
settlements, roads, on the paper.
on the cross section.
• Construct a frame with two vertical lines whose baseline is equivalent to the
length of the line between the two points marked on the map.
• Join all the points with a curved line following the dots on the paper to decide
the bends of the line.
An example of drawing a cross section is given on the map below. The area of study
is represented by the line between point A and point B.
• Transfer the information on the paper to your graph paper and mark the point
To do this exercise, consider the following steps:
• Place a piece of paper on a map above along the line marked A and B
• Mark all the contours heights on the paper as shown below
The map below indicates further steps followed in recording contours on paper.
These contours indicate elevation and distances in two dimensions.
• Draw two vertical lines at each end of the horizontal line, these will represent
the Y -axes where the mark off the vertical scale will be made.
• Label point A and point B on the other end of the horizontal axis.
• Use the information on the paper to mark the highest and lowest heights
marked along the line A and B.
• Label the horizontal axis by writing the horizontal scale
• Label the vertical axis by writing the vertical scale.
• Write a title of the cross-section: cross-section from point A to point B
• Join all the dots on the graph using a free-hand line.
• Make sure the cross-section line links to points A and B on the vertical axes.
Application activity 1.2
Use a topographic map provided below to draw a cross section between point P and C.
1.3. Determining vertical exaggeration, the gradient, amplitude and inter-visibility
Learning activity 1.3
1. Using different types of maps identify and explain different types of scale.
2. Make research and explain the meaning of these terms:
a) Vertical exaggeration
1.3.1. The vertical exaggeration
The vertical exaggeration is the relationship between the vertical scale and horizontal scale. It is calculated after drawing a cross section.
Using the cross section drawn in figure showing a complete cross-section between points A and B presented above, vertical exaggeration is calculated as follows:
0.5 cm = 20 m0.5cm = 2000cm5cm = 20,000cm1cm = 4000cmTherefore, 1cm represents 4000 cm
- To determine the vertical scale, refer to the scale given when asked to draw a cross-section, e.g. 0.5 cm represents 20 m. This scale needs to be converted into centimeter units to be used in the formula.
The vertical exaggeration (VE) = 12.5 times1.3.2. The gradientGradient refers to the steepness of a slope between two places expressed as a proportion between the vertical intervals (VI) and horizontal equivalent (HE).When two places are located at different heights (altitude), the difference in height between them is known as the vertical rise or the vertical interval (V.I).The horizontal distance is the distance between the two places represented on a map which is corresponding with the real distance on the ground.Gradient is calculated as follows:• Plot the two points on the map which are needed to determine the gradient. Name them for instance as A and B or X and Y.• Join those two points by a straight line.• Use the scale to measure the distance between A and B (H.E). Let us consider the distance to be 8 cm.• Calculate the actual ground distance using the map scale. If the scale of the map is 1:50,000 meaning that 1 cm on the map represents 50,000 cm on the ground. Therefore, the ground distance of the represented area would be 8 x 50,000 which 400,000 cm = 4000 m.• Calculate the difference in height between points A and B, using the contours. For example, the difference in height between A and B is 200 m.• The formula for calculating the gradient is:1.3.3. AmplitudeAmplitude refers to the difference between the highest altitude and the lowest altitude along the cross section.1.3.4. Inter visibilityInter visibility refers to whether one point on a map can be seen from another point.• A point is inter visible when it can be seen from another point, i.e. there is no higher land between the two points.• A point is not inter visible when there is higher land or some obstruction that blocks being able to see one point represented on a map from another point.• Inter visibility can be established by drawing an inter visibility line between two points on a cross- section.• Inter visibility can also be established by looking at the contour heights on a topographic map between two points to see if there are any higher areas of land blocking the view between these two points.• On the cross-sections in Figure 1.6 and 1.7 draw a straight line (an inter visibility line) between the two points A and B on each sketch.• In Figure 1.6, the inter visibility line is above the cross- section at all heights. This means that point A is inter-visible from point B.• In Figure 1.7, the inter visibility line is below the cross-section in one section. This means that point A is not inter visible from point B.When checking for inter visibility on a topographic map, join the two places with a straight light line using a pencil or place a ruler along a line between the two points. Check the heights all along this line to see if there is higher land blocking the view between the two points.Application activity: 1.3Use the figure below to calculate the amplitude of the area indicated1.4. Drawing sketch mapsLearning activity 1.4Move around your school and draw the site of the school compound in your note book and explain the steps followed in drawing that sketch map.A sketch map is a simple representation of part or whole of a sheet map drawn on a piece of paper without using a given scale.When drawing a sketch map, use the following procedures:• Identify and critically observe the area to be sketched on the map given.• Measure the edges of the map.• Make a frame by either reducing or enlarging the map as instructed.• Indicate both physical and human features as requested.• Provide the key for the sketch map.The following are examples of a map and a drawn sketch map of Gituza respectively:Application activity 1.4Make a field trip around your school and draw a sketch map of the nearby market place.1.5. Enlargement and reduction of a mapLearning activity 1.51. Draw a sketch map of your school, identify, mark and name the features found there.2. Use the same sketch map drawn in (a) above:i. To reduce it by 2 timesii. To enlarge it by 2 times1.5.1. Map EnlargementMap enlargement refers to the changing of the size of a given map to a bigger one. It becomes bigger depending on the number of times it is enlarged. For example, it may be decided that part of a map is enlarged, and its outline drawn.The following steps should be followed:• Identify an area of the original map or part of the map to be enlarged.• Measure the length and width of the original map or identified part of the map.• Multiply the length and width by the number of times the map is to be enlarged.• Draw an outline that has new dimensions.• Mark and label the features in their relative positions.• The scale also changes (becomes bigger).The following map of Africa has been enlarged by 2 times from the above map.1.5.2. Map reductionMap reduction refers to the changing of the size of the map to a smaller one. Below are the steps to follow for map reduction:• Measure the length (L) and width (W) of the given part or whole map. For example, L=11cm and W=10.8cm.• Divide the length and width by the number of times the given map is to be reduced or as directed by the demands of the question. For example, by 2 times.• Draw an outline that has the new length and width. For example, L=5.5cm, W=5.4cm.• Mark and label the features in their relative positions.• Use a key to label features in the map.• The scale changes (becomes smaller).An example of a reduced map can be seen below:Application activity 1.5Reduce by two the administrative map of Rwanda provided below and draw its outline.End unit assessment1. Explain how contours help to describe the relief of an area.2. Make a visit of the school compound; draw its sketch map indicating on it all observed physical and human features.3. Use the topographic map provided below and answer the following questions:i. Draw a cross section between point X and Y, make a class presentation.ii. Enlarge two times the area on the map south of the northings 85 and draw its outline.Files: 2URLs: 4
- On topographic maps, the horizontal scale is most of time the same 1: 50,000, i.e. 1 cm represents 50000cm.
- As all units have been converted into centimeters, insert these measurements into the formula.
Key Unit Competence:
By the end of this unit, I should be able to interpret photographs, video images and draw sketches by reduction or enlargement of the photographs.
In the previous unit, it was shown that maps are very important tools to indicate and to describe physical and human features. Describe other ways used in geography to show physical and human features.
2.1. Definition and types of photographs
Learning activity 2.1
Provide the difference existing between the two photographs provided below
A photograph is a picture of an object or environment taken by a camera at a particular time in a given place. Photographs are ways of recording geographical information. They enhance the understanding of reality. However, when a photograph is taken, some parts of the object or environment are seen while others may not be seen clearly. A hidden ground/area which cannot be seen by a camera when a photograph is taken is called a dead ground.
2.1.2. Major types of photographs
There are two major types of photographs: Terrestrial/ close or ground photographs and Aerial photographs.
1. Ground Photographs
Ground photographs: These are photographs taken from the ground level. They record exactly what a person would see if he / she was standing on the ground level. A ground photograph gives a horizontal view, great details of the landscape and covers a small area. There are two categories of ground photographs:
i. Ground horizontal photograph: This is a photograph taken when a camera is held horizontally to the ground.
ii. Ground oblique photograph: This is a photograph taken when the camera is titled at an angle facing the ground.
2. Aerial Photographs
Aerial photographs: These are photographs taken from aerial station using aircrafts, satellites, and other flying objects. They cover a wide area, features are greatly reduced, show the top of the object, do not show the horizon. There are two categories of aerial photographs:
i. Vertical aerial photographs: These are photographs taken when the camera is directly above (overhead) the objects or when it is perpendicular to the ground.
ii. Oblique aerial photographs: These are photographs taken when the camera is titled at an angle below 90 degrees.
Application activity: 2.1
Identify the types of the photographs A and B below and describe them
2.2. Sections of a photograph and interpretation of physical and human aspects
Learning activity 2.2
Observe the photograph below and answer the following questions:
1. Identify the physical and human features shown on the above photograph.
2. Indicate the respective parts where these features are found in the above photograph.
2.2.1. Sections of a photograph
From a horizontal perspective, photographs fall under three categories as indicated below:
• The foreground: It is the part of the photograph located nearest to the camera.
• The middle ground: It is the central part of the photograph.
• The back ground: It is the farthest part of the photograph that includes the horizon.
From vertical perspective, photographs are also divided in three parts: left, center and right.
Combining both horizontal and vertical perspectives, the photographs can be put into the following categories:
Categories of photographs depending on the position of photography
2.2.2. Interpretation of physical and human aspects on photographs and video images
Physical and human aspects on photographs and video images can be interpreted as follows:
a. Interpretation of physical aspects
i. Climate: Climate in a photograph is indicated by rainfall and temperature. Heavy rainfall can be observed by presence of dense forests and crops like sugar cane, rice and tea while high temperature may be observed by the presence of poor vegetation, people wearing light clothes etc.
ii. Relief: The features of the relief depicted on a photograph include mountains, hills, valleys, escarpments, plateaus and plains. A hilly or mountainous landscape is indicated by the presence of steep slopes, presence of terraces, snow and glaciers on the top. Plateaus and plains are identified by a uniformly flat land with sloping edges and pools of water or irrigated land. Wide valleys with meanders and flood plains also suggest the presence of plain land.
Relief on vertical aerial photographs can be interpreted by observing the following:
• Flat areas can be identifiable by the presence of meandering rivers, straight roads and gentle bends.
• Plateaus can be indicated by presence of flat topped hills.
iii. Vegetation: This is the plant life that covers the earth surface; it is both natural and artificial. When describing vegetation on a photograph, the aspects to consider are the type of vegetation whether grassland, scrub or thicket; the tree species such as baobab, acacia, eucalyptus; the density of the vegetation whether trees are close together or scattered; and the nature of the vegetation whether human made or natural.
iv. Drainage: Drainage is shown by the presence of water bodies on a photograph, such as streams, rivers, lakes, swamps, seas, and oceans. Others are man-made water features like wells, ponds, valley dams and boreholes. In photographs, drainage is interpreted in the following ways:
• Rivers appear with meandering channels with swampy vegetation along them.
• Swamps appear with luxuriant vegetation dominated by papyrus reeds.
v. Soils: The types of soils can be identified by observing the types of crops grown there because there are crops that grow well in specific types of soils, for example, tea and coffee grow well in fertile volcanic soils. Where erosion took place, the soils are exposed.
b. Interpretation of human aspects
Photographs and video images can be very useful in the interpretation of human activities such as:
i. Forestry: A forest is evidenced by the presence of both artificial and natural forests.
ii. Agriculture: Agricultural activities can be observed by the presence of food crops and cash crops as well as animals like cattle both exotic and traditional breeds.
iii. Transport and communication: Both transport and communication networks are evidenced by presence of motor vehicles, bicycles, roads, ships, airports, and communication facilities such as telephone lines and masts.
iv. Mining: This is shown by Open pits, people undertaking mining or a mineral processing plant show that there is mining taking place in that area.
v. Industry: Industrialization is shown by the presence of industries emitting smoke from huge chimneys.
vi. Trade or commerce: the commerce is evidenced by trading centers with congested buildings and at times presence of markets.
vii. Settlement: It is evidenced by the presence of houses in different patterns.
Application activity 2.2
Observe the photograph below and describe the physical and human aspects represented on it.
2.3. Drawing sketches of photographs by reduction or enlargement
Learning activity 2.3
Draw a sketch of the photograph provided below and explain the steps involved in drawing it.
A sketch of a photograph focuses on the identification, marking using symbols and labeling marked features in their relative positions. Sketching takes into account physical and man-made features and should reflect the proportional size of features.
To draw a sketch of a photograph by enlargement or reduction requires the following steps:
i. Draw a rectangle and a square of the size as requested on a piece of paper.
ii. Draw horizontal lines across the photograph by using a pencil to subdivide it into three equal sections. These will be the foreground, middle ground and background either reduced or enlarged as instructed.
iii. Draw vertical lines across the photograph by using a pencil. These will be left, center and right.
iv. Place the framework of a photograph onto the prepared rectangle or square. The framework could be the guider in placing the various features in their respective positions.
v. Enlarge or reduce the size of features and the frame as requested.
vi. When filling in the main features, it is better to start with the background or right by drawing the skyline as it appears on the photograph.
vii. It is better to place and label all important features either physical or human as they appear on the photograph, reduce or enlarge them as required.
viii. Choose a suitable title, key, orientation of a sketch. It is possible to put on a sketch other elements of a sketch map which are useful in reading and interpreting it.
Therefore, a sketch of a photograph can be enlarged or reduced as shown below:
Application activity 2.3
Draw a reduced sketch by 2 times of the photograph provided below and indicate all features represented on the photography.
