Topic outline

  • General

  • UNIT 1: INTERPRETATION OF PHOTOGRAPHS AND VIDEO IMAGES

    Key Unit competence:
    By the end of this unit, I should be able to interpret photographs, video and 

     images.

     Introductory activity
     In the previous units, it was shown that maps are very important tools to 
    indicate and to describe physical and human features. Identify and describe 
    other techniques used in geography to locate and display physical and 

    human features.

     1.1. Definition and types of photographs
     Learning activity 1.1

     Describe  the difference between the two photographs provided below:

    1.1.1. Definition
     A photograph is a picture of an object or environment taken by a camera at 
    a particular time in a given place. Photographs are techniques 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 appear clearly. A hidden ground or area which cannot be 

    seen by a camera when a photograph is taken is called a dead ground.

     1.1.2. Major types of photographs
     There are two major types of photographs: Terrestrial / close or ground 
    photographs and Aerial photographs.

     

    1) Ground Photographs


     Also called terrestrial or close photographs, ground photographs are 
    photographs taken from the ground level. They record targets exactly what a 
    person would see if he or 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 types 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 are images taken from aerial station such as aircrafts, 
    satellites and other flying objects. They cover a wide area where features are 
    greatly reduced. They show the top of the object and do not view objects in a 
    perfect horizontal perspective. 

    There are two categories of aerial photographs: 
    i) Vertical aerial photographs are images taken when the camera is 
    directly located above or overheading the target 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 1.1

     Identify the types of the photographs A and B below and describe them

    1.2. Sections of a photograph and interpretation of 
    physical and human aspects
     Learning activity 1.2
     Observe the photograph below and answer the following questions:
     1) Identify the physical and human features shown on the below 
    photograph.
     2) Indicate the respective parts where these features are found in the 

    below photograph. 

    1.2.1. Sections of a photograph
     From a horizontal perspective, photographs have three parts described 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 a vertical perspective, photographs are also divided in three parts: left, 
    centre and right.

    Combining both horizontal and vertical perspectives, the photographs can be 

    put into the following categories:

     1.2.2. 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 landforms 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 1.2
     Observe the photograph below and describe the physical and human 

    aspects represented on it.

     1.3. Relationship between physical and human aspects on 

    photographs and video images

     Learning activity 1.3
     Describe the relationship between physical and human features represented 

    on the photograph below:

    Photograph showing physical features (down-left) and human 
    features (up-right):     the arrow indicates the position of a river which 

    drains the valley that appears on the photograph.

    Some photographs and video images help in illustrating the relationship 
    between human and physical aspects. Basing on the figure provided above, the 
    relationship between human and physical aspects can be explained as follows: 
    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 1.3
     Observe the photograph below and describe how physical features have 

    influenced human activities in the area.

    Skills Lab
     With help of the knowledge, skills, attitudes and values acquired in this unit, 
    suggest ways of conserving the physical features and promoting economic 

    activities in your school environment for sustainable development. 

    End unit assessment

     Study the photograph provided below and answer the following questions:

    1) Identify the economic activities taking place and describe their 
    importance to the people living in the area. 
    2) Suggest ways of conserving the area in the background of the 
    photograph for environmental sustainability.
     3) Identify the human features which are predominant in the foreground 

    of the above photograph

  • UNIT 2 :THE ORIGIN AND DISTRIBUTION OF THE CONTINENTS

    Key Unit competence: 
     By the end of this unit, I should be able to discuss theories of the origin and 
    the distribution of continents.
     Introductory activity
     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.
    2.1. Concept and theories of continental drift
     Learning activity 2.1
     • Make research using books and internet to explain the theory of 
    Alfred Wegener on the continental drift.
     2.1.1. 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. 
    2.1. 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 and Frank Taylor’s theory.
    The theory of the origin and distribution of the continents and ocean 
    basins according to Alfred Wegener
     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 break-up of Pangaea and periods of disintegration
    The theory of continental drift traces the origin and distribution of continents 
    through five major steps:
     i) The supercontinent 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 above). 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.   
    Application activity 2.1
     1) Explain the concept of continental drift 
    2) Explain why Taylor’s theory on the origin and distribution of the 
    continents and ocean basins was initially criticized.
     2.2. Evidences of continental drift
     Learning activity 2.2
     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. 

    iii) 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 2.2
    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. 
    2.3. Effects of continental drift on the evolution of physical 
    features
     Learning activity 2.3
     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 parts 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 highest 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 
    trenches, mountain range formation, and other geologic phenomenon 
    which created the new landscapes on the earth’s surface;
     Application activity 2.3
     Explain the effects of continental drift on the evolution of physical landscape 
    of the earth.

     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
     2.4.1. 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. 

     2.4.2. 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:

     2.4.3. 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.  

     i) 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 landforms (folded mountains, volcanoes, insular 
    arcs, deep sea trenches, and batholith intrusion) are found at plate 
    boundaries.
    Major landforms resulting from plate movements:


     Application activity 2.4
     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.
    2.5. Major plates and effects of plate tectonics
     Learning activity 2.5
     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.

    2.5.1. 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). 

     2.5.2. Effects of plate tectonics

     The following are the main effects of plate tectonics:

     i) Earthquake
     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.

     iii) Tsunamis
     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 sometimes be imperceptible 

    when their magnitude is low. 

    Application activity 2.5
     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 continents, 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 tectonically 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: 
    Knowing the causes of the earthquake, explain how Rwandans can 

    cope with it and its impacts and other resulting natural hazards.  

    Skills Lab
     Basing on the distribution of continents and oceans basins, discuss the 

    geological evidences  of continental drift.

     End unit assessment
     1) What is the contribution of Wegner’s theory 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.

  • UNIT 3:EXTERNAL LANDFORM PROCESSES AND RELATED FEATURES

    Key Unit competence:
    By the end of this unit, I should be able to demonstrate an understanding of 

     different landforms resulting from the external processes.

     Introductory activity
    Observe the photographs below and explain the processes that affected 

    the rocks that appear on them.

    3.1. Definition, types and process of weathering

     Learning activity 3.1

     1. Differentiate physical weathering from chemical weathering

     2. Outline the processes of chemical weathering

     3.1.1. Definition of weathering

     Weathering refers to the process of disintegration and decomposition of rocks 

    into small particles by the action of weather and living organisms.

     Agents of weathering include the temperature, rainfall (water), wind, animals 

    and plants (vegetation).

     3.1.2. Types of weathering and processes
     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.

    i) Physical weathering
     Physical weathering refers to the breaking down 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:
    1) Thermal expansion or insolation 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

     2) Exfoliation
     Exfoliation occurs when there is expansion of rocks during the day and 
    contraction of rocks during the night due to repeated temperature changes. 
    This process 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.

     3) 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. 