2.4. Relationship between physical and human aspects on photographs and video images
Learning activity 2.4
Describe the relationship between physical and human features represented on the photograph below.
Some photographs and video images help in showing the relationship between human and physical aspects. The relationship between human and physical aspect is discussed basing on the photograph below:
i. Relief and transport: Transport routes occur on gentle slopes and avoid steep slopes and valleys since it is very expensive to construct roads in hilly areas.
ii. Relief and agriculture: On steep slopes, less agriculture takes place while on gentle slopes most agricultural practices are observed. The low lands are usually reserved for growing of vegetables, sugar cane, rice, and other crops that need enough water.
iii. Relief and settlement: Settlements are commonly found in gentle slopes and are few in steep slopes and valleys because of the problem of severe soil erosion and flooding in valleys.
iv. Drainage patterns and settlement: Settlement occurs in well drained areas and avoids lake shores or river banks because of floods and associated problems.
v. Drainage and transport: Transport routes are usually found in well drained areas. For example, roads cannot be constructed in swampy areas due to excessive water. Water transport occurs on water bodies like rivers, lakes, oceans and seas.
Application activity 2.4
Observe the photograph below and describe how physical features have influenced human activities in the area.
End unit assessment
1. Explain the key guidelines followed in drawing a sketch of a photograph.
2. Study the photograph provided below and answer the following questions:
a. Identify the economic activities taking place and describe their importance to the people living in the area.
b. Suggest ways of conserving the area in the background of the photograph for environmental sustainability.
Observe carefully the maps provided below and answer the following questions:
1. How many oceans do you find on map a
2. How many continents do you see on map b
3. How many continents do you see on map c
4. Explain the processes which led to the separation of the unique initial landmass into various continents as they appear today.
3.1. Concept and theories of continental drift
Learning activity 3.1
Make research using books and internet to explain briefly the theories related to the continental drift.
3.1.1. Concept of continental drift
The term continental drift refers to the study of causes and consequences of the distribution of continents and ocean basins. It is defined as a slow movement of the Earth’s continents towards and away from each other. The differential movement of the outer shell resulted into fragmentation by rifting, followed by drifting apart of individual masses of the broken outer shell.
3.1. 2. Theories of the origin and distribution of the continents and ocean basins
There are several theories of continental drift that were developed at the beginning of the 20th century. The following are the four main theories of continental drift:
• Alfred Lothar Wegener’s theory
• Maurice Ewing’s theory
• Harry Hammond Hess’ theory
• Frank Taylor’s theory
a. Alfred Lothar Wegener’s theory
According to Wegener’s theory, there was a breakup of the single super continent block called Pangaea“pan JEE uh”, which means “all land” into multiple continents, as they appear today, that moved apart in a process called continental drift. That movement took place about 200 million years ago. The map provided below fits together the continents whose breaking up resulted in today’s continents.
The theory of continental drift traces the origin and distribution of continents through five major steps:
i. The super continent Pangaea was surrounded by an extensive water mass called the ‘Panthalassa’ (pan means all and Thalassa means oceans) or the primeval Pacific Ocean. During the Carboniferous period (about 250 million years ago), the South Pole was near Natal (South African coast) and the North Pole was in the Pacific Ocean.
ii. In about 200 million years, Pangaea broke up to form Laurasia (North America, Greenland, and all of Eurasia north of Indian subcontinent), and Gondwanaland (South America, Africa, Madagascar, India, Arabia, Malaysia, East Indies, Australia, and Antarctica). These two blocks were separated by a long shallow inland sea called Tethys Sea.
iii. In about 145 million years ago, the drifting of the southern landmasses continued. India drifted northwards.
iv. In about 65 million years ago, Australia began to separate from Antarctica.
v. The present shapes and relative positions of the continents are the result of fragmentation of Laurasia and Gondwanaland by rifting and drifting apart of the broken landmasses following the formations of oceans and seas (see figure 3.24). South America separated from Africa, North America separated from Europe, while Antarctica, Australia, India and Madagascar formed a single unit with South America.
However, Wegener’s theory was initially criticized because he could not explain how solid continents have changed their positions. His theory has been revived by other researchers after discovering new evidences.
f. Maurice Ewing’s theory
Maurice Ewing confirmed the existence of Mid-Atlantic Ridge which is a mountain range extending the entire length of the ocean bed which is about 1000 km wide and rises 2500 m in height. Also, Ewing’s studies argue that rocks of this range were volcanic and recent in origin. Similar ranges were later discovered on other oceans’ floors.
g. Harry Hammond Hess’s Theory: Sea-Floor Spreading
The Seafloor spreading theory suggests that magma from earth’s mantle rises to the surface at mid-ocean ridges and cools to form new seafloor, which new magma pushes away from the ridge.
The Sea-Floor Spreading theory was put forward by an American Geologist, Harry Hess. Sea-floor spreading occurs along mid-ocean-ridge; when the tectonic plates slowly moves away from each other, hot magma from the mantle comes up to the surface. As magma cools by the seawater the rock forms a new part of the crust.
The interior of the Earth is in a molten (semi-fluid) state because of great heat resulting from radioactivity within the asthenosphere. This tremendous heat causes melting, or near-melting of rocks of the interior of the Earth. The molten rocks tend to rise from within the mantle in form of convection currents.
Material heated by radioactive elements in the earth’s interior slowly rise in the crust. This magma reaches the surface along the Mid-oceanic ridges and flows away from them, cooling and hardening to form the rigid lithosphere.
New lava emerging from the ridges attaches itself to the near solidified older lava plates and forces them to move laterally. Hess’s studies demonstrated that after millions of years the lithospheric plates will have moved thousands of miles by constant additions of new lava at their rear.
The leading edges were eventually forced to sink down into the lithosphere under the continental crust block thus forming deep ocean trenches along the edge of continents. In this “recycling” process, later named “seafloor spreading”, older sediments and fossils are carried off in the subduction zone, and continents are moved as new ocean crust spreads away from the ridges.
Hess explained how the once-joined continents had separated into the seven that exist today. The newest rocks were in the center of the ocean, and were still being formed in Iceland, and that the oldest rocks were those nearest to the USA and the Caribbean. He also suggested that the Atlantic could be widening by up to 5 cm a year. This process produced by mantle convection currents was named the “Sea floor spreading”.
h. Taylor’s theory
Frank Taylor’s theory states that the original Laurasia was located near the current North Pole, whereas Gondwanaland was located near the South Pole. Both landmasses radially moved to the Equator. Their collision would have resulted in the formation of folded mountains, such as Atlas, Alps mountain ranges and others.
He suggested that Laurasia and Gondwanaland were forced to move from their former positions because of the moon’s tidal attraction. According to this theory,the moon came very close to the earth during the cretaceous period.
This closeness of the moon to the earth exerted powerful tidal attraction, which pulled the landmasses from their polar position towards the Equator. Where there was resistance to the outward spread of landmasses, the crust usually would fold, raising mountain ranges in front, while resulting in stretches (troughs and basins).
The present basins of Southern Atlantic and Indian Oceans were formed in this way.
Taylor’s arguments about continental drift have however been criticized:
• The theory doesn’t clearly demonstrate how the causes of the movement of continents from their polar positions ought to have been from within the earth and not outside it.
• The theory was rejected because researchers of his time doubted how the moon could ever exert enough force to pull the huge landmasses (continents) as they are known today.
• Finally, Taylor doesn’t explain the formation of earlier fold mountains like the Caledonian system of SiluroDevonian times while explaining the possible formation of the fold mountains Atlas and Alps.
Application activity 3.1
1. Discuss the contribution of each researcher’s findings described in this section to the confirmation of continental drift.
2. Referring to the different theories of continental drift, explain why Taylor’s theory about Moon’s tidal attraction has been rejected.
3.2. Evidence of continental drift
Learning activity 3.2
Observe the map provided below and answer the following questions:
1. Describe the edges of the continents.
2. What suggests the distribution of the same animal and vegetation species over the different continents?
Many evidences of continental drift exist, but they can be summarized in four major categories:
i. Geological evidence
A good fit of edges of continents and similar rock structures are found on different continents. For example:
• East coast of South America and the Western Coast of Africa have good visual fits, both at the surface (1000 m) and depth (2000 m).
• Both Africa and South America are composed of rocks of varying ages and there is a convincing boundary joint across the two continents between Accra and Sao Louis in Brazil and, dividing Pan-African rocks and Elaurean rocks. This evidence constitutes what is commonly known as “matching geology”
• Parts of Appalachian Mountains of the United States of America are similar to those found in Greenland and Western Europe;
• The fact that rock particles have magnetic properties allowed geophysicists to reconstruct the position of the poles in past times and also the probable climatic lay belts of the past. From this, it appears that Southern Africa and South America lay within the Arctic circle of Permian and carboniferous times and that during the Triassic period, the continents had moved some 40° closer to the Equator.
ii. Biological evidence
There is similarity in the fossils and vegetation remains found on the eastern coast of South America and the Western coast of Africa. For example;
• Mesosaurus was small reptile living in Permian time (280 million of years before present); its remains have been found only in South Africa and Brazil.
• Remains of Glossopteris, a plant which existed when coal was being formed has only been located in India and Antarctica. These animals and plants could not have swum across oceans if continents were separated by water bodies, so continents must have been close together for them to occur on different continents which probably had a similar climate.
ii. Climatic evidence
Coal formed under warm and wet conditions was found beneath the Atlantic ice-cap, and evidence of carboniferous glaciation had been noted in tropical and central India. For example;
• Coal could not have been formed in Britain with its present climate.
• Peninsular India, Australia and Antarctica further prove the unification of all landmasses in one landmass (Pangaea) during carboniferous period.
• Groves curved on rocks by glaciers in the southern parts of landmasses forming Gondwanaland shown by arrows on the figure below provided evidence for continental drift.
iv. Geodetic evidence
Geodetic evidence has revealed that Greenland is drifting westward at the rate of 20 cm per year. This is one of the scientific evidences arising from measurement and representation of the earth that confirm the spread of the sea floor.
Application activity 3.2
1. Describe the rocks at the edge of the continents and show how all continents formed a unique block.
2. Using some examples, compare the fossils of animal species and vegetation species found on different continents by showing how they indicate the continental drift.
3.3. Effects of continental drift on the evolution of physical features
Learning activity 3.3
Make a research and describe at least four major effects of continental drift.
The continental drift has had many effects on the evolution of physical features but the most important are the following:
• Pangaea split apart into a southern landmass, Gondwanaland and the northern landmass called Laurasia; later the two super continents split again into land masses that look like present day continents.
• Continental drift has also affected the earth’s climate. The climate of different part of the world has changes throughout the year;
• Continental drift has affected the evolution of animals. The rearrangement and displacement of huge landmasses has helped create the diversity which we see present in modern day animals.
• Collision of earth crusts. The collision of the Indian subcontinent and Asian continent created the Himalayan mountain range, home to the world’s high-est mountain peaks.
• Formation of rift valleys. Rift valleys are sites where a continental landmass is ripping itself apart. Africa, for example, will eventually split along the western Great Rift Valley system.
• Continental drift is the major cause of earthquakes, volcanoes, oceanic trench-es, mountain range formation, and other geologic phenomenon which created the new landscapes on the earth’s surface;
Application activity 3.3
Explain the effects of continental drift on the evolution of physical landscape of the earth.
3.4. Plate Tectonics
Learning activity 3.4
Observe the illustration below and answer the following questions:
1. Identify the types of crust found on the map
2. Describe the difference between lithosphere and asthenosphere
3. Differentiate collision, constructive, and destructive processes
4. Determine the position of plate movements
5. Explain how convection cells cause the movement of plates
3.4.1. The concept of plate tectonics
The concept suggests that earth’s crust and upper mantle (lithosphere) are broken into sections, called plates that slowly move on the mantle.
The word tectonic comes from the Greek word ‘tektonikos’ meaning building or construction; this means how the earth crust is constructed. Therefore, plate tectonics refers to the deformation of the earth’s crust, because of internal forces, which can form various structures in the lithosphere.
The plate size can vary greatly, from a few hundred to thousands of kilometers across. Plates are moved by the energy originating from the earth interior. This energy is a result of convection currents which form convection cells. Tectonic plates are irregularly shaped slabs of solid rocks, generally presenting two types: Continental crust and Oceanic crust as shown on the figure below.
Tectonic processes include tension when plates diverge and compression when plates converge. These processes result in deformation of the earth crust. Tension causes fracturing and faulting of the crust while compression produces folds and over thrust faults.
3.4.2. Types of Plate Tectonics
There are two types of plate tectonics: continental plate and oceanic plate.
i. Continental crust is composed of older, lighter rock of granitic type: Silicon and Aluminum (SIAL).
ii. Oceanic crust consists of much younger, denser rock of basaltic composition: Silicon and Magnesium (SIMA). The major differences between the two types of plates are summarized in the table below:
Difference between continental plate and oceanic plate
3.4.3. Boundaries and movement of tectonic plates
i. Tectonic Plate boundaries
Boundaries of plate tectonic include the subduction zone, the mid-ocean ridge and the transform boundary.
• Divergent boundary (Mid-ocean ridge): It is an underwater mountain range which is formed when forces within earth spread the seafloor apart. It is created when convection currents rise in the mantle beneath where two tectonic plates meet at a divergent boundary, thus forming the oceanic ridge.
• Transform boundary (Transform fault): It is a boundary which exists between two plates that are sliding horizontally past one another, thus forming the transform faults (see the figure below).
• Convergent boundary (Subduction zone): This is the area where an ocean-floor plate collides with a continental plate and the denser oceanic plate sinks under the less dense continental plate, thus forming the oceanic trench.
ii. Tectonic plate movements
Plate movements include convergence, divergence and way past movement along the transform fault.
• Convergence is a movement whereby two crustal plates are colliding or one subsiding beneath the other. The margin where this process occurs is known as a destructive plate boundary. This boundary is a region of active deformation.
• Divergence is a movement whereby two crustal plates are moving away from each other. The margin where this process occurs is known as a constructive plate boundary. It initially produces rifts which eventually become rift valleys.
• Way past is plates’ movement predominantly horizontal, where crust is neither produced nor destroyed as the plates slide horizontally past each other.
The plate movements are characterized by the following:
• Due to its relatively low density, continental crust does not sink; but it is the oceanic crust which is denser that can sink. Oceanic crust is then formed and destroyed, continuously;
• Continental plates, such as the Eurasian plate, may consist of both continental and oceanic crust;
• Continental crust may extend far beyond the margins of the landmass;
• Plates cannot overlap. This means that either they must be pushed upwards on impact to form mountains, or one plate must be forced to downwards into the mantle;
• No gap may occur on the earth’s surface so, if two plates are moving apart new oceanic crust originating from the mantle is formed;
• The Earth is neither expanding nor shrinking in size. Thus, when the new oceanic crust is being formed in one place, older oceanic crust is being destroyed in another;
• Plate movement is slow and is usually continuous. Sudden movements are detected as earthquakes;
• Most significant land forms (folded mountains, volcanoes, insular arcs deep sea trenches, and batholith intrusion) are found at plate boundaries.
Major land forms resulting from plate movements:
3.4.4 Characteristics of plate tectonics
Tectonic plates are characterized by the construction and destruction of land forms at margins of plates. However, at some boundaries, the construction or destruction may not occur. These are called passive margins or conservative boundaries.
i. Constructive land forms
Constructive land forms occur where two plates diverge, or move away from each other, and a new crust is created at the boundary. They are formed in the following ways:
• This occurs when a continent ruptures and the two new plates move apart and create a new ocean.
• The crust is uplifted and stretched apart, causing it to break into blocks that become tilted on faults. Eventually a long narrow rift valley appears.
• Magma rises up from the mantle to continually fill the widening crack at the center (A) as presented on figure below.
• The magma solidifies to form new crust in the rift valley floor.
• Crustal blocks on either side slip down along a succession of steep faults, creating mountains.
• A narrow ocean is formed, floored by new oceanic crust (B) as presented on figure below.
• The ocean basin can continue to widen until a large ocean has been formed and the continents are widely separated.
• The ocean basin widens, while the passive continental margins subside and receive sediments from the continents.
• As the plates diverge, molten rock or magma rises from the mantle to fill any possible gaps between them, creating new oceanic crust.
• The magma initially forms submarine volcanoes which may in time grow above sea-level. Volcanic islands are created by the submarine volcanism at the vertical of oceanic ridge, e.g. Iceland (see the figure below).
ii. Destructive land forms
Destructive land forms occur where continental and oceanic plates converge. They are formed in the following ways:
• The oceanic plate that is denser is forced to dip downwards at an angle to form a subduction zone with its associated deep-sea trench.
• The sunk plate will melt and transformed into magma as the pressure and the temperature rise.
• The newly created magma will try to rise to the earth’s surface. Where it does rich surface volcanoes will occur. This process will either create a long chain of fold mountains (e.g. the Andes) or, if the eruptions take place off shore, an Island arc will be created(e.g. Japan, Caribbean).
iii. Passive or conservative margins
Passive continental margins are:
• The areas which are lacking active plate boundaries at the contact of continental crust with oceanic crust.
• The transform faults which are large cracks produced at right-angles to the plate boundary because neither land form is constructed nor destroyed.
Application activity 3.4
1. Describe SIAL and SIMA in terms of thickness, age, weight and nature of rocks
2. Explain the difference between convergent movement, divergent movement and way past movement
3. Describe the subduction, collision, spreading processes and give their effects and corresponding motions in relation to plate tectonic movements.
4. Explain the processes that lead to constructive and destructive land forms
3.5. Major plates and effects of plate tectonics
Learning activity 3.5
1. Make research using books and a printed hand out and represent on the world map the major tectonic plates.
2. Identify the effects of the plate tectonic?
3.5.1. Major tectonic plates of the world
The following are the major tectonic plates of the world:
i. The Pacific plate which covers a large part of the basin of Pacific Ocean.
ii. The Eurasian plate located between the northern mid-ocean ridge of the Pacific Ocean and the Pacific and Philippines Plates margins.
iii. The North American plate bordered by the eastern margin of the Pacific plate in the West and mid-ocean ridge of the Atlantic Ocean in the East.
iv. The South American Plate located between the subduction zone of Nazca plate in the West and the mid-ocean ridge of the Atlantic Ocean in the East.
v. The African plate located between the mid-ocean ridge of the Atlantic Ocean in the West and the mid-ocean ridge of Indo-Australian plate in the East.
vi. The Indo-Australian plate extends around the Australian subcontinent, between the Pacific plate and the African Plate.
vii. The Antarctic plate corresponds with the Antarctic continent around the South Pole.
viii. The Nazca Plate which is located between the Pacific plate and the South American plate.
However, several minor plates, about 20 have been identified (e.g. Arabian plate,Bismarck plate, Caribbean Plate, Carolina plate, Cocos plate, Juan de Fuca plate, Nazca or East Pacific plate, Philippines plate, Scotia plate among others).
3.5.2. Effects of plate tectonics
The following are the main effects of plate tectonics:
This is a series of vibrations induced in the earth’s crust by the abrupt separation and echo of rocks in which elastic strain has been slowly accumulating. This sudden violent shaking of the ground typically causes great destruction, because of movements of seismic waves within the earth’s crust.
Most earthquakes occur as the result of the sudden movement along a fault line between two adjacent tectonic plates. These have several impacts like landscape modification, destruction of houses, tsunamis, etc.
ii. A volcanic eruption
A volcanic eruption occurs when hot materials (molten materials) are thrown out of a volcano. Lava, rocks, dust, and gas compounds are some of these materials which are ejected out during volcanic eruption. Volcanic eruption take place when a plate moves over the top of another plate, then the energy and friction melt the rock and push it upwards.
Tsunamis are giant waves, often generated at destructive plate margins that can cross oceans. They occur when a sudden, large scale change in the area of an ocean bed leads to the displacement of a large volume of water and the subsequent formation of one or more huge waves.
When a major seismic tremor occurs underneath a body of water, the energy from that tremor is released into the surrounding liquid. The energy spreads out from its original site, traveling through the water in the form of a wave.Tsunamis have exceptionally long wave-length up to 10 km and can cross oceans at speeds of up to 700 km/hour but can sometime be imperceptible when their magnitude is low.
Application activity 3.5
1. Conduct your own research to identify the minor tectonic plates of the world and locate them geographically.
2. Apart from the distribution of the continent, what are other effects of plate tectonics.
3. Identify the major seismic and volcanic zones in the world and explain the impact of those natural hazards referring to the tectonic plates.
4. Our country, Rwanda, is in a region which is tectonic ally active and subjected to earthquakes events. The more documented earthquake is the one which occurred on 3rd and 4th February 2008. It occurred on Sunday about 09h31 with the magnitude of 6.1 and 5, and on Monday the 4th February 2008 and affected mostly Nyamasheke and Rusizi Districts, Western Province. 37 people died, and 643 injured including 367 traumatized. Many houses were destroyed in these two Districts where 1,201 families were rendered homeless:
5. Knowing the causes of the earthquake, explain how Rwandans can cope with it and its impacts and other resulting natural hazards.
3.6. The theory of Isostasy
Learning activity 3.6
1. Make research and explain the isostasy theory.
2. Explain isostasy based on the figure below.
3.6.1. Meaning of Isostasy
The concept of Isostasy comes from “iso” = equal, and “stasis” = equilibrium. It describes how various continental and oceanic crusts, stay in equilibrium over the asthenosphere. The following are the main characteristics of isostasy:
• By isostasy, the lighter crust must float on the denser underlying mantle.
• It explains how different topographic heights can exist on the earth’s surface.
• Isostatic equilibrium is an ideal which states where the crust and mantle would settle in equilibrium in absence of disturbing forces.
• Isostasy theory is concerned with vertical movements of plates which depend on lithospheric masses.
• The loading of crust by ice or sediments may cause the subsidence of lithosphere, whereas the discharge resulting from ice melting or erosion may cause the uplift of lithospheric compartment.
• The waxing and waning of ice sheets erosion, sedimentation, and extrusive volcanism are examples of processes that perturb isostasy.
• Isostasy controls the regional elevations of continents and ocean floors in accordance with the densities of their underlying rocks.
3.6.2. Main theories of Isostasy
There are two main theories which have been developed to explain how Isostasy acts to support mountain masses
i. Pratt’s theory: The theory stipulates that there are lateral changes in rock density across the lithosphere (crust). If the mantle below is uniformly dense, the less dense crustal blocks float higher to become mountains, whereas the denser blocks form basins and lowlands.
ii. Airy’s theory: According to Airys’s theory, the rock density across the lithosphere is approximately the same but the crustal blocks have different thicknesses. Therefore, mountains that shoot up higher also extend deeper base into the denser material beneath.
Both theories predict a relative deficiency of mass under high mountains. Airy’s theory is now known to be a better explanation of mountains within continental regions, whereas Pratt’s theory essentially explains the difference between continents and oceans, since the continent crust is largely of granitic composition which is less dense than the basaltic ocean basin.
Application activity 3.6
Referring to Pratt’s theory and Airy’s theory, explain the principle of Isostasy.
End unit assessment
1. What is the contribution of Wegner’s theory and others on the distribution of continents?
2. Basing on the knowledge acquired in this unit, explain the relationship between the earthquakes which occur in the region of the western rift valley of Africa where Rwanda is located, with the continental drift.
3. Using a map represent graphically the main tectonic plates of the world map.
4. Discuss the consequences of the plate tectonics on population in some specific areas of the world.
Key unit competence:
By the end of this unit, I should be able to demonstrate an understanding of different features resulting from external processes and their relationships with human activities.
Observe the photograph below and explain the processes that affected the rock shown
4.1.1. Types and processes of weathering
Learning activity 4.1
a. Making good use of the diagrams below explain the processes involved in both physical and chemical weathering.
b. Make a research and compare the processes of soil formation and the processes of weathering
a. Definition of weathering
Weathering refers to the process of disintegration and decomposition of rocks ‘in-situ’ into small particles by the action of weather and living organisms.
Agents of weathering: temperature, rainfall (water), wind, animals and plants (vegetation).
b. Types of weathering
There are three types of weathering namely physical or mechanical weathering, chemical weathering and biological weathering which cuts across each of the physical and chemical weathering.
1. Physical weathering
Physical weathering refers to the breakdown or disintegration of rocks, without any change in the chemical or mineral composition of the rock being weathered. Rocks disintegrate into smaller particles but maintain their previous chemical characteristics. Only the physical size and shape change. Physical weathering is mostly influenced by temperature changes.
Processes of physical weathering include:
i. Thermal expansion or isolation weathering
This process is caused by the changing of temperature ranges which causes differential heating of minerals forming the rock. When heated dark minerals expand, faster than others resulting in cracking and fragmentation of the rock.
This occurs when there is expansion of rocks during the day and contraction of rocks during the night due to repeated temperature changes. It is common in arid and semi-arid regions. This results into rocks of a few centimeters thick to start peeling off (breaking away) leaving behind exfoliation domes.
ii. Freeze thaw
This process also called frost weathering (or frost shuttering) occurs due to water that enters into the cracks of the rocks; this water freezes and expands exerting pressure within cracks. Water from rain or melting snow and ice is trapped in a crack or joint in the rock.
If the air temperature falls below freezing point, the water freezes and expands. As a result, the rock becomes weak and breaks. This process is common in cold regions, especially glacial, periglacial and high mountainous zones. The figure below shows steps from infiltration of water into the rock to the condensation within rock fissure which result in the fragmentation.
iii. Pressure release
The process of pressure release known as the unloading or dilatation weathering occurs when materials on top are removed by erosion. This releases pressure, which causes the materials below to expand and crack parallel to the surface.
iv. Salt crystallization
The process of salt crystallization weathering illustrated on the figure below occurs when saline water (or water carrying salts in solution) passes through cracks and joints in rocks. As it evaporates, the dissolved salts change into salt crystals. These crystals expand within cracks as they are heated up and apply pressure on the rock leading to its breaking up.
v. Shrinkage weathering
Some clayey rocks expand after absorbing water. For instance, there are some clays which swell when they absorb water during rainy seasons. This results in increase in volume. During dry seasons, they massively lose this water through evaporation and they contract. This process is known as shrinkage. This alternating expansion of these rocks during the wet season, and contraction during the dry season, creates stresses and later cracks the rock.
vi. Granular disintegration
This takes place almost in the same way as exfoliation except that in this type, rocks disintegrate into small particles called granules. It is produced either by differences in thermal expansion and contraction, or through the frost heaving process (congeliturbation).
2. Chemical weathering
This is a type of weathering which involves a complete change in the chemical and mineralogical composition of the rock resulting into the disintegration of rocks. It is common in areas which experience alternating wet and dry seasons.