     4) 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 
    (decreases) pressure, which causes the materials below to expand and crack 

    parallel to the surface.

     5) 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. 

    6) Shrinkage weathering
    Some clay 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 their volume. During dry seasons, they massively lose this water 
    through evaporation and they contract. This process of alternation of expansion 
    of these rocks during the wet season and contraction of clay during the dry 
    season is known as shrinkage. This creates stresses and weakness of rocks 
    causing cracks within the rock.

    7) 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).

    ii) 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: 
    1) 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.

    2) Carbonation: This is the process through which rain water dissolves the 
    atmospheric gases of carbon dioxide (CO2 ) to form a weak carbonic acid 
    which reacts rocks to wear (weather) them away especially in limestone 
    areas. After reaction, new compounds are produced as it is shown by the 

    following equation:

     3) 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.

    4) 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.

    5) 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.

    6) 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).

     7) Chelation: Chelation is a form of chemical weathering by plants. It 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. 

     iii) 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 rocks.

     Application activity 3.1
    Use your local environment to identify the evidences of biological weathering.

    3.2. Factors influencing weathering and interdependence 
    of physical and chemical weathering
     Learning activity 3.2
    Using the diagram below, explain how these elements influence the rate of 

    weathering in your local area.

    A number of factors are required for weathering to occur in any environment. 
    The major factors of weathering include relief, living organisms, time, climate 

    and rock (parent material),

    i) Relief
    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 leeward 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.

     iii) Time
    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.  

    iv) Climate
     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 3.2
    Visit your local environment and explain how relief and climate have influenced 

    the rate of weathering.

     3.3. Landforms associated with weathering and their 
    importance
     Learning activity  3.3
     1. Identify the features associated to weathering
     2. Analyse the importance of the following weathering features
    a) Cave
    b) Oasis

    Landforms processes may be similar of different depending on whether rocks 
    have the same or different mineralogical compositions.  The major landforms in 

    different geological structures are briefly presented in the following paragraphs.

     3.3.1. Landforms associated to weathering in limestone regions

    Limestone is a sedimentary rock in which calcite (calcium carbonate: CaCO    3 ) 
    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 landforms associated with weathering in limestone regions are Karsts 
    landforms that include: caverns, stalagmites, stalactites, pillar, dolines, limestone 

    pavements (uvalas), poljes.

     1) Caverns
     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.

    2) Doline
    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.

    3) Uvala
    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.

    4) 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. 

    5) Stalactites
    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.

    6) Stalagmites
    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.

    7) Pillars
    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.

    3.3.2. Landforms associated with weathering in arid regions 
    The features formed in these regions as a result of weathering are both erosion

    al and depositional. 

    a) Erosional features
    1) Inselbergs
    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.

    2) Bornhardts
     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.

     3) Tor
     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

     4) Pediment
    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.

    5) Deflation basins
     Deflation is the process whereby loose or non-cohesive sediment are blown by 
    the wind. Depressions formed in the deserts due to removal of sand through the 
    process of deflation are called Deflation Basins.  They are also called blow
    outs     or deserts hollows. The depth of deflation is determined by groundwater 

    table. 

    6) 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.

     7) Demoiselles
     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.

    8) Zeugen
    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. 

    9) Yardangs
    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.

    10) Reg
    Reg is a desert surface covered 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.

    11) Oases
    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).

    b) Depositional features in desert
    1) Dunes
     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.

    – Barkhans
     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.

    – Seifs
     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.

    2) Loess
     Loess is a wind-blown deposit of fine silt and dust. It is unstratified, calcareous, 

    permeable, homogenous and generally yellowish in colour. 

     3) Erg 
    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.

     3.3.3. Importance of landforms resulting from weathering
    – This soil supports poor scrub vegetation as well as some shrubs and 
    grasses.
    – Chalk landscapes are characterized by undulating topography.
    – The surface and underground landforms of karsts appearance are 
    beautiful to attract tourists.
    – Limestone blocks are used for building houses.
    – They are also raw materials for cement manufacturing.
    – Weathering results into soil formation.
    – It produces a number of landforms which modify the nature of landscape
    – It produces lateritic soils, which are important in road construction.
    – It helps to expose mineral rock on the surface.

    – It produces clay which is important in pottery industry

    Application activity 3.3
     1. Examine the contribution of weathering on human activities

     2. Humid tropical regions are the most affected by weathering. Discuss

     3.4. Mass wasting
     3.4.1. Definition and types of mass wasting
     Learning activity  3.4
     Study the photograph below taken in northern part of Rwanda and describe 

    the cause of the phenomena which happened. 

    i) Mass wasting
     Mass wasting, also called 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. 
    ii) Types of mass wasting
     Mass wasting is classified into two major categories: Slow movement and 

    rapid movement.

     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 climates. 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
     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.

     3.4.2. 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.

     Application activity  3.4
     1. Examine the major causes of mass wasting

     2. Using diagrams distinguish between slumping to rock fall

     3.5.  Effects and control measures for mass wasting
     Learning activity  3.5
     Observe the photograph below showing the effects of mass wasting and 

    answer questions:

     1. Analyse the effects of mass wasting.

     2. Suggest any three measures to control mass wasting.

    3.5.1. 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: I    n 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.

     3.5.2. Control measures for mass wasting
     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 3.5
    Make a field trip to observe different areas affected by mass wasting. 
    Analyse the causes of mass wasting and propose the sustainable measures 

    to control it.

     Skills Lab
    Identify any area mostly affected by mass wasting, examine how the 

    Community Work / Umuganda may help you to fight against it.

     End unit assessment
     1. Give the reasons why highlands are the most affected by mass wasting. 
    2. How have topography and parent rock influenced the rate of 
    weathering in your area? 
    3. Explain how the weathering landforms identified in your area affect 

    positively and negatively human activities.

  • UNIT 4 : WAVE EROSION AND DEPOSITION

     Key Unit competence
     By the end of this unit, I should be able to categorise different features 
    resulting from the wave action and their relationships with the human 

    activities

     Introductory activity

    Use the pictures provided below and answer the following questions: 

    1) Identify the coastal landforms found on figure above. 
    2) Explain the factors for formation of the coastal landforms identified 

    on the figure.

     4.1. Coastal landforms: Definition of key terms and types of waves
     1. Make a research and show the difference between the following 
    terms related to coastal landforms:
     a. Coast
     b. Shore
     c. Wave
     d. Longshore drift

    2.  Mention the type of waves 
    Learning activity  4.1
     4.1.1. Definition of key terms
     The following are definitions of some terms related with coastal landforms: 
    • 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 .Waves are created 
    by the transfer of energy from the wind blowing over the surface of 
    the sea or from submarine shock waves by earthquakes or volcanic 
    activities (e.g. Tsunami).
    • When waves appear with high frequency they demonstrate the short 

    wavelengths. 

    Structure of wave
     Longshore drift, often used interchangeably with beach drifting, is a general 
    term for sediment transport parallel to shore in the nearshore zone due to 
    incomplete wave refraction. In this process sediments transported by the river 
    systems are moved by ocean waves and ocean currents to form beaches. 

    Beach drifting,     also called littoral drifting, is a process in which waves breaking 
    at an angle to the shoreline move sediment along the beach in a zigzag fashion 

    in the swash zone.  Both processes are illustrated on figure below.

     A wave approaching a straight coastline at a large angle will have velocity 
    progressively decreasing. This will cause the wave to swing around, but it may 
    not have enough time to conform fully to the shape of the shoreline before 

    breaking, leading to littoral drifting.

     4.1.2. 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. 
    There are four types of breaking waves: spilling, plunging, collapsing, and surging.
     Spilling Waves 
    Spilling waves are waves that are produced when the ocean floor has a gentle 
    slope. As the wave approaches the shore, it slowly releases energy, and the 
    crest gradually spills forward down its face until it is all whitewater. These waves 
    take more time to break than any other wave. Surfers usually call these waves, 
    “mushy waves.”

    Plunging Waves
     Plunging waves are formed when the incoming swell hits a steep ocean floor or a 
    sea bottom with sudden depth changes. As a result, the wave’s crest curls over 
    and explodes on the trough. The air under the lip of the wave is compressed, 
    and a crashing sound is often heard. Plungers are more common in offshore 
    wind conditions.
     
    Surging Waves 
    Surging waves are produced when long period swells arrive at coastlines with 
    steep beach profiles. The base of the wave moves fast and does not allow the 
    crest to evolve. As a result, the wave almost doesn’t break, and there is little 
    whitewater. Surging waves look friendly, but can be quite deadly because of the 
    backwash associated with them.

    Collapsing Waves
     Collapsing waves are a blend between surging and plunging waves. The crest 
    never completely breaks, and the bottom face of the wave gets vertical and 
    collapses, resulting in whitewater.
    – 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.

     Application activity 4.1
     1. Differentiate a constructive wave from a destructive wave.
     2. If you find an occasion to visit the ocean coast as an East Africa 
    person, describe the coastal features you would be interested to 

    discover and explain why.

     4.2. Factors determining the strength of waves and wave 
    action processes
     Learning activity  4.2
     1. Analyse the factors that determine the strength of waves on the coast

     2. Explain how waves can cause erosion along the coast.

     4.2.1. 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, wavelength and height increase. Therefore waves 
    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.

     4.2.2. 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 landforms such as wave-cut, sea cliffs, and even 
    unusual landforms such as caves, sea arches, and sea stacks can form.

    They erode in four ways as:
    1) Solution: it is also called corrosion. It is common on coasts composed 
    of soluble rocks such as limestone and rock salt.
    2) Corrosion or abrasion: this is a type of wave erosion in which the load 
    already weathered down and hence being transported drag itself on the 
    bed of the coast and hence wears away some rock particles.
    3) Attrition: this is a process of wave erosion which involves the reduction 
    in size of eroded particles by themselves.
    4) Hydraulic action: this is the direction of breaking waves that push water 
    on a cliff. As this water retreats during a backwash, pressure is suddenly 
    released and this generates shock waves that weaken rock particles and 
    make them easily eroded by a backwash. 
    • 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. Longshore drift, longshore 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 
    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 silt, sand and shingle.

     Application activity 4.2
     1) Wave erosion is done in four ways, differentiate them

     2) Explain the impact of wind and tides on the strength of the waves.

    4.3. Factors for Formation of coastal landforms and 
    landforms produced by wave and their importance
     Learning activity  5.3
     In section 4.2, we have defined key terms related to coastal landforms. 

    Observe carefully the following figure and answer the following questions:

     1) Identify the landforms produced by wave erosion on the figure above.

     2) Explain the factors that result in the formation of coastal landforms.

     4.3.1. Factors influencing the formation of coastal landforms
     The following are the major factors influencing the formation of coastal landform:
    – Tides: Tides are greatly influencing forces of coastal landforms. 
    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 keeps redefining the shoreline of coastal 
    landforms.
    – Nature of rocks at the coast: Wave erosion is more pronounced on 
    areas that are weak and soluble e.g. jointed and consolidated rocks. 
    Rocks which are strong and highly consolidated are hard to erode. The 
    hard and resistant rocks stand as headlands while easily eroded rocks 
    become bays.
    – Openness of the shore to wave attack: Coasts which are totally 
    exposed to wave attack are easily undermined by wave attack while 
    those which are sheltered by coastal reefs and islands are protected 
    from direct wave attack and are hence less eroded.
    – 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.
    – Abundance and size of loads which is used as an abrasive 
    tool:     When materials e.g.; boulders, sands, etc. are in abundance, the 
    coast line will be easily eroded through corrosion. In the absence of 
    such materials, wave erosion becomes meager.
    – Longshore currents: Parallel movement of water is known as 
    longshore 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 landforms. 
    Winds are directly related to the intensity of waves. Landforms 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 
    landforms. 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.

    4.3.2. Landforms produced by wave erosion (destructive wave)
     The coastal features formed due to marine erosion by sea waves and other 
    currents and   solution processes include cliffs, caves, geo, stacks, blowhole, 
    arch, 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 faces 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.

    – Headland:     Is a projection of land into the sea or lake. Where alternate 
    hard and soft rocks occur at the coast, the weak material is eroded to 
    form a bay while the harder rock resists erosion and remains extending 
    out into the water as a headland.

    – 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.

     4.3.3. Landforms produced by wave deposition (constructive wave) 

    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.

    Different elements of a 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 4.3
     1) Describe landforms produced by wave deposition.
     2) Explain the factors influencing the formation of coast landforms.
     3) According to you, which landforms are likely to be found around lakes 

    in Rwanda ?

     4.4. Importance of coast landforms produced by wave 
    action and type of coasts
     Learning activity  4.4

     Study the following photograph and answer related questions:

     1. Describe the types of coasts.

     2. Describe the economic activities that can be carried out in this area

    4.4.1. Importance of coast landforms produced by wave
     Coastal landforms 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 landforms are very favourable for tourism that contributes 
    to the economic development of countries.
    – Marine, estuary and coastal wetland areas often benefit from flows of 
    nutrients from the land and also from ocean upwelling which brings 
    nutrient-rich water to the surface. They thus tend to have particularly 
    high biological productivity.
    – The world’s fish production is dependent on the nature of coastal 
    landforms like bays and headlands.
    – Beaches support leisure, recreation, trade and mining of sand
    – Mud flats and sand dunes have fine silt which attracts mangrove 
    swamps used in crafts industry.
    – Features produced are important in agriculture development
    – Cliffs protect the land from wave attack.
    – These landform features are used in study purposes.