The following are the chemical reactions that take place during weathering:
i. Oxidation: oxidation is one of the varieties of chemical weathering in which oxygen dissolved in water reacts with certain rock minerals, especially iron, to form oxides.
ii. Carbonation occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid.
H2O +CO2 H2CO3 (weak carbonic acid)
This reacts with calcium carbonate (the limestone) to form calcium bicarbonate which is soluble in water.
iii. Dissolution: Dissolution is one of the less important forms of chemical weathering, in which solid rocks are dissolved by water. When water (e.g. rainwater) mixes with carbon dioxide gas in the air or in air pockets in soil, a weak acid solution, called carbonic acid, is produced. When carbonic acid flows through the cracks of some rocks, it chemically reacts with the rock causing some of it to dissolve.
iv. Hydrolysis: Hydrolysis involves water combining with rock minerals to form an insoluble precipitate like clay mineral. Compared to hydration-a physical process in which water is simply absorbed – the hydrolysis process involves active participation of water in chemical reactions to produce different minerals.
v. Hydration: Hydration is one of the major processes of mechanical weathering, involving the addition of water to a mineral, causing it to expand and thereby initiate stress within the rock. For example the conversion of hematite to limonite. Once minerals have experienced hydration, they become more susceptible to the effects of chemical weathering, especially those of carbonation and oxidation.
vi. Solution: is a process in which the minerals in the rock directly dissolve in water without their chemical and mineralogical composition being altered. e.g. olivine, Rock salt (calcium chloride) and calcium bicarbonate are easily weathered in solution.
e.g. NaCl + H2O → Na+, Cl- (dissolved ions with water).
vii. Chelation: Chelation is a complex organic process by which metallic cations are incorporated into hydrocarbon molecules. In fact, the word chelate means a coordination compound in which a central metallic ion is attached to an organic molecule at two or more positions. Chelation is a form of chemical weathering by plants.
3. Biological weathering
Biological weathering is a process of rock disintegration (decay) due to the influence of living organisms both growing plants and animals. The diversity of life in soil includes plants, algae, fungi, earthworms, flatworms, roundworms, insects, spiders and mites, bacteria, and burrowing animals.
Plants wear away the rocks by their roots which widen the rock joints hence allowing in other weathering agents like water to disintegrate the rocks. Some plant roots also have chemicals at the tips of their roots which are acidic and hence cause rock weathering.
Tree roots find their way into cracks or joints in the rocks. As they grow, they cause the joints to become bigger. The end result is that the rocks break into smaller pieces at some points.
Burrowing animals like rodents and moles, warthogs (wild pigs) and wild animals in game parks like the chimpanzee, excavate the rocks and as such, they break up the rocks hence weathering them. Man also disintegrates rocks through his activities.
Man’s activities such as mining, construction, quarrying, agriculture, etc. result in such a fast rate of disintegration of geo materials (rocks).
Application Activity: 4.1
Use your local environment to identify the evidences of biological weathering.
4.2. Factors influencing weathering and interdependence of physical and chemical weathering
Learning activity 4.2
Using the illustration below, identify the missing factor and explain how it influences the rate of weathering.
A number of factors are required for weathering to occur in any environment. The major factors of weathering include:
The term relief refers to the nature of landscape or topography. It influences significantly the weathering process because it controls the flowing of run-off and infiltration of water through slope exposition, steepness and length. In mountainous regions, the windward slopes receive heavy rainfall which may speed up chemical weathering, whereas the leeward sides receiving little amount of rain becoming arid. This favors physical weathering to dominate on the lee ward part.
ii. Living organisms
Living organisms include plants and animals. They both contribute to weathering in a number of ways. Growing roots of trees widen and deepen into the ground and open up joints. Animals ranging from the big to small, including man affect the rate of weathering both mechanically and chemically. Animals and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about
The longer a rock is exposed to agents of weathering, the more weathered it is likely to be and vice-versa. Young rocks such as solidified volcanic rock after a fresh volcanic eruption are likely to be less weathered than rocks formed long ago.
The key components of climate in weathering are moisture and temperature. The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles. High temperatures and heavy rainfall increase the rate of chemical weathering. Arid and semi-arid areas are associated with physical weathering since there is low rainfall and high temperature. As the rocks expand during a period of high temperature and contract during a period of low temperature they develop cracks. In addition, equatorial regions with high rainfall and high temperature experience fast and deep chemical weathering.
v. Nature of rocks
Nature of the rock determines the rate at which it may break down. Their nature depends on rock forming minerals. Some minerals are easily soluble. Also environmental condition such as organic acids and temperature may increase the rate of weathering of rocks. Soft rocks, for example, break down more easily than hard rocks. Similarly, jointed rocks (rocks with cracks) break down faster than rock substances without joints.
vi. The interdependence of physical and chemical weathering
There is interdependence between mechanical and chemical weathering. Chemical weathering to occur needs first mechanical process which provides fragmented pieces of rocks. These rock fragments are then attacked by the chemical process of weathering. Many reasons can be advanced to justify their interdependence:
• The joints and crack found in a rock as a result of physical weathering allow deeper penetration of water which leads to chemical weathering.
• Some rocks are dissolved in water and weathered away in solution. The solutions formed may later undergo precipitation leading to the formation of crystal. These crystals will exert a lot of pressure that will disintegrate the rocks physically.
• Hydration (chemical process) results in a high rate of absorbing water by rocks .e.g.: hematite, limonite which makes these rocks to peel off in a physical process called spheroidal weathering The physical process of frost shattering opens up cracks in the rock and when these cracks are occupied by water, chemical weathering process takes place. e.g. carbonation.Roots of plants which expand within bedding planes of rocks and burrowing animals which drill holes in rocks allow water entry into these rocks which accelerates chemical weathering.
Application activity 4.2
Make a field study around your school and explain how relief and nature of the rock have influenced the rate of weathering.
4.3. Weathering in limestone regions
Learning activity 4.3
1. Differentiate the types of weathering.
2. Describe the type of rock associated with limestone regions.
Limestone is a sedimentary rock in which calcite (calcium carbonate: CaCO3) is the predominant mineral, and with varying minor amounts of other minerals and clay. Limestone rocks are very sensitive to organic acids derived from the decomposition of living organisms.
The major land forms associated with weathering in limestone regions are Karsts land forms that include: caverns, stalagmites, stalactites, pillar, dolines, limestone pavements (uvalas), poljes.
Caverns or caves are also one of the important characteristic features of groundwater in limestone regions. Caverns are formed in several different ways. The rocks in which most caverns occur are salt, gypsum, dolomite and limestone, with the latter by far the most important.
Doline also called Dolina is a round or elliptical hollow on the surface of a limestone region which is formed when several small hollows merge. The small hollows are formed when water starts acting on the points of convergence of joints on the surface.
Uvala is a large surface depression (several km in diameter) in limestone terrain (karst region). It is formed by the coalescence of adjoining dolines and has an irregular floor which is not as smooth as that of Polje.
Polje is a large depression in a karst region with steep sides and flat floor. If it is drained by surface water sources, it is termed as open Polje.
Stalactites are protrusions on top of limestone cave formed as results of water dissolving some rocks which form a solution that leaks from the roof.
Stalagmites are formed like a columnar concretion ascending from the floor of a cave. It is formed from the re-precipitation of carbonate in calcite form perpendicularly beneath a constant source of groundwater that drips off the lower tip of a stalactite or percolates through the roof of a cave in a karst environment. It may eventually combine with a stalactite to form a pillar.
Pillars are formed within the weathered limestone cave after the joining together of stalactites from up and stalagmites from down. The two may finally meet forming a pillar.
For karst land forms to be formed the following conditions must be in place:
• Precipitation: the major types of precipitation which contribute to groundwater are rainfall and snowfall.
• Slope: infiltration is greater on flat areas since water is likely to remain in one place for a long time given that other factors are favorable. On steep slopes, a lot of water is lost through surface run-off with little infiltrating in the ground.
• Nature of the rock: For groundwater to percolate and accumulate there must be spaces within the rocks for it to pass through as well as to occupy further beneath.
• Vegetation cover: the presence of vegetation increases the rate of infiltration.
• Level of saturation of the ground: The rate of water infiltration is high when the ground is very dry and the soil is dry; all the air spaces in it are wide open.
Application activity 4.3
In groups make a field trip to any limestone region, observe karst land forms and present your findings in class.
4.4. Weathering in humid tropical and arid regions and resultant land forms
Learning activity 4.4
Choose any climatic region (Humid tropical/Arid) and identify the type of weathering which will dominate the area
4.3.1. Humid tropical regions
The tropical climate is characterized by high amount of rainfall (more than 1000mm)and high temperature of up to and (more than 18° C) respectively. Weathering is favored in equatorial and tropical regions where the wetness and high temperature are permanent. During the rainy season, chemical weathering dominates through the process of hydration, hydrolysis, solution, oxidation, and reduction. In areas with alternating seasons, chemical weathering is temporary interrupted during drought periods because of lack of moisture. Physical weathering processes such as exfoliation, granular disintegration and block disintegration dominate. Therefore, in tropical (savanna) climate, both physical and chemical weathering processes dominate in dry and rainy seasons alternatively.
4.3.2. Arid and desert regions and resultant land forms
The features formed in these regions as a result of weathering are both erosional and depositional.
1. Erosional features
An inselberg (island hill or mountain in German) called Monadnock in the United States, is an isolated hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. These forms are characterized by their separation from the surrounding terrain and frequently by their independence of the regional drainage network.
These are dome-shaped and steep-sided rocks that rise up to 30 meters. They are massive rock, commonly granite comprised of bare rock that stretches several hundred meters. They take many shapes such as oranges. A good example of where Bornhardts are found is Central Australia.
A tor is a pile like hill of rocks or rock peak. It is a product of massive weathering and comes in all manner of shapes.
This is a rock that is gently inclined at an angle of 0.5 to 7 degrees. It is concave in shape and is found at the base of hills where rainfall is heavy and falls over a short period of time.
v. Deflation basins
Depressions are formed in the deserts due to removal of sand through the process of deflation and are called deflation basins or blow-outs, or deserts hollows. The depth of deflation is determined by groundwater table.
vi. Mushroom rock
The rocks having broad upper part and narrow base resembling an umbrella or mushroom are called mushroom rocks or pedestal rocks. These undercut, mushroom-shaped pedestal rocks are formed due to abrasive works of wind.
Demoiselles represent rock pillars having relatively resistant rocks at the top and soft rocks below. These features are formed due to differential erosion of hard rocks (less erosion) and soft rocks (more erosion). The demoiselles are maintained so long as the resistant cap rocks are seated at the top of the pillars.
Rock masses of tabular form resembling a capped inkpot standing on softer rock pedestal of shale, mudstone is called Zeugen. The bases of such features are broader than their tops.
These are formed always in the same way as Zeugens except that yardangs only develop on landscapes which have alternating rock layers with different resistance to erosion parallel to the direction of prevailing winds. Winds enter and scour up rock particles from the soft bands, thus digging depressions within the soft bands. The resistant hard bands therefore remain standing high up as raised ridges.
Reg is a desert surface armored with a pebble layer, resulting from long continued deflation; found in the Sahara desert of North Africa. Often the winds blow off all the smaller fragments, and leave the bigger size pebbles and gravels over an extensive area.
These are depressions that have water in deserts. These are created by strong winds which remove rock particles from a particular place until a depression is excavated (created).
2. Depositional features in desert
Dunes are mounds or ridges of wind-blown sand. They are depositional features of the sandy deserts and are generally mobile. They vary in size and structure. The main types of sand dunes are Barchan, Transverse Dunes, and Seifs.
Also called Barchans, these are typical crescent shaped sand dunes. The windward slope of barchans is gentle and convex, and the leeward slope is steep and concave. Barchans move slowly, at a rate of meters per year in the direction of the prevailing winds.
These are long and narrow sand ridge which grow parallel to the direction of the prevailing or dominant wind.
- Transverse dune
Transverse dune is an alongated dune lying at right angles to the prevailling wind direction. They have a gentle sloping windward side and a steep sloping leeward side, they are commmon in areas with enough sand and poor vegetation.
Loess is a wind-blown deposit of fine silt and dust. It is unstratified, calcareous, permeable, homogenous and generally yellowish in colour.
Erg is also called sand sea or Dune Sea. It is a large, relatively flat area of desert covered with wind-swept sand with little or no vegetative cover.
Application activity 4.4
Explain the reasons why the erosive power of wind is high in arid regions than in tropical regions.
4.5. Weathering in the glaciated (cold) regions
Learning activity 4.5
Observe the photographs provided below and answer the questions that follow:
Photograph A Photograph B
1. Explain the difference between photograph A and B.
2. Why is the top of mountain in photograph A white in color?
A glacier is a mass of ice of limited width, which moves outwards from a central area of ice accumulation. In other words, a glacier is a mass of ice produced by the accumulation and compression of snow, which moves slowly downhill or sea ward due to its weight.
Glaciation or glacial activity refers to the work done by glaciers or moving ice. It is a process of movement of ice usually from mountain tops downhill which leads to erosional and depositional glacial land forms. Snow/ice is formed when temperature falls under 0oC.
The permanent ice sheets occur in Greenland, Antarctica, and on high mountain tops. The level above which there is perpetual snow cover is called a snow line.
In temperate regions, ice accumulation occurs in winter as the temperature falls under 0oC, and melts later in summer. In tropical regions, snow accumulates on top of mountains of about 4800m above sea level.
4.5.2. Types of glaciers
The main types of glaciers include the following:
i. Valley glaciers: these are also called alpine or mountain glaciers. They move down slope and occupy former river valleys under the influence of gravity and size. e.g. Glaciers on Rwenzori, Kilimanjaro, and Mount Kenya.
ii. Continental glaciers: are alternatively called ice sheets or ice caps. These cover large areas of the plateau surface. They accumulate from a common area and spread towards continental margins with massive movement. e.g.: Glacier found in the polar regions of Greenland, Antarctica, Arctic, Northern Canada and north Western Europe.
iii. Piedmont glaciers: these are produced when mountain glaciers move down below the snow line and spread in the low lands of foot hills of glaciated mountains. They merge to produce large mass of ice.
iv. Cirque glaciers: these are small accumulations of ice which occupy Cirque basins on the mountain sides.
4.5.3. Types of glacial flow
Glacial movements are categorized into two types: gravity flow and Extrusion flow.
a. Gravity flow
In this process, glaciers move down slope under gravity and it usually affects low-lying valleys. This kind of gravity flow includes the following various types:
i. Plastic flowage: ice usually behaves as an elastic brittle solid. When more ice accumulates, internal stress forces the ice to spread and therefore to move like a highly viscous liquid.
ii. Regelation: when ice accumulates, pressure is created inside the ice sheet. This pressure forces some ice to melt and this molten water moves down-slope. When this water derived from the melting of ice reaches in an area of low pressure, it freezes again and solidifies to produce ice.
iii. Intergranular translation: this involves the movement of crystals or granules down slope due to pressure from overlying ice. Melt water lubricates these ice crystals making it easy for them to slide past each other.
b. Extrusion flow
As the accumulation of snow on ice caps increases, there will be an automatic side-ways displacement of ice in all directions following increased accumulation. Hence ice does not flow necessarily down slope as under gravity flow. It flows in all directions as a thick porridge spreads in all directions as more is added. This is how ice sheets which cover large areas of plateau surfaces move.
Application activity: 4.5
Using examples distinguish between valley glaciers and continental glaciers.
4.6. Factors influencing the formation and movement of glaciers
Learning activity 4.6
Why is glaciation dominant in high altitude regions?
There are many factors that influence the formation of glaciers in an area. The most important are briefly described in the following paragraphs:
The effect of altitude: Following the principle of altitude increase and temperature decrease, glaciers usually form in areas of higher altitudes. e.g. Everest, Kilimanjaro mountains.
• The factor of latitude: Areas that lie astride the equator within the tropics have high temperatures that limit ice accumulation. On the other hand areas far away from the equator have low temperature which favor ice formation.
• Precipitation of snow: Glaciers are formed from the condensation of water vapor. This results in the formation of ice crystals which fall as snow. The progressive accumulation and their compaction result in thick and continued glaciers that cover the surface.
The rate at which glaciers move is different from glacier to glacier and is determined by a number of factors. The most important are highlighted below:
- Nature of slope: when the slope is steep enough, glacier moves faster than when slopes are gentle.
- The amount of ice or size of the glacier: when the glacier thickness is big, there will be more pressure to generate quick motion than when the thickness is low.
- Temperature: The glaciers are faster in warm climate conditions due to the presence of enough melt water than in regions of low temperatures. High temperatures quickly produce melt water, which lubricates the ground for quick basal slippage and inter granular translation.
- The amount of load: Load is the eroded materials carried by a moving glacier. The more the load the slower the glacier due to increase in friction and the lesser the load the faster the glacier will be.
Application activity 4.6
Make research on other factors that influence ice accumulation and make a class presentation.
4.7. The work of glaciers and resultant land forms
Learning activity 4.7
From the experience you acquired in previous lessons, make a difference between ice and glacier
4.7.1. Processes associated with glacial erosion
Glaciers perform a triple function. These are erosion, transportation, and deposition.
Many processes are associated with glacial erosion but the most important are the abrasion, plucking and the frost shattering. They are detailed below:
- Abrasion also known as grinding process is the sandpapering effects of angular material embedded in glacier as it rubs the valley sides and floor. Glacial abrasion is caused by the rock debris embedded in the glacier.
- Plucking is also referred to as sapping or quarrying. It occurs when the ice at the base and sides of the glacier freezes onto rock outcrops. The rocks are then pulled and carried away by the moving ice.
- Frost shattering: this process produces much loose material which may fall from valley sides onto the edges of the glacier to form lateral moraine.
4.7.2. Land forms produced by glaciers
There are two types of land forms performed by glacial processes:
1. Glacial erosional features
The most common glacial erosional land forms include:
- Cirque: also called corrie is a steep-sided rock basin with a semi-circular shape. It starts from a small depression which is gradually enlarged. Frost shattering helps shatter the rocks on the edges of the depression and as they break, the depression is enlarged.
- Arêtes: an arête is a narrow ridge with steep sides developing between two corries
.- Pyramidal peak: A pyramidal peak also called horn is a surviving top mountain mass that is not yet worn down by erosion. It is shaped like a pyramid hence the term “pyramidal peak”. It is formed at the junction of arêtes.
- Tan: also called tarn, is a cirque lake produced when the ice melts and the melt water occupy the cirque depression.
- U-Shaped valley or glaciated trough: is formed when a glacier passes through a preexisting river valley to a characteristic of U shape profile. The over deepening and widening of these former river valleys is a product of abrasive action of ice using large amounts of moraine as its tool.
- Hanging valleys: these are valleys associated with glacial troughs. They are small valleys whose floors are found at higher level than the floor of the main valley to which they are tributaries. The floor of the main valley is at a lower level due to greater erosion than the floor of tributary valleys where there is less erosive power.
• Ribbon lakes: the floor of a glacial trough is often eroded very unevenly, and long depressions may be formed at the U-shaped valley floor. These depressions may become sites of long narrow lakes called Ribbon lakes, for example, Lake Noir in France.
• Roche montane (roche moutonée): this is a mass of more resistant rock that projects above the general level of a glaciated valley floor. In most glaciated valleys, it is possible to find rock surfaces that have been grooved and scratched.
◊ Crag & Tail: This is an elongated rock mass which is formed when a flowing glacier meets a resistant rock protecting a soft rock on its leeward side. The soft rock on the leeward side is called a tail.
2. Glacial depositional features
Deposition of debris is among processes performed by glaciers. Debris are preferably deposited in depression or lowlands. Glacial deposits are generally called drifts. They include sands, gravels and rock boulders...
The major glacial depositional features are:
They refer to materials (debris) carried and later deposited by a glacier as it stagnates or decay. Moraines can be classified into the following types:
- Terminal moraine: these are deposited on the mouth of a glacier.
- Lateral moraine: these are deposited on the sides of a glacial trough and from elongated ridges on the sides of valley gorges.
- Medial moraine: These are materials that were originally carried by the valley sides of two small valleys which after emerge into one valley. These materials found themselves in the Center of a glacier.
- Ground moraine: This type of moraine covers the entire width of the valley floor.
• Till plains: these are extensive lowland areas covered by till or a till covered plain.Fluvial glacial deposits: Fluvial glacial deposits are those materials de-posited by melt water from a stagnant glacier. They lead to the formation of the following depositional land forms:
• Out wash plain: is a wide gentle sloping plain which is composed mainly of sand and gravel which were deposited by unevenly melt water
• Kame: is an irregular mound of sand and gravel deposited by melt water, they are short lived and can collapse any time.
• Kame Terrace: is a flat topped ridge formed between a valley glacier and the valley slopes. It is composed of materials deposited by melt water streams flowing laterally to the glacier.
• Esker: is an elongated, narrow ridge which is made up of sand and gravel.
• Kettle holes: is a depression or hole formed by glacial deposits when a block of ice detached from the main glacial while the latter is retreating.
• Drumlins: these are low, rounded smooth, elongated mounds or hills of till rising up to 50m or 1km long. They are products of glacial deposits which flattened the landscape
Application activity 4.7
a. Account for the limited coverage of glaciation in East Africa.
b. Make a research and illustrate the major glacial depositional features.
4.8. Impact of glaciation on the landscape and to human activities
Learning activity 4.8
Using the experience acquired in previous lessons, identify different human activities carried out in glaciated mountainous regions.
There are many impacts of glaciation on the landscape and human activities. Some are positive while others are negative. The main impacts are described below:
4.8.1. Positive impacts
- Crop farming: the till and out wash plains contain fertile soils. These are some of the richest agricultural areas in the world.
- Livestock rearing: the glaciated uplands provide suitable grazing lands since they form fine benches on which pastures thrive in summer.
- Tourism: glaciated landscape has features such as arêtes, pyramidal peaks and cirques that attract tourists.
- Natural harbors: fiords provide ideal sites for the development of natural harbors, for example, the port of Rotterdam in Netherlands, natural habors in Norway and Sweden.
- Fishing grounds: fiord coastlines such as those in Norway provide suitable fishing grounds since they are deep and well sheltered.- Provision of water: glacial lakes provide water for domestic and industrial use.
- Transportation: glacial lakes provide natural waterways, for example, the Great Lakes of North America.
- Mining: glacial erosion exposes minerals to the surface making their exploitation easy, for example, gold and copper in the Canadian Shield of North America.
- Generation of hydro-electric power: waterfalls formed by rivers flowing through hanging valleys are suitable for the generation of hydro-electric power, for example, in Switzerland.
4.8.2. Negative impacts
- Production of bare land: in some instances, the land surface has been scrapped and polished to bare rock. Such regions are of no economic use.
- Discourage settlement: the cold temperatures especially at high altitude limit settlement and other economic activities. They therefore remain as wastelands.
- Transport barrier: the rugged landscape produced by glaciers makes it difficult to establish infrastructure such as roads and railways.- Hindrance to agriculture: sand and gravels deposited on out wash plains make the land unsuitable for agriculture.
Application activity 4.8
Make research using geographical documents and internet on negative effects of glaciation apart from those mentioned in the content.
4.9. Mass wasting
Learning activity 4.9
Make research using geographical documents and internet on negative effects of glaciation apart from those mentioned in the content.
4.9. Mass wasting
Learning activity 4.9
Observe and explain the phenomena that occurred in the photograph below
Mass wasting or mass movement is defined as the creeping, flowing, sliding or falling of rocks and weathered materials down slope under gravity. It is different from erosion in a sense that, in erosion water physically transports away the soil particles, in mass wasting water does not wash away but assists the rock to slide down under the influence of gravity. Mass wasting is classified into two major categories: Slow movement and rapid movement
.4.9.1. Slow movement
Also called creep movements, they are very slow in their motion and they may occur without being noticed. These slow movements’ include:
• Soil creep: This is the most common and the most widely spread type, because it is found in both tropical and temperate climate. The movement of materials is so slow that they may move a few centimeters per day. It can be detected by leaning of trees, electric poles and fencing poles in the direction of the slope.
• Solifluction: This is limited to glaciated mountainous regions and cold climatic areas where thawing causes the saturated surface layer to creep as a mass over underlying frozen ground.
• Talus creep: This is a down slope movement of mainly screes that are relatively dry. It occurs almost in the same way as soil creep and it also occurs under tropical and temperate climate.
• Rock glacier creep: This is a slow process of slope failure in which individual rock boulders with very little soil but with some ice embedded within them slowly move down slope confined within a channel.
• Rock creep: This is the movement of individual rock boulders slowly down slope.
4.9.2. Rapid movement
• Earth flows: These are the rapid down ward movements of clayish or silty soils along a steep slope.
• Mud flows: These are similar to earth flows but they are muddy and occur on slopes that receive heavy rainfall. They are very fast. In Rwanda they are common in the Northern and Western-provinces.
• Debris avalanches: This is the most form of rapid flowage due to the fact that slopes are very steep and there is enough rain to soak slopes. It occurs on very steep slopes that occur in humid climate.
• Slumping: This is the downward slipping of one or several units of rock debris, usually with a backward rotation with respect to the slope over which movement takes place. Undercutting of slopes by streams and man are the main causes of slumping. The surface of the slumped mass has a number of step-like terraces.
• Rock slide: This is the type of sliding in which individual rock masses fall from vertical cliffs or faces of slopes or jointed cliffs.
• Rock fall: Here, individual boulders fall freely from a steep rock face.
Landslides: These are also called landslips. They are down-slope gravitational movements of a body of rock or earth as a unit. It may be induced by natural agencies (like heavy rain, earthquake) or it may be caused by human interference with the slope stability.
Application Activity 4.9
Make research and analyze the types of mass wasting common on hilly areas of Rwanda.
4.10. Causes, effects and control measures for mass wasting
Observe the photograph below taken in Gakenke district and describe the phenomenon that took place.
4.10.1. Causes of mass wasting
The following are the major causes of mass wasting:
- The degree of slope: The steeper the slope, the higher are the chances of material movement. Mass wasting is almost nil in gentle and flat areas.
- The structure and lithology of rocks: Alternating hard and soft rock layers on a slope can be a cause of slope fall. For example, a layer of clay on top of limestone layer can easily slide down.
- The degree of lubrication: Most mass wasting processes occur after a heavy down pour. Water assists to lubricate rock particles and the layers of rock on top of a slope. Therefore, water provides a medium of sliding because it reduces internal friction between rock particles and layers.
- The amount of load on a slope: Slopes which are light rarely fall compared to those which are heavy. Therefore, additional load on a slope increase chances of slope fall.
- Tectonic movements: Earthquake and Volcanic eruptions cause vibrations of the earth which often trigger off widespread movements of materials such as landslides
- Climate: The amount and nature of rainfall received in an area determines the kind of movement that occurs.