    – Cliffs may produce waterfalls important in generation of power. 

    4.4.2. Types of coasts
     There are two types of coasts: Submerged coasts and Emerged coasts.
     i) 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: 
    1) 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.

     2) 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.

     3) 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.

     4) 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. Delta: Is a large, flat and 
    low lying plain of river deposits laid down where a river flows to the sea or lake.

     A delta is a large area covered by river deposits (alluvium) formed at the mouth of a river

     Ii) Emerged coasts
     Emerged coasts comprise emerged highlands coasts and emerged lowland 
    coasts.
     A. Emerged upland coasts
     Raised beaches: when the sea level drops, wave activity also drops to lower 
    levels. The wave deposition will be at a new point of low tide level forming a new 
    beach there, hence leaving the old beach up high at a former point of sea. These 
    types of beaches are usually evident on land that is far away from the present 
    edge of the water. They may have been formed at the head of a bay but they are 
    now isolated on land. Most raised beaches are colonized by vegetation.
     Raised cliffs: this is formed when there is a relative fall in the level of the sea.
     A raised terrace: the drop in sea level produces a wave cut platform down to 
    a new level of the sea leaving the former terrace suspended up to the original 

    level of the sea before emergence.

     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 
    costs are: the northern shore of the Gulf of Mexico and the southern shore of 

    the Rio-de-la Plata in Argentina.

    Application activity 4.4
     1. Give five examples of cities located in coastal areas, including at least 
    two cities located in East African Community.
     2. Indicate the type of submerged coast, and describe its characteristics.
     3. Suppose that you live nearby the coast, explain the business 
    opportunities that you may carry out there and the challenges you can face.  

    4.5. Coral reefs: Nature, types and formation of coral reefs
     Learning activity  4.5
     Observe the figure below of a coral reef and answer the following questions: 

    answer the following questions: 



    1. What do you think are the elements that constitute a coral reef?
     2. Analyze the processes in which coral reefs are formed.
     3. What do you think are the problems related to coral reefs formation?
     A coral is a hard limestone rock made up of the skeletons of tiny (very small) 
    marine organisms, known as coral polyps. Also coral reefs are limestone rocks 
    which are formed from dead animals called corals. Corals have a hard shell of 
    calcite, formed by the extraction of calcium carbonate from sea water.

    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.

    1) 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.

     2) 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. 

    3) 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
     1) 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. 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.

     1) 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 doesn’t 
    allow 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 rainforests of the oceans. These cannot survive in extreme 

    warm environment.

    Application activity 4.5
     1) Using illustrative graphics, differentiate the types of coral reefs.

     2) Explain the conditions for coral reefs formation.

    4.6. Theories of the origin of coral reefs, Problems facing 
    the development and growth of coral reefs, Impact of coral reefs
         Learning activity  4.6
     1. Using concrete examples, show how coral reefs are important

     2. Mention the theories explaining the formation of coral reefs

    4.6.1. 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 Theory or 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 theory or antecedence theory 
    The Antecedent-Platform or uplift theory for the origin of coral reefs stipulates 
    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. 
    – The theory agrees that atoll coral reefs formed when the tops of islands 

    were undergone wave action resulting to a platform.


         C. Daly’s theory or 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 


    4.6.2. Impact of coral reefs
     Coral reef landforms 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.

    4.6.3. Problems facing the development and growth of coral reefs
     The following are the major problems facing the development and growth of 
    coral reefs:
    – Overfishing: Increasing demand for food fish and sea tourism has 
    resulted in over fishing of not only deep-water commercial fish, but 
    key reef species as well. This affects the reef’s ecological balance and 
    biodiversity. 
    – Coral disease: coral diseases contribute to the deterioration of coral 
    reef communities 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 favours 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 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.


         Application activity 4.6
     1. Establish the similarities of the subsidence, antecedence and 
    glaciated control theories of coral reefs formation.
     2. Account for the negative impacts of human activities on the coral 
    reefs growth.

     3. Describe the economic importance of coral reefs.


         4.7. Sea level change
     Learning activity  4.7

     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 any three features observed on this picture
     
    4.7.1. Meaning of sea level change and its resulting features 
    The sea level change is the variation and fluctuation of the sea level throughout 
    time. 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.

    4.7.2. Types of sea level changes
     There are two types of sea level changes which are:
     • Submergence or Rise of sea level: This is 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.
     • Emergence or fall of sea level: This is referred to as the 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.

    4.7.3. Causes of sea level change
     The sea level changes daily because of the following causes:
    – Eustatic variations in sea leve    l 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 process 
    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.

    4.7.4. 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.

         4.7.5. 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 farther up 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 4.7
     1. Explain the causes of sea level change
     2. According to you, which feature is more attractive to tourism. Defend 
    your view

     3. Explain the environmental effects of sea level changes.

     Skills Lab
     With help of knowledge and skills acquired in this unit, suggest ways 

    beaches may be preserved and more productive.

    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:
     i) Examine the economic activities that should be carried out in the 
    regions demonstrated on photographs.
    ii) According to you, what are the advantages of coast or shore to 
    people living nearby?
     3) Demonstrate the impacts of sea level change to the environment.

  • UNIT 5 : ROCKS AND MINERALS

     Key Unit competence
     By the end of this unit, I should be able to compare different types of rocks 

    and minerals and evaluate their importance.

     Introductory activity

    Observe the rock provided below and answer the following questions: 

    1. Identify the types of rocks given above.
    2. In which category can they be classified? 
    3. Which properties can help to identify these rocks and their minerals? 

    4. Explain the economic advantages of the rocks and minerals.

     5.1. Rocks: Definition, types and characteristics
    Learning activity  5.1
     Make a field trip in your environment; observe the rock and identify their 
    types and distinctive characteristics.
     
    5.1.1. Definition 
    A rock is a natural aggregate of minerals in the solid state; usually hard and 
    consisting of one, two, or more mineral varieties. Rocks form the solid part of the 
    earth’s crust. Rocks may also include substances like clay, sandstones, 
    shells and corals. Rocks which contain metallic compounds are called ores. 