- Grazing: The grazing of cattle, movement of elephants and other animals can cause some tremors on slopes hence making them fall.
- Nature of soil: soils which are infertile and therefore unable to support vegetation in enough quantities, are more susceptible to mass wasting compared to soils, which are fertile and therefore able to support dense vegetation.
- Influence of vegetation: Vegetation help to hold rock materials together thus reducing their movement on the surface.
- The work of animals: Animals and micro-organisms facilitate deep weathering which results into the reduced cohesion of the rock particles on slopes. This therefore leads to easy movement.
- Vulcanicity: Volcanic eruption on the ice capped highlands cause ice to melt and therefore soak the slopes. This lubrication greatly increases the chances of slope movement.
4.10.2. Effects of mass wasting
The following are some of the effects of mass wasting:
- Threat to life and property: There are several serious incidents of landslides and rock slides every year. They cause loss of life and property. In a minor incident they may block only one line of a road, but in severe cases entire blocks of buildings collapse.
- Loss of vegetation: Mass wasting and soil erosion result in the loss of surface topsoil which is essential for vegetation. As a result, more areas become barren.
- Scars and Gullies: In areas where topsoil and vegetation are removed, bare spots form scars in the landscape. Gullies form on weathered slopes through rain action and mass wasting in areas with little or no vegetation. Intense gully cuts up the landscape into large scale gullies and ridges and destroys the area. Gullying is common in the bare, granitic areas.
- Pollution of water: large amounts of geologic materials enter streams as sediments as a result of this landslide and erosion activity, thus reducing the potability of the water and quality of habitat for fish and wildlife.
- Wildlife destruction: Although most kinds of wildlife are able to retreat fast enough to avoid direct injury from all but the fastest-moving landslides, often are subject to habitat damage by landslides.
It is noticed that mass wasting especially landslides, has severe impacts on humans and environments. For this reason, measures have to be taken for preventing or mitigating them. Some of the measures are highlighted below:
- Gradients of steeper slopes could be reduced by constructing terraces.
- Retaining walls can be built to stabilize the slope.
- Steep slopes should be inspected regularly, especially during periods of intense or prolonged rainfall to identify areas prone to mass wasting for preventive measures.
- More surface drainage channels and ditches can be constructed to reduce overflowing discharge
- Legislation can restrict development and building in zones prone to mass wasting.
- Trees can be planted on steeper slopes to stabilize the soil and the slope.
- Appropriate instruments can be installed to monitor slope stability, providing early warning in areas of concern.
- Mass education of people
Application Activity 4.10
Make a field trip to observe different cases of mass wasting in your area. Analyze its causes and propose the sustainable measures to control it.
4.11. The relationship between weathering land forms and human activities
Learning activity 4.11
Are land forms resulting from weathering important in your area? Support your answer.
Weathering affects human activities in various ways as follows:
• Weathering provides a basis for the development of construction industry in an area. e.g. marrum soil and laterite are good for road construction.
• Weathering can also produce land forms that offer important touristic opportunities.
• Weathering facilitates soil formation, this directly provides a basis for the development of agriculture in the region.
• Weathering affects limestone regions (calcium carbonate) that are important for cement production.
• Building stones in urban areas are subjected to the weathering processes as natural outcrops but with additional influences.
• Weathered shales also produce good brick clays, whereas the weathered ba-salt produces fertile soils based on montmorillonite.
• On weathered rocks, weathering often improves the grade of economic de-posits by concentrating desirable elements such as copper around the water table.
Application Activity 4.11
Make research in your area; describe how weathering land forms have benefited the people for their sustainable development.
End unit assessment
1. Describe the main causes of mass wasting that usually occur in north- western part of Rwanda. How does the community work (umuganda) contribute to the reduction of mass wasting in your area?
2. With reference to East Africa explain the formation of glacial land forms in mountain areas.
3. How have topography and parent rock influenced the rate of weathering in your area?
4. Make a field trip in your local environment and explain how the weathering land forms identified in your area affect positively and negatively human activities
.5. Referring to above questions suggest ways of sustainable environmental protection.
Key unit competence:
By the end of this unit, I should be able to categorize different features resulting from wave action and their relationships with human activities.
Use the pictures provided below and answer the following questions:
1. What is the type of water body represented on the pictures above?
2. Identify the coastal land forms found on both pictures.
3. Explain the factors of the formation of the coastal land forms identified on the map at right.
4. According to you, do you think the level of water in ocean is always the same? Justify your answer.
5. If you need to be a business man / woman at the coastal regions, explain the business opportunities that you may carry out there and the challenges you can face.
5.1 Coastal land forms: definition of key terms
Learning activity 5.1
1. Using internet and different books, research and describe the difference between the following terms related to coastal land forms:
d. Long shore drift
2. Identify the parts of a wave
Definition of key terms:
Below are defined different terms associated with the coastal land forms:
• Coast: A coast refers to the land that borders the sea or the ocean. It is a narrow zone where the land and the sea overlap and directly interact. Some coasts are made up of broad sandy beaches, while others form rocky cliffs or low-lying wetlands. The shape of the coastline is determined by factors such as the types of rocks present, the forces of erosion, and the changes in sea level.
• Shore: This is the area where land meets the sea or ocean. Different features are found in this area resulting from erosion and deposition of sediments, ocean or sea waves, as well as the effects of rivers as they join the sea. It is also called coastline.
• Waves: Waves are defined as undulations of sea/lake water characterized by well-developed crests and troughs (see the figure below). Waves are created by the transfer of energy from the wind blowing over the surface of the sea. When waves appear with high frequency they demonstrate the short wave-lengths. When they appear with low frequency they demonstrate long wave-lengths. However, there are special waves like Tsunamis that result from sub-marine shock waves by earthquakes or volcanic activities.
• Long shore drift: This refers to the movement of sediments along the shore in a zigzag pattern. Sediments produced by the erosive action of waves or sediments transported by the river systems are moved by ocean waves and ocean currents to form beaches. Sediments are as well moved offshore onto the continental shelf. Most waves reach the shore at an angle of about 10° but this can change depending on the wind direction. Each successive wave moves sand at an angle along the beach face. Consequently, currents within the surface zone flow along the shore as long shore drift (see the figures below).
Application activity 5.1
1. By using a diagram, demonstrate the parts of a wave.
2. Differentiate a shore from a long shore drift.
3. If you find an occasion to visit the Indian ocean coast in East Africa, describe the coastal features you would be interested to discover.
5.2 Types, factors and action processes of waves
Learning activity 5.2
1. Using internet and other resources, describe the types of waves.
2. A coast may have the steep slope like cliff or gentle slope like beach. Explain different ways by which waves may hit the coast depending on its slope.
3. Visit a water body in your local area, observe the waves movement and do the following:
i. Find out the causes of wave movement
ii. Identify the direction of wind on water surface
iii. Determine the direction of waves in water
iv. Describe the effect of wind on wave
4. Explain how waves can cause erosion along the coast.
5.2.1 Types of waves
There are two main types of waves: constructive waves and destructive waves.
1. Constructive waves: These are waves whose swash is more powerful than backwash. They are depositional in nature.
2. Destructive waves: These are waves whose backwash is more powerful than swash. They are erosional in nature.
Other types of waves:
• Breaking waves: Formed when the wave collapses on top of itself. They are of two types: (i) Plunging breaker: The wave reaches a steeper beach and curls, moving over a pocket of air. It travels very fast.
(ii) Spilling breaker: The wave reaches a sloping sandy beach, dispersing the energy over a large area.
• Deep water waves/Swell waves: Are made up of a number of waves of different lengths superimposed on each other. They are straight and long, powerful, and travel great distance.
• Inshore waves: These waves drain the beach as a backwash.
• Internal waves: Formed due to the disturbances found between two water masses of different density. They are high and become turbulent currents when they hit a landmass.
• Kelvin waves: Formed due to lack of winds in the Pacific Ocean. They are high and wide waves, warmer than the surrounding water.
• Progressive waves: Move with a steady speed, so they are called Progressive Waves. They are of two types:
• Capillary waves: Formed when wind creates pressure over capillarity, the binding force that holds the water molecules of the ocean surface together.
• Orbital progressive waves: Formed at the boundary of two liquids with different density.
• Refracted waves: Travel in shallow water when they approach the shore. The shallowness decreases the power of the wave and causes a curve. These are usually seen near headlands and bays.
• Seiche waves: Caused due to the movement within a confined space. These have long wavelengths and rarely result in any damage as their height is generally short.
• Shallow water waves: Move in shallow waters. They are of two kinds:
• Tidal waves: Formed due to the gravitational pull of the sun and the moon on the ocean.
• Seismic sea Waves/tsunami: Caused due to earthquakes beneath the ocean. They travel extremely fast in open water, have significant height in shallow water, and are very dangerous and devastating.
• Swell waves/Surging waves: Intense waves generating from the center of a storm where the winds are strong. These expel little energy, travel long distance, and break on distant shores.
5.2.2 Factors determining the strength of waves
The following are the major factors determining the strength of waves.
- Wind strength: Wind must be moving faster than the wave crests for energy transfer to continue;
- Wind duration: Winds that blow for a short time will not generate large waves;
- Fetch: The uninterrupted distance over which the wind blows without changing direction;
- Depth of water or roughness of sea bed: As waves enter shallow water their speed decreases, wavelength decreases, and height increases. Waves therefore tend to break in shallow water, for example over a bar at the entrance to a harbor;
- Direction and speed of tide: If the tide direction is against the wind, this will also increase wave height and decrease wavelength.
5.2.3 Wave action processes
The wave action includes erosion, transportation and deposition.
• Erosion: Several mechanical and chemical effects produce erosion of rocky shorelines by waves. Depending on the geology of the coastline, nature of wave attack, and long -term changes in sea-level as well as tidal ranges, erosional land forms such as wave-cut, sea cliffs, and even unusual land forms such as cases, sea arches, and sea stacks can form
• Transportation: Waves are excellent at transporting sand and small rock fragments. These, in turn, are very good at rubbing and grinding surfaces below and just above water level in a process known as abrasion. Long shore drift, long shore currents, and tidal currents in combination determine the net direction of sediment transport and areas of deposition.
• Deposition: Sediments transported by the waves along the shore are deposited in areas of low wave energy and produce a variety of land forms, including spits, tombolo, beaches, bars and barrier islands. Different types of pediments are deposited along a coast, sometimes in the form of an accumulation of unconsolidated materials such silt, sand and shingle.
Application activity 5.2
1. Explain the wave action processes.
2. Why do you think in some areas the wave action processes may occur differently in nature?
3. Carry out a research in your local region to identify if there are wave actions taking place.
4. Describe the particularities of Tsunami compared to other types of waves.
5. Explain the impact of wind and tides on the strength of the waves.
5.3. Formation of coastal land forms
Learning activity 5.3
1. Observe the following picture showing a coastal land form and answer the questions that follow
a. What are the major factors for this land form to be formed?
b. Water level on photograph may increase or decrease. What are the causes of such phenomena?
2. Using the internet and other geographical resources, describe the following:
i. Wave erosion features
ii. Wave deposition features
5.3.1. Factors influencing the formation of coastal land forms
The following are the major factors influencing the formation of coastal land form:
- Tides: Tides are greatly influencing forces of coastal land forms. They are commonly semi-diurnal (12-hour cycle). The rise and fall of water levels produce oscillating currents known as tidal streams. Tidal currents can transport large quantities of sediments, especially at the mouths of estuaries. The tidal amplitude also determines the sediments deposition or erosion and keep redefining the shoreline of coastal land forms.
- Waves: Waves contribute to the erosion of shore. The greater the wave action, the higher is the erosion and sediment movement. Where the shoreline is long and flatter, the wave energy gets dispersed. Wherever there are rock formations, cliffs and short shore area, the wave energy is high. Strong waves can pick up sediments from deeper waters and make them available for transportation by the coastal currents. The larger the wave, the larger the particle it can move. Storm waves can even move boulders. Even small waves can lift the sediments and deposit along the coastal shoreline.
- Long shore currents: Parallel movement of water is known as long shore current and it extends up to the zone of breaking waves from the coastal shoreline. As the long shore currents are formed by refracting waves, the direction of flow will depend upon the angle of the wave which in turn depends upon the wind directions. If the wind direction is balanced, the sediment movement is also balanced. If the wind movement and resultant wave action dominate in one direction great volumes of sediment may be moved in one direction.
- Weather elements: The elements of climate, such as wind, rainfall and temperature play an important role in formation of coastal land forms. Winds are directly related to the intensity of waves. Land forms like coastal dunes are created by wind action. Temperature is required for physical weathering of sediments. Rainfalls provide runoff for producing and transporting sediments from land to seashore.
- Gravity: Gravity is an important factor for the development of coastal land forms. Gravity is indirectly involved in the movement of wind and waves as well as in downward movement of sediments.
- Nature of coastal rocks: Soft rocks are easily eroded hence forming erosional features like bays while hard or resistant rocks lead to the formation of headlands.
5.3.2 Land forms produced by wave erosion
The coastal features formed due to marine erosion by sea waves and other currents and solution processes include cliffs, caves, indented coastline, stacks, chimneys, arch, inlets, wave-cut platforms.
- Cliffs: A cliff is a steep rocky coast rising almost vertically above sea water. Cliffs are very precipitous with overhanging crest. The steepness of vertical cliffs depends on the following: lithology of the area, geological structure, weathering, erosion of cliff face and marine erosion of cliff base.