    5.1.2. Types of rocks  
    There are three major groups of rocks namely igneous rocks, sedimentary rocks 
    and metamorphic rocks. Their classification is based on the mode of formation 
    and the nature of constituting minerals. Characteristics of each rock group are 
    briefly described below.  

    i) Igneous rocks
    The word igneous comes from the Latin word ignis, which means fire. Igneous 
    rocks are rocks formed by cooling of molten material from a volcano or from 
    deep inside the earth. This molten material from inside the earth is known as 
    magma. Igneous rocks are also called magmatic rocks or volcanic rocks. Their 
    formation is associated with the cooling and hardening of molten material from 
    the interior of the earth. 

    ii) Sedimentary rocks
    Sedimentary rocks are the result of the accumulation of small pieces broken 
    off from pre-existing rocks (igneous rocks, metamorphic rocks and sedimentary 
    rocks) or precipitation of dissolved minerals. Sedimentary rocks form when 
    sediments become pressed or cemented together or when sediments precipitate 

    out of solution. 

    iii) Metamorphic rocks
     The metamorphic rocks get their name from “meta” (change) and “morph” (form). 
    Metamorphic rocks are formed from pre-existing rocks due to increases in heat 
    and pressure which alter rock structure and chemical composition. Therefore, 

    sedimentary and igneous rocks can become metamorphic rocks.   

    There are four factors that contribute to the formation of metamorphic rocks:  
    – Heat or high temperature: this speeds up the chemical reactions that 
    result in metamorphic rocks. The heat is from magma, steam from hot 
    water and rocks sinking deeper into the warmer layer of the crust
    – High pressure which changes the mineral and feel of the original rock.
    – Nature of the parent rock which determines how resistance it is to change.
    – Time which determines the period required for the chemical 

    reactions to take place.

     5.1.3. Characteristics of rocks
     A. Characteristics of igneous rocks
     Igneous rocks have the following characteristics:
    – They are hard, and water does not pass through their joints easily, that 
    is why they are less affected by erosion;
    – Igneous rocks have a lot of minerals;
    – They do not have strata or layers;
    – They do not contain fossils (fossils are remains of plants and animals 
    fixed in rocks);
    – The number of joints increases upwards in any igneous rock;
    – Igneous rocks are mostly associated with volcanic activities and are 
    mainly found in the volcanic zones. That is why they are also called 
    volcanic rocks.

    Igneous rocks can also be classified based on the chemical and mineralogical 
    compositions, texture of grains, forms and size of grains, and the mode of origin, 

    igneous rocks are classified as follows:

     1) Classification based on the amount of silica
     • Acidic igneous rocks: they contain more silica: (≥65% of SiO2 );

     • Basic igneous rocks: they contain low amount of silica (≤ 45% of SiO2 ).

     2) Classification based on the chemical and mineral composition
     • Felsic igneous rocks: they are composed of the dominant minerals of 
    the light group (e.g. Silicon, Aluminum).
     • Mafic igneous: they arecomposed of the dominant mineral of dark 
    group (magnesium and iron).
    3)  Classification based on texture of grains 
    • Pegmatitic igneous rocks: they are very coarse grained: (e.g. granite);
     • Phaneritic igneous rocks: grains of minerals are of intermediate size;• 
    Aphanitic igneous rocks: they are fine grained igneous rocks);
     • Glassy igneous rocks: they don’t contain a defined grain size;

     • Porphyritic igneous: they have mixed graine sizes.

    4) Classification based on the mode of occurrence
     • Intrusive igneous rocks: They are formed when the rising magma, 
    during a volcanic activity, does not reach the earth’s surface but rather 
    cools and solidifies below the surface of the earth. Intrusive igneous 
    rocks fall into two categories:
     a) Plutonic igneous rocks: they are formed due to the cooling of 
    magma very deep inside the earth.
     b) Hypabyssal igneous rocks: they are formed due to the cooling 
    and solidification of rising magma during volcanic activity in cracks, 
    pores, crevices and hollow places just beneath the earth’s surface.
     •  Extrusive igneous rocks: They are formed due to the cooling and 
    solidification of hot and molten lava on the earth’s surface (examples 
    are basalt, Gabbro). Extrusive igneous rocks are further divided into 

    two major subcategories:
    a) Explosive type: The igneous rocks formed by a mixture of volcanic 
    materials ejected during explosive or violent volcanic eruptions.
    b) Quiet type: The appearance of lava through minor cracks and 
    openings on the earth’s surface is called ‘lava flow’. The lava forms 

    basallic igneous rocks after cooling and solidifying.

     B) Characteristics of sedimentary rocks 
    Sedimentary rocks have the following characteristics: 
    – They are the product of other rocks that have already formed; 
    – They appear in the form of layers or strata; 
    – Sedimentary rocks are formed of fragment from materials from older 
    rocks, plant and animal remains;
    – They are found over the largest surface area of the earth;
    – Sedimentary rocks have various minerals because they are a product 
    of different sources; 
    – Most of the sedimentary rocks allow liquids and gases to pass through 
    them (permeable and porous);
    – Sedimentary rocks are characterized by different sizes of joints;
    – Sedimentation units in the sedimentary rocks having a thickness of 
    greater than one centimetre are called beds;
    – As highlighted in the figure below, the composition of sedimentary 
    rocks includes clay, sand, rounded pebbles, angular fragments, calcium 

    deposits and organic carbon.

     C) Characteristics of Metamorphic Rocks 
      The following are the characteristics of metamorphic rocks:
    – They are harder than the original rocks. Therefore, they are not easily 
    eroded;– They do not split easily; 
    – They contain minerals;
    – Some are made up of just one mineral, for example, marble; 
    – They have a different texture or feel from the original rock. 

    Metamorphic rocks present two distinctive physical characteristics:     Foliated 
    metamorphic rocks     and Non-foliated metamorphic rocks. Foliated 
    metamorphic rocks such as gneiss, phyllite, schist and slate have a layered or 
    banded appearance that is produced by exposure to the heat and pressure. 
    Non-foliated metamorphic rocks such as hornfels, marble, quartzite do not have 

    a layered or banded appearance. 

    Application activity 5.1
     1. In which area of Rwanda do we find igneous rocks? Explain their 
    characteristics. 
    2. Observe rocks found in your environment and examine their major 

    rock groups.

     5.2. Composition, properties of rocks and Impact of rocks
         Learning activity  5.2
     Rocks are composed of physical and chemicals elements. describe the 

    physical and chemical properties of rocks.

     5.2.1. Composition of rocks  
    All rocks are composed of minerals. Composition refers to both the types of 
    minerals within a rock and the overall chemical makeup of the rock. The mineral 

    that compose the three types of rocks are presented in the table below. 

    Types of rocks and their forming minerals


    5.2.2. Properties of rocks
     i) Physical properties of rocks
    Physical properties of a rock can be intensive (hardness and softness) and 
    extensive (volume, total mass and weight). Rocks, whether igneous, sedimentary 
    or metamorphic, are subject to powerful stress or pressure by tectonic forces 
    and the weight of overlying rocks. The physical properties of rocks determine 
    their behaviour and respective deformations when a rock is subject to stress 
    such as folding, faulting or warping, and their resulting landscape deformation. 
    – Stress refers to forces that constantly push, pull, or twist the earth 
    crust. There are three types of stress: tension (stretching), compression 
    (shortening), and shear (twisting or tearing). 
    – Strain is how rocks respond to stress whether by stretching, shortening, 
    shearing. 
    – The surface expressions refer to the structure of landforms resulting 
    from the stress depending on whether the rock is brittle (hard) or ductile 
    (pliable). Surface expressions can be folding (bending) or faulting 
    (breaking). Brittle rock breaks (brittle deformation) while ductile 
    rocks like clay bend or flow (ductile deformation).

    The figure below presents different types of stresses that are naturally applied 

    on rocks, their resulting strains and surface expressions. 


    ii) Chemical properties of rocks
    A) Sedimentary rocks

    All water falling onto the earth as rain and running over the earth surface carries 
    minerals in solution. These minerals may precipitate by direct evaporation of 
    water, chemical interaction or by the release of pressure where underground 
    water reaches the surface. Sedimentary rocks formed as chemical precipitates 
    include halite, gypsum, silcretes, ferricretes, limestone, and dolomite. The table 

    below gives details on their chemical composition. 

    Chemically formed sedimentary rocks and their composition


    B) Metamorphic rocks
     Metamorphism involves the alteration of existing rocks either by excessive 
    heat and pressure or through the chemical action of fluids. This alteration can 
    cause chemical changes or structural modification to the minerals making up 
    the rock. Metamorphism process results in the creation of new minerals by 
    the substitution, removal, or addition of chemical ions. Metamorphism may 
    consist of three minerals, kyanite, andalusite and sillimanite. These are 
    all aluminium silicates having the same chemical formula (Al2SiO5 ) but different 
    crystal structures and physical properties. 

    Below is an example of a simplified representation of sediments products and 

    resulting metamorphic rocks from sea beaches to far shelf.

    Lateral representation of metamorphic rocks form beach to far sea shelf


    Igneous Rocks
    The major indicator for the chemical classification of igneous rocks is the amount 
    of Silica (SiO2). Igneous rocks with a high proportion of silica exceeding 65% 
    are said to be acidic or felsic, for example, the granite found on an extensive 
    part of Muhanga District of the Southern Province. Where the amount of silica 
    is very low (less than 45%), the rocks are said to be ultramafic or ultrabasic. 
    Rock having intermediate silica content comprised between 65% and 45% are 
    said to be mafic or basic rocks.     

    Igneous rocks are classified according to their forming minerals (see the table 
    below). Mineral groups include Felsic minerals (feldspars and silica), mafic 
    minerals (magnesium and iron), and ultramafic minerals (low silica content). 
    Some of these rocks form underneath the earth’s crust and are known as 
    intrusive magmatic rocks, whereas other form from the volcanic lave that 

    reached the earth’s surface, forming extrusive volcanic rocks.

    Families of igneous rocks and constituting minerals




    5.2.3. Impact of rocks: advantages and disadvantages on the 
    landscape and man
     A) Advantages of rocks on the landscape and on the man
     Rocks have a wide variety of uses. Many of them are used as building materials 
    of houses and infrastructures such as roads and rail ways.
    – Some rocks are more resistant to weathering and others are less resistant. 
    This difference in rock resistances provides various landscapes such 
    as alternation of elevated topographies (hills, mountains or interfluves) 
    and depressions (valleys and low-lying areas) which are sometimes 
    drained; 
    – Gravel and sand, being among products of rock weathering make 
    beautiful landscape at some location of the earth. Also, the weathering 
    of rocks provides different types of soils including sand, silt and clay 
    which are useful at varying points for agriculture. 
    – Some rocks present beautiful landscapes which may attract tourists; 

    – Some rocks store, purify water and act as water sources to rivers.

    The table below shows usages of rocks.





    B) Disadvantages of rocks on the landscape and man
    – Hard and resistant rocks hinder the penetration of plant roots hence, 
    limiting the weathering process or hindering the growth of vegetation;
    – Rock forming minerals have different colours. The difference in colours 
    makes minerals to absorb differently the heat. Dark-coloured minerals 
    absorb much heat during daytimes and therefore expand, causing the 
    cracking and fragmentation of rocks.
    – The sand can blow; rocks can roll risking injury to people;
    – Light-coloured rocks reflect sunlight and increase the temperature 
    around the plants during the daytime;
    – Some environments such as sand rocks (dunes, erg, etc.) are not 
    suitable for human settlement because of lack of water and soils;
    – Some rocks may reflect landscape with steep slopes where human 
    activities such as agriculture or settlement cannot be possible.
     
    Application activity 5.2
     1) Referring to the properties of rocks, explain how rocks react to the 
    stress and the resulting landscapes? 
    2) Analyse a sedimentary rock in your local environment and describe 
    the process under which it might have been formed. 
    3) With relevant examples, discuss the disadvantages of rocks on 

    landscape and society.

    5.3. Minerals
     Learning activity  5.3
     1) Account for the types and characteristics of minerals.
     2) Examine the use of minerals to your society.
     
    5.3.1. Definition, types and characteristics of a mineral
     A mineral is a solid inorganic substance that occurs naturally in the earth’s 
    crust. A mineral deposit is a concentration of naturally occurring solid material 
    in or on the earth’s crust. Mineral resources are non-renewable. 
    There are five characteristics shared by all minerals.  
    – All minerals are formed by natural processes. They can form when 
    magma cools, when liquids containing dissolved minerals evaporate, or 
    when particles precipitate from solution. 
    – Minerals are inorganic. They are not alive and are not made by life 
    processes. Coal, for instance, is made of carbon from living things. 
    Although geologists do not classify coal as a mineral, some people do. 
    Miners, for example, generally classify anything taken from the ground 
    that has the commercial value as a “mineral resource”.   
    – Minerals are solid and have a definite volume and shape. A gas such 
    as air and a liquid such as water aren’t minerals because they do not 
    have definite shape. 
    – Every mineral is an element or a compound with a chemical 
    composition unique to that mineral.
    – The atoms in a mineral are arranged in a pattern that is repeated 
    over and over again    .
    The table below shows two examples of mineral crystals (salt and quartz) with 
    defined shapes:

    Examples of mineral crystals with defined shapes 

    5.3.2. Types of minerals and ores

    The wide varieties of minerals that have been explored by man for general and 
    commercial purposes to satisfy his needs are of two types: metallic minerals 

    and non-metallic minerals. 

    1) Metallic minerals
     Metallic minerals include:
    – Industrial metallic minerals: iron ore
    – Ferroalloy metallic minerals: manganese, chromium, cobalt, molybdenum, 
    vanadium, nickel.

    – Precious metallic minerals: gold, silver and platinum

    2) Non-metallic minerals
     This category of non-metallic minerals includes salt, tin, potash, asbestos and 
    sulphur.