- Wave-cut platform: Rock-cut flat surfaces in front of cliffs are called wave-cut platforms or simply shore platforms. They are slightly concave upward. The origin and development of wave-cut platforms is related to cliff recession. The plat-form is composed of bare rock or it may contain a temporary deposit or rock debris, pebbles or sand.
- Sea caves: A sea cave is a natural cavity or chamber which develops along the coast due to gradual erosion of weak and strongly jointed rocks by up rushing breaker waves (surf currents). Sea caves are more frequently formed in carbonate rocks (limestone and chalks) because they are eroded more by solution processes. However, sea caves are not permanent as they are destroyed with time.
• - Blowhole: This is a vertical shaft linking the cave to the surface. It is formed when wave action attacks the back part of the roof of the cave. At the same time, weathering by solution acts on the line of weakness from the surface downwards to form a blowhole.
- Geo: Wave erosion may continue on the roof of the cave along the blowhole. Hence, the roof of the cave may collapse to form a long and narrow sea inlet known as Geo.
- Stack/ Column/Pillar: A stack is an isolated rock monolith or pillar rising steeply from the sea. It is a former part of the adjoining land that has become isolated from it by wave erosion, probably after having formed part of a marine arch.
- Sea arch: A sea arch is a natural opening through a mass of rock limestone or boulder clay. It is most commonly seen on the sea coast where waves have cut through a promontory. When the keystone of the marine arch collapses, the feature will become a stack.
5.3.3 Land forms produced by wave deposition
Sediments transported along the shore are deposited in areas of low wave energy. They produce a variety of landforms, including spits, tombolo, beaches, bars and barrier islands. Different types of pediments are deposited along a coast, sometimes in the form of an accumulation of unconsolidated materials such as silt, sand and shingle.
• Spits: A spit is an embankment composed of sand and shingle attached to the land on one end and projecting seaward. It may form parallel to the coast and stretch several kilometers. It may also grow at an angle across an estuary. Spits are formed when materials are transported and deposited by the long shore drift, mostly where the orientation of the coast changes.
• Tombolo: It is a spit which grows seawards from the coast and joints to an island.
• Beaches: A beach is located on a wave-cut platform of solid rock and is generally of a low gradient with a gently concave platform. Beaches may extend for hundreds of kilometers. Beaches are classified into: sand beach, shingle beach, and boulder beach.
• Bar: A bar is an elongated deposit of sand, shingle or mud occurring in the sea. It is more or less parallel to the shoreline and sometimes linked to it. Bars may be of submerged or emergent embankments of sand and gravel built along the shore by waves and currents. One of the most common types of bars is the spit.
• Barrier Islands: Barrier Islands are long, offshore islands of sediments tending parallel to the shore. They form long shorelines adjacent to gently sloping coastal plains, and they are typically separated from the mainland by a lagoon. Most barrier islands are cut by one or more tidal waves.
• Cuspate foreland: This is a large triangular-shaped deposit of sand, mud and shingles projecting seaward. It is a rare feature formed when two adjacent spits growing towards each other at an angle join and enclose a shallow lagoon.
• Mud flats: These are platforms of mud, silt and river alluvium kept by salt-tolerant plants to form a swamp or marshland. They are formed when tides deposit fine silts along gently sloping coats in bays and estuaries.
• Coastal dunes: These are low-lying mounds of fine sand, deposited further inland from a wide beach by strong onshore winds. They are common in arid and semi-arid coasts.
Application activity 5.3
1. Describe land forms produced by wave deposition.
2. Explain the factors influencing the formation of coast land forms.
3. Describe the formation of features produced by wave erosion.
4. According to you, which land forms are likely to find around lake Kivu?
5.4 Importance of coast land forms produced by wave action
Learning activity 5.4
Observe the following picture and describe the economic activities that can be carried out in this area.
Coastal land forms produced by wave action are very important in different ways as follows:
• Many of the world’s major cities are located in coastal areas, and a large portion of economic activities, are concentrated in these cities.
• There are different activities that take place in coastal zones including coastal fisheries, aquaculture, industry, and shipping.
• Many of coastal land forms are very favorable for tourism that contributes to the economic development of countries.
• The peculiar characteristic of coastal environments is their dynamic nature which results from the transfer of matter, energy and living organisms be-tween land and sea systems, under the influence of primary driving forces that include short-term weather, long-term climate, secular changes in sea level and tides.
• Marine, estuary and coastal wetland areas often benefit from flows of nutrients from the land and also from ocean up welling which brings nutrient-rich water to the surface. They thus tend to have particularly high biological productivity.
• It is estimated that 90 percent of the world’s fish production is dependent on the nature of coastal land forms.
• The coastal land forms attract people as at present, two-thirds of the world’s cities with a population of 2.5 million or more are situated near tidal estuaries.
Application activity 5.4
1. Give five examples of cities located in coastal areas, including at least two located in East African Community.
2. Describe the main activities that are related to the Lake Kivu.
5.5. Types of coasts
Learning activity 5.5
1. Make your own research using internet or other geography resources and identify different types of coasts.
2. Describe the areas subjected to fiords coasts.There are two types of coasts: Submerged coasts and Emerged coasts.
5.5.1. Submerged coasts
Submerged coasts fall into two categories: Submerged upland coasts and submerged lowland coasts.
a. Submerged upland coasts
When the margin of an irregular upland area is submerged, a more or less indented coastline is produced. It appears with islands and peninsulas representing the former uplands, and with inlets indicating the former valleys. The following are the three types of submerged coasts:
i. Ria coasts: Ria is a Spanish term widely used to describe a submerged coastal valley or estuary resulting from a rise of the sea level. In the case of a Ria coast, hills and river valleys meet the coastline at right angles. The rias are characterized by funnel-shaped which decreases width and depth as they run inland. The head of a stream which is small is responsible for the formation of the valley at the inlet.
ii. Fiord (Fjord) coasts: A long, narrow inlet of the sea bound by steep mountain slopes. These slopes are of great height and extend to considerable depths (in excess of 1,000 m) below sea level. It is formed by the submergence of glacially over deepened valleys due to a rising sea level after the melting of the Pleistocene ice sheets. Fiords occur in western Scotland, Norway, Ireland, Greenland, Labrador, British Columbia, Alaska, Southern Chile and New Zealand. The main reason for their existence is the submergence of deep glacial troughs and that is why fiords have many characteristics of glaciated valleys.
iii. Dalmatian or longitudinal coasts:Dalmatian is a term derived from the Yugoslavia Adriatic in which the coast runs parallel with the lineament of the topography and probably with the underlying geological structure. A rise of sea level (estuary) has drowned the coastal area, resulting in a coastline of narrow peninsulas, lengthy gulfs and channels and linear islands. The Dalmatian coast tends to be straight and regular.
b. Submerged lowland coasts
These are formed when a rise in the sea level drowns a lowland coast. The sea penetrates deep inland along rivers to form estuaries. The rise in base level causes an increase in deposition by rivers leading to formation of mud flats, marshes, and swamps which are visible at low tides.
5.5.2. Emerged coasts
Emerged coasts comprise emerged highlands coasts and emerged lowland coasts.
a. Emerged upland coasts
The chief feature of an emerged upland coast is a raised beach or cliff-line, found above the present zone of wave action. The coastlines are revealed as distinct notches in the slope, backed by a cliff, often with distinct caves. They are fronted by a wave-cut rock platform covered with each material such as shell banks and shingles. Many parts of the world show evidence of this emergence. The western coast of Malta is a typical example of emerged upland coast.
b. Emerged lowland coasts
An emerged lowland coast has been produced by the uplift of part of the neighboring continental shelf. The landward edge of such coastal plain is found in the southern of USA. It is formed by the fall-line where rivers descend from the Appalachian in a series of waterfalls. Other examples of emerged lowland coasts are: the northern shore of the Gulf of Mexico and the southern shore of the Rio-de-la Plata in Argentina.
Application activity 5.5
1. With help of diagrams describe different types of emerged coasts.
2. In a field trip at the lake shore in Rwanda (if any), indicate the type of the visited submerged coast, and describe its characteristics.
5.6 Coral reefs
Learning activity 5.6
Observe the figure below of a coral reef and answer the following questions:
1. What do you think are the elements that constitute a coral reef?
2. Make research on internet and find out the illustrations of the process of coral reefs formation.
3. What do you think are the problems related to coral reefs?
5.6.1 Nature, types and formation of coral reefs
Coral reefs are significant submarine features. They are formed due to the accumulation and compaction of skeletons of dead marine organisms known as coral polyps. Coral polyps thrive in the tropical oceans. Numerous coral polyps live at a place in groups in the form of a colony.
They are generally attached to submarine platforms or islands submerged under seawater.
a. Types of coral reefs
On the basis of the location of the main types of reefs, we distinguish tropical coral reefs and marginal belt coral reefs. But, by categorizing on the basis of the nature, the shape and the mode of occurrence, we have three types of coral reefs which are: fringing reefs, barrier reefs and atoll.
i. Fringing reefs (Shore Reefs):These are the coral reefs developed along the continental margins or along the islands. The seaward slope is steep and vertical while the landward slope is gentle. A fringing reef runs as a narrow belt which grows from the deep sea bottom sloping steeply seaward side. It is separated from the main land by a narrow and shallow lagoon.
ii. Barrier reefs:Barrier reefs are extensive linear reef complexes that are parallel to a shore and are separated from it by a deep and wide lagoon.
iii. Atoll: An atoll is a roughly circular (annular) oceanic reef system surrounding a large and often deep central lagoon. There are three types of atolls, namely, true atolls, island atolls and coral island or atoll islands.
• True atolls are characterized by circular reef enclosing a shallow lagoon but without an island;
•Island atolls have an island in the central part of the lagoon enclosed by circular reefs;
• Coral islands or atoll islands do not have islands in the beginning but later on islands are formed due to erosion and deposition by marine waves.
b. Formation of coral reefs
i. The process of coral reefs formation
Coral reefs start to form when the free-swimming coral larvae attach to the submerged rocks or other hard surfaces along the edges of islands or continents. This continues to grow under the influence of coral reefs conditions to grow in any types accordingly. The coral reef formation takes three stages: fringing, barrier and atoll.
Concerning the process, a typical fringing reef is attached to or borders the shore of a landmass, while a typical barrier reef is separated from the shore by a body of water and an atoll began as a fringing reef around a volcanic island. Over time, the volcano stopped erupting, and the island began to sink. Over time, coral growth at the reef ’s outer edge would push the top of the reef above the water. As the original volcanic island disappeared beneath the sea, only an atoll would remain.
The general conditions influencing coral formation
• Corals are found mainly in the tropical oceans and seas because they require high mean annual temperature ranging between 20°C and 21°C for their survival. They cannot survive in the waters having either very low temperature or very high temperature.
• Corals do not live in deep waters, that is, not more than 60-77 meters below the sea level.
• There should be clean sediment-free water because muddy water or turbid water clogs the mouths of coral polyps resulting into their death.
• Though coral polyps require sediment-free water, fresh water is injurious to their growth. This is why corals avoid coastal lands and live away from the areas of river mouths.
• High salinity is injurious to the growth of coral polyps because such waters contain little amount of calcium carbonates whereas lime is important food of coral polyps. The oceanic salinity ranging between 27% and 30% is most ideal for the growth and development of coral polyps.
• Ocean currents and waves are favorable for corals because they bring necessary food supply for the polyps.
• There should be extensive submarine platforms for the formation of colonies by the coral polyps. Besides, polyps also grow outward from the submarine platforms.
• Human activities like deforestation, industrialization cause global warming, which adversely affects corals in their habitats. Corals are more susceptible to long-term climatic change. Corals are generally termed as rain forests of the oceans. These cannot survive in extreme warm environment.
5.6.2 Theories of the origin of coral reefs
There are three main theories about the origin of coral reefs that are:
• The subsidence theory by Darwin,
• Antecedence theory by Murray,
• Glaciated control theory by Daly.
a. Darwin’s Subsidence Theory
• Darwin, a British naturalist developed his theory as follows:
• Darwin’s theory starts with a volcanic island which becomes extinct.
• As the island and ocean floor subside, coral growth builds a fringing reef, often including a shallow lagoon between the land and the main reef.
• As the subsidence continues, the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside.
• Ultimately, the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon.
b. Murray’s antecedence theory
The Antecedent-Platform or uplift theory for the origin of coral reefs holds that:
• Any bench or bank that is located at a proper depth within the circum-equatorial coral-reef zone is potentially a coral-reef foundation.
• If ecological conditions permit, a reef may grow to the surface from such a foundation without any change in sea-level.
• Reef foundations, or platforms, are formed by erosion, deposition, volcanic eruption, or earth movement or by combinations of two or more of these processes.
• Murray agreed that atoll coral reefs formed when the tops of islands were undergone wave action resulting to a platform.
• Platforms originating below the depth limit of reef corals were veneered with tuffaceous limestone and built to the zone of reef-coral growth by organisms other than corals, chiefly foraminifera and algae.
c. Daly’s glaciated control theory
• Daly studied the coral reefs of Hawaii and he was greatly impressed by two things:
• the reefs were very narrow and there were marks of glaciations
• there should be a close relationship between the growth of reefs and temperature.
• According to Daly’s hypothesis, in the last glacial period, an ice sheet had developed due to the fall in temperature. This caused a withdrawal of water, equal to the weight of the ice sheet. This withdrawal lowered the sea level by 125-150 m.