    Rocks or minerals worked because they contain valuable (profitable) elements 
    are usually called ore-deposits. Minerals are extracted in a mineral ore. For in
    stance, Aluminum comes from the ore bauxite. The iron comes from the mineral 
    ore Hematite. A mineral can also be called an ore, for example Hematite is a 
    mineral that can also be called an ore. A mineral is an ore if it contains useful 
    substance that can be mined at a high profit and be processed and refined into 
    more useful materials. For instance, Aluminum can be refined from bauxite, and 
    made into the useful products. These products are worth more money than the 

    cost of the mining, so bauxite is an ore. 


    5.3.3. Physical properties of minerals
     The most common minerals in earth’s crust can often be identified in the field 
    basing on their basic physical properties such as their form, hardness, fracture, 
    cleavage, colour, streak, density, luster, mass, taste, odour, feel, magnetism as 
    described below:

    1) Form:     Definite geometrical forms called crystals can be recognized 
    in minerals. These are for example: cubic, acicular (needle shaped), 

    columnar, fibrous, reniform (kidney shaped) and nodular forms.

    Pyrite (left) has a cubic form; Tourmaline (middle) is prismatic; azurite and 
    malachite (right) are often amorphous.

    2) Hardness:     The hardness of a mineral can be tested in several ways. 
    Most commonly, minerals are compared to an object of known hardness 
    using a scratch test developed by Friendrich Mohs. He assigned integer 
    numbers to each mineral, where 1 is the softest and 10 is the hardest. 

    This scale is shown below.

    If the gem minerals are excluded, the scale has only 7 numbers. Substitutes may 
    be used when the scale minerals are not available:
    – Easily scratched by nail;
    – Not so easily scratched;
    – Can be scratched by a piece (a copper coin);
    – Scratched easily by knife;
    – Can be scratched by knife with difficulty;
    – Scratched by window-glass;
    – Window-glass is scratched by the mineral.
    3) Fracture: Freshly broken surfaces of minerals present characteristic 
    fracture surfaces. The following important types are noted:
    – Conchoidal (vitreous):  the fracture surfaces are curved with a concave 
    or convex form; for example, quartz.
    – Even: the fracture surfaces are nearly flat; for example, in chert.
    – Uneven: the fracture surface is formed of minute elevations and 
    depressions; for example, most of minerals.
    4) Cleavage: This is how the mineral breaks. Certain minerals split easily 
    along certain planes called cleavage-planes. These planes are parallel 
    to certain faces of the mineral crystal, or to faces of a form in which the 
    mineral may crystallize.
    5) Colour: When a body absorbs all the seven colours that make up white 
    light it appears black, and when it reflects all the colours it appears white. 
    When a body reflects the green vibrations of white light and absorb the 
    other vibrations it appears green. Thus, the colour of a body depends on 
    the selective reflection and absorption of the different vibrations of white light.
    6) Streak: The colour of the powder of minerals sometimes differs from 
    the mineral in mass. Different specimens of the same mineral might show 
    variation in colour, yet the streak is fairly constant.
    7) Luster: The amount and the type of reflection from the surface of a 
    mineral determine its brightness. 
    8) Mass: The mass of a mineral can be used to identify its type.
    9) Density: The density of a mineral can also be used to determine its type.
    10) Taste: Some of the minerals which are soluble in water give distinctive 
    taste but the character is not very useful in identification of minerals 
    because there are only a few minerals which are soluble is water. For 
    example, we get a saline taste in case of common salt, and alkaline in 
    case of soda or potash.
    11) Odour: Only a few minerals give characteristic odour, e.g. the odour of 
    garlic from arsenic compounds.
    12) Feel: Minerals differ in the sensation they give by touch, e.g. minerals are 
    smooth, greasy or rough.
    13)  Magnetism: Generally, iron bearing minerals are magnetic, but not 
    necessarily all iron bearing minerals are magnetic.  Some non-magnetic 
    minerals like monazite are also slightly magnetic. The electromagnetic 
    minerals depend on the varying magnetism of different minerals. 

    5.3.4. Chemical properties of minerals
     Some minerals are affected by the variations in temperature and the pressure on 
    the earth’s surface. Others vary in the structure depending on the percentage of 
    water that they loose with the change of the temperature and the pressure. The 
    chemical composition influences the destruction of the rocks and development 
    of new minerals. 

    Chemical properties of minerals are identified from their chemical composition. 
    We refer to two elements that are Silicon and oxygen. These are the two most 
    abundant elements in the earth crust. They constitute approximately 90% of 
    the crust of the earth. Then we distinguish silicate minerals and non-silicate 
    minerals. Silicate minerals (silicates) are minerals containing Silicon and Oxygen 
    atoms usually with one or more other elements. Non-silicates are minerals other 

    than silicate minerals.

    Chemical properties of minerals


     5.3.5. The importance of minerals and manufactured products
     Minerals provide the material used to make most of the things of industrial-based 
    society; roads, cars, computers, fertilizers, etc. In more than 1600 minerals 
    identified in earth crust, only 200 are extracted for commercial and industrial 
    purposes and less than 1/3 are the most economically significant. 

    Some minerals have high economic value because of their uses or they are rare 
    and beautiful. For example, germs or Gemstones is a mineral with a distinctive 

    colour which makes it expensive. That is why it is used for jewellery. 

    The table showing manufactured products from minerals




    Application activity  5.3
    1. What are the five characteristics shared by all minerals?
    2. Differentiate a mineral from an ore.  
    3. Identify minerals that are extracted in your district and describe their 

    advantages and disadvantages.   

    Skills Lab
    Show how you are going to use available rocks for the economic 

    improvement of your    society.

     End unit assessment
     1. Classify the different types of rocks and their characteristics.
     2. Evaluate the economic importance of rock and minerals in your society.

     3. Identify the physical and the chemical properties of the minerals.

  • UNIT 6 : CLASSIFICATION OF SOILS AND SOIL FORMATIO

    Key Unit competence
    By the end of this unit, I should be able to explain the classification of soils 

    and factors responsible for the formation of the soil.

     Introductory activity
     Read the passage below and answer the questions that follow:
     Soil is defined as the thin layer of material covering the earth’s surface and 
    is formed from the weathering of rocks. It is composed of mineral particles, 
    organic materials, air, water and living organisms all of which interact slowly 
    but constantly.

    Most plants get their nutrients from the soil and they are the main source 
    of food for humans, animals and birds. Therefore, most living things on land 
    depend on soil for their existence.

    Soil is a valuable resource that needs to be carefully managed as it is easily 
    damaged, washed or blown away. If we understand soil and manage it 
    properly, we will avoid destroying one of the essential building blocks of our 

    environment and our food security.

     1. Identify major types of soil in the world
     2. Describe factors responsible for soil formation
     3. Assess the importance of soil to man
     4. Discuss the major causes of soil erosion and suggest what should 

    be done to prevent it

     6.1. Definition of the soil
    Soil is a dynamic natural body capable of supporting a vegetative cover. It 
    contains chemical solutions, gases, organic refuse, flora, and fauna. The physical, 
    chemical, and biological processes that take place among the components of a 

    soil are integral parts of its dynamic character.

     6.2. Classification of the major types of soil in the world, 
    factors and processes of soil formation
    This section presents briefly the classification of the major types of soil in the 

    world, factors and processes of soil formation are briefly described.

     Learning activity  6.1
     1. Make research on the major types of soils in the world.
     2. Identify factors influencing soil formation

     3. Discuss on processes leading to the formation of the soil

     6.2.1. Classification of the major types of soil in the world
     The classification of soils is either based on geographic regions, where 
    the soils are well-developed from the parent material by the normal soil
    forming action of climate and living organisms.  Another way of classifying 
    the soil is based on the level of weathering, which is related to geographic 
    environments, but also under the same geographical region you can find 

    different types of soils which reflect the level of weathering.   

    A) Soil classification based on geographical regions
     The soil classification based on geographical regions, include three soil 
    classes: zonal soil, intrazonal soil and azonal soil.

     i) Zonal soils 
    These are soils that cover a wide geographic region in the world. They depend 
    on the major climatic zones, vegetation and living organisms in areas where the 
    landscape and climate have been stable for a long time.  They are common on 
    gentle slopes. They are found both in tropical and temperate regions. 
    This kind of soil has the following types: Tundra Soils, Podzols, Brown forest 
    Soils, Lateritic Soils / Latosols / Ferralsols, Chernozem / Prairie / Steppe, 
    Grumusol / Reddish Brown Soils, Desert (Seirozems and Red Desert) Soils.

    ii) Intrazonal soils
     These are soils that mainly develop due to relief of the area and the nature of 
    parent rock. These soils reflect the dominance of a single local factor, such as 
    parent rock or extremes of drainage that prevail over the normal soil-forming 
    factors of climate and living organisms. They are divided into three types:
     • Calcimorphic or calcareous soils which develop on limestone 
    parent rock (rendzina and terra rossa);
     • Halomorphic soils which contain high levels of soluble salts (e.g. 
    sodium ions) which render them saline. 
    • Hydromorphic soils that have constantly high water content which 
    tends to suppress aerobic factors in soil-formation.
     
    iii) Azonal soils 
    Azonal soils have a more recent origin and occur where soil-forming processes 
    have had insufficient time to operate fully. They lack well-developed horizons 
    because of immaturity or other factors that have prevented their development 
    such as excessive soil erosion. They are skeletal soils resulting from erosion and 
    deposition. They lack clear soil horizons. They are common in volcanic regions, 
    glaciated regions and areas blown by winds. They include dry sand, loess, 
    moraine soils, and marine soils, alluvial and volcanic soils. 

    The map below shows the major soil types of the world

    B) Soil classification based on level of weathering 
    Basing on the level of weathering, the American soil taxonomy has classified 
    soils into 12 soil orders which reflect the level of weathering (slight, intermediate 

    and strong) plotted on the chart and briefly described below:

    Table: Major soil orders according to American classification

    6.2.2. Soil formation factors
     Soil formation is a function of five factors which include parent material, 
    climate, biology (living organisms), relief (topography), and time. They are 
    classified passive (parent material, relief “topography” and time) and dynamic 
    (climate and biology “living organisms)”. Recent studies have shown that human 
    activities can have an impact on soil development. These factors interact as a 

    system to form soils. The roles of these factors are briefly hereafter described:  

    Parent rock
     Physical and chemical weathering of rocks in the upper lithosphere provides 
    the raw mineral ingredients for soil formation. This helps to determine the type 
    of soil, mineral composition and texture. For instance, granite and sandstone 
    disintegrate to form sandy soils rich in quartz, volcanic lavas form clay soils with 

    low quartz content and plants decompose to form loam rich in humus.

    Climate 
    The moisture (rainfall), evaporation and temperature changes determine the 
    chemical reactions and physical breakdown of rocks. Climate also affects rate 
    and type of weathering. For example, heavy rainfall results into deep soils due 
    to heavy weathering and leaching, wind in deserts is responsible for formation 

    of loess soils. 

    Living organisms 
    Plants, animals and microbes are living organisms that affects soil development. 
    Dense vegetative cover protects a soil from being eroded away by running water 
    or wind.  .  Burrowing animals and worms mix organic remains with mineral soil 
    component.  - Roots penetrate and add more porosity, improve soil depth and 
    aeration. Micro-organisms such as bacteria cause plant and animal remains to 

    decay into humus

     Topography 
    The topography represents the slope of the relief. The slope of the land and its 
    aspect (the direction it faces) all influence soil development. Steep slopes are 
    generally subject to rapid surface runoff of rainfall and less infiltration of water, 
    whereas on gentler slopes runoff decreases with an increasing infiltration.  As a 
    consequence, rapid runoff on steep slopes can erode soils as fast, or faster 
    than soil can develop on them. Steep slopes result in shallow immature soils 
    due to severe erosion and prevent the formation of a soil that would support 
    abundant vegetation, 

    On     gentler slopes there is higher infiltration and less runoff. More water is 
    available for soil development and to support vegetation growth, so erosion is 
    not as intense. Well-developed soils typically form on land that is flat or has a 

    gentle slope.

     Time
    All of the mentioned above natural factors in soil development require time to 
    operate. This determines the depth of weathering and the period of operation 
    of soil formation processes. Briefly, the longer the time taken by soil forming 

    processes the deeper and well developed soil is.

     6.2.3. Processes of the soil formation
    The formation of soil requires numerous processes.  Soil is said to be formed 
    when organic matter has accumulated and colloids are washed downward, 
    leaving behind deposits of clay, humus, iron oxide, carbonate, and gypsum, 

    producing a distinct layer called the ‘B’ horizon.     

    Weathering: Weathering is the process by which the rocks break down into 
    small particles to form soil. It is the combined action of physical weathering, in 
    which rocks are fractured and broken, and chemical weathering, in which rock 
    minerals are transformed to softer or more soluble forms.

    Mineralization:     This is the process through which organic matter is further 
    decomposed into mineral compounds. Mineral content in humus may be further 
    converted to inorganic matter e.g. silica.
     Humification: Humification is the process by which organic matter is 
    decomposed to form humus, a task performed by soil organisms. 

    Eluviation:      Eluviation is the downwards movement of fines particles such as 
    clay and the leached soluble materials from upper layers of the soil (‘A’ horizon) 
    to another lower layer within the soil