• The corals which existed prior to the ice age had to face this fall in temperature dining this age and they were also exposed to air when the sea level fell. As a result, the corals were killed and the coral reefs and atolls were planed down by sea erosion to the falling level of sea in that period.
• When the ice age ended, the temperature started rising and the ice sheet melted. The water returned to the sea, which started rising. Due to the rise in temperature and sea level, corals again started growing over the platforms which were lowered due to marine erosion.
• As the sea level rose, the coral colonies also rose. The coral colonies developed more on the circumference of the platforms because food and other facilities were better available there than anywhere else.
• Hence, the shape of coral reefs took the form of the edges of submerged platforms, a long coral reef developed on the continental shelf situated on the coast of eastern Australia. Coral reefs and atolls developed on submerged plateau tops. After the ice age, the surface of platforms was not affected by any endogenic forces and the crust of the earth remained
5.6.3. Problems facing the development and growth of coral reefs
The following are the major problems facing the development and growth of coral reefs:
• Over fishing: Increasing demand for food fish and tourism curios has resulted in over fishing of not only deep-water commercial fish, but key reef species as well. This affect the reef ’s ecological balance and biodiversity.
• Coral disease: coral diseases contribute to the deterioration of coral reef com-munities around the globe. Most diseases occur in response to the onset of bacteria, fungi, and viruses.
• Destructive fishing methods: Fishing with dynamite, cyanide and other methods that break up the fragile coral reef are highly unsustainable. Dynamite and cyanide stun the fish, making them easier to catch. Damaging the coral reef habitat on which the fish rely reduces the productivity of the area.
• Unsustainable tourism: Physical damage to the coral reefs can occur through contact from careless swimmers, divers, and poorly placed boat anchors. Hotels and resorts may also discharge untreated sewage and wastewater into the ocean, polluting the water and encouraging the growth of algae, which competes with corals for space on the reef.
• Coastal development: The growth of coastal cities and towns generates a range of threats to nearby coral reefs. Coral reefs are biological assemblages adapted to waters with low nutrient content, and the addition of nutrients favors species that disrupt the balance of the reef communities.
• Pollution: Coral reefs need clean water to thrive. From litter to waste oil, pollution is damaging reefs worldwide. Pollution from human activities inland can damage coral reefs when transported by rivers into coastal waters.
• Marine debris: It is any solid object that enters coastal and ocean waters. Debris may arrive directly from a ship or indirectly when washed out to sea via rivers, streams, and storm drains. Human-made items tend to be the most harmful such as plastics (from bags to balloons, hard hats to fishing line), glass, metal, rubber (millions of tires!), and even entire vessels.
• Dredging operations. They are sometimes completed by cutting a path through a coral reef, directly destroying the reef structure and killing any organisms that live on it. Operations that directly destroy coral are often intended to deepen or otherwise enlarge shipping channels or canals, due to the fact that in many areas, removal of coral requires a permit, making it more cost-effective and simple to avoid coral reefs if possible.
• Global Aquarium Trade: It is estimated that nearly 2 million people worldwide keep marine aquariums. The great majority of marine aquaria are stocked with species caught from the wild. This rapidly developing trade is seeing the movement of charismatic fish species across borders. Threats from the trade include the use of cyanide in collection, over-harvesting of target organisms and high levels of mortality associated with poor husbandry practices and insensitive shipping. Some regulation is in place to encourage the use of sustainable collection methods and to raise industry standards.
• Alien invasive species: Species that, as a result of human activity, have been moved, intentionally or unintentionally, into areas where they do not occur naturally are called “introduced species” or “alien species”. In some cases where natural controls such as predators or parasites of an introduced species are lacking, the species may multiply rapidly, taking over its new environment, often drastically altering the ecosystem and out-competing local organisms.
• Climate change: Rising sea levels due to climate change requires coral to grow to stay close enough to the surface to continue photosynthesis. Also, water temperature changes can induce coral bleaching coral bleaching as happened during the 1998 and 2004 El Niño years, in which sea surface temperatures rose well above normal, bleaching or killing many reefs.
• Ocean acidification: results from increases in atmospheric carbon dioxide. The dissolved gas reacts with the water to form carbonic acid, and thus acidifies the ocean. This decreasing pH is another issue for coral reefs.
• Coral mining: Both small scale harvesting by villagers and industrial scale mining by companies are serious threats. Mining is usually done to produce construction material which is valued as much as 50% cheaper than other rocks, such as from quarries. The rocks are ground and mixed with other materials, like cement to make concrete. Ancient coral used for construction is known as coral rag. Building directly on the reef also takes its toll, altering water circulation and the tides which bring the nutrients to the reef'
5.6.4 Impact and problems related to coastal land forms
Coastal land forms have crucial impact in world economic activities. These are:
• Tourist attraction: Coastal features like caves, beaches and arches are tourist attractions.
• Development of harbors: Rias and fiords favor the development of deep sheltered harbors.
• Industrial raw materials: Coral limestone provides raw materials for the manufacture of cement. This is obtained from raised coral reefs.
• Fishing grounds: Fiords contain sheltered waters which are suitable for feeding and development of fishing ports. Continental shelves contain shallow waters which favor growth of planktons. This makes them rich fishing grounds.
• Habitat for marine life: Lagoons, mud flats and mangrove swamps are good habitats for marine life. This has promoted the development of research on marine life and establishment of marine parks.
• Impact on agriculture: emerged coasts have sand, gravel and bare rock. These inhibit agriculture, especially crop farming.
• Transport barrier: coastal features such as sandbars and coral reefs inhibit water transport and development of ports.
Application activity 5.6
1. Using illustrative graphics, differentiate the types of coral reefs.
2. Explain the conditions for coral reefs formation.
3. Identify the problems related to coral reefs.
4. Establish the similarities of the subsidence, antecedence and glaciated control theories of coral reefs formation.
5. Describe the economic importance of coral reefs.
5.7 Isostatic and Eustatic changes on the coast
Learning activities 5.7
1. Basing on your knowledge on the concept of isostasy, do you think it can be related to the isostatic and eustatic changes on the coast? Explain.
2. Differentiate isostatic from Eustatic sea level change.
The sea level is not static, which causes the level on coast changing regularly. These changes are discussed below:
5.7.1 Isostatic change
Isostatic sea level change is the result of an increase or decrease in the height of the land. When the height of the land increases, the sea level falls and when the height of the land decreases the sea level rises. Isostatic change is a local sea level change whereas Eustatic change is a global sea level change.
• During an ice age, isostatic change is caused by the build-up of ice on the land. As water is stored on the land in glaciers, the weight of the land increases and the land sinks slightly, causing the sea level to rise slightly. This is referred to as compression.
• When the ice melts at the end of an ice age, the land begins to rise up again and the sea level falls. This is referred to decompression or isostatic rebound.
• Isostatic rebound takes place incredibly slowly and to this day, isostatic rebounding is still taking place from the last ice age.
5.8. Sea level change
Learning activity 5.8Observe the following picture and answer the question that follow:
• Isostatic sea level change can also be caused by tectonic uplift or depression. As this only takes place along plate boundaries, this sort of isostatic change only takes place in certain areas of the world.
5.7.2 Eustatic Change
Eustatic change is when the sea level changes due to an alteration in the volume of water in the oceans or, alternatively, a change in the shape of an ocean basin and hence a change in the amount of water the sea can hold. Eustatic change is always a global effect.
During and after an ice age, Eustatic change takes place.
• At the beginning of an ice age, the temperature falls and water is frozen and stored in glaciers inland, suspending the hydrological cycle. This results in water being taken out of the sea but not being put back in leading to an overall fall in sea level. Conversely, as an ice age ends, the temperature begins to rise and so the water stored in the glaciers will re-enter the hydrological cycle; and the sea will be replenished, increasing the sea levels.
• Increases in temperature outside of an ice age also affect the sea level since an increasing temperature causes the ice sheets to melt, putting more water in the sea.
• The shape of the ocean basins can change due to tectonic movement. If the ocean basins become larger, the volume of the oceans becomes larger but the overall sea level will fall since there’s the same amount of water in the ocean. Conversely, if the ocean basins get smaller, the volume of the oceans decreases and the sea level rises accordingly
Application activity 5.7
1. Describe the isostatic and eustatic changes on the coast.
2. Describe the effects of Eustatic change on the environment.
5.8. Sea level change
Learning activity 5.8
Observe the following picture and answer the question that follow:
1. Find evidence that the level of water on this picture changes.
2. What do you think can cause that change?
3. Describe the features portrayed on this picture.
5.8.1 Meaning of sea level change and its resulting features
The sea level change is the variation and fluctuation of the sea level throughout time. On a day to day basis, the sea level changes from the tide action but the sea level also changes on a much grander time scale too. These changes in sea level are normally caused by ice ages or other major global events. The sea level change results from eustatic and isostatic changes. It can contribute to the formation of submergent landforms such as Ria (a river valley that’s been flooded by the eustatic rise in sea level), fjords and dalmatian coastline, and emergent landforms such as raised beaches. These are wave-cut platforms and beaches that are above the current sea level. There are also some old cliffs (relic cliffs) behind these raised beaches with wave-cut notches, arches and stacks along them.
5.8.2 Types of sea level changes
There are two types of sea level changes which are:
• Rise of sea level: Thisis referred to as an increase in global mean sea level as a result of an increase in the volume of water in the world’s oceans. This leads to the formation of coastal features of submergence.
• Fall of sea level: This is referred to as decrease in global mean sea level as a result of a decrease of the world’s oceans. This leads to the production of emergence coastal landform.
5.8.3 Causes of sea level change
The sea level changes daily because of the following causes:
• Eustatic variations in sea level are the effects of external forces. Most experts agree that human induced global warming is the force behind the current global sea-level rise. There are three factors that primarily affect eustatic sea level change that are: thermal expansion of the ocean, melting of nonpolar glaciers, and change in the volume of the ice caps of Antarctica and Greenland.
• The changes in global temperature affect the amount of ice stored on land as water, thus changing the sea levels. A rise in temperatures causes the ice caps to melt, and sea levels rise, and vice versa.
• The changes in sea levels are also affected by the steric effect. This is where the density of the water increases or decreases as the temperature rises or falls. If the temperature rises the water expands and if it falls the water contracts. It is estimated that sea levels can rise up to 0.4 mm per year.
• The mass of ice adds weight to the earth’s crust causing it to sink lower into the mantle resulting into relative rise in the sea-level during glacial period.
• Isostatic re-adjustment; at the end of glacial period, the mass of ice melts and the weight is lost from crust which then rises. When the ice melts the land begins to rise as the weight is removed. This results in a relative fall in sea-level. This is called isostatic re-adjustment
• Uplift/mountain building due to plate movements may also result in a relative fall in sea-level as land rises up.
• Pre-glacial erosion of a coastline causes the coast rise and end-up to the sea level change.
5.8.4 Evidences of sea level changes
The following are evidences of sea level changes:
• The presence of old coastline high above the present sea level: During the Ice Age the sea levels fall leaving the old coastline. Since the end of the Ice Age, sea levels have risen again, but not to their previous levels. The raised beaches continue to be above the present sea level by quite a distance.
• The estuaries and inlets flooded: Sea level rise after the last Ice Age caused estuaries and inlets to be flooded. This occurred in South West England, drowning many river valleys around the coasts of Devon and Cornwall, and creating Rias. In other, more northern areas, glacial valleys were drowned to create Fjords.
• Isostatic re-adjustment phenomenon: Some places in Scotland still undergoing isostatic re-adjustment up to 7 mm per year in some areas.
5.8.5 Effects of the sea level changes
Rising sea level has many impacts on coastal areas. The following are some of them:
• Erosion of beaches and bluffs: Beach erosion is the most common problem associated with rising sea level. Depending on beach composition, beaches erode by about 50 to 200 times the rate of sea level rise. That translates a 2-millimeter (0.08-inch) per year increase in sea level eroding from 10 to 40 centimetres (3.9 to 15.6 inches) of coastline per year. Beach erosion has not only a strong ecological impact, but also a profound economic impact.
• It increases the flooding and storm damage caused by changes in sea level.
• Contamination of drinking water: as the rising sea crawls farther and fartherup the shore, in many places it will seep into the freshwater sources in the ground that many coastal areas rely on for their drinking water. Saltwater is unsafe to drink, and while it is possible to remove the salt from water, doing so is an expensive and complicated process.
• Interference with farming: Those same freshwater sources we use for drinking also supply the water we use for irrigation. The problems here are the same: The intruding sea could make these groundwater sources saltier. Saltwater can stunt or even kill crops, but creating freshwater from saltwater is a costly and unsustainable practice.
• Change in coastal plant life: more saltwater hitting the shores changes the soil composition on the coast, meaning the plant life there will most likely change as well.
• Threating the wildlife population: Many forms of wildlife make their home on the beach. As the rising ocean erodes the shoreline and floods the areas in which coastal animals live, animals like shorebirds and sea turtles will suffer and die and others will migrate.
• Hurting the economy: the tourism and real-estate industries in coastal areas are likely to take a hit as prime beachfront properties and recreational areas are washed away by rising waters. This is a fact that some involved in these industries are finding hard to swallow.
Application activity 5.8
1. Explain the causes of sea level change
2. Describe the evidences of sea level change
3. Basing on the study of sea level change. Visit a local water body and identify the evidences of its water level change.
4. According to you, which feature is more attractive to tourism. Defend your view
5. Explain the environmental effects of sea level changes.
End unit assessment
1. Describe the major features resulting from wave erosion and deposition processes.
2. Observe the following photographs and answer the questions that follow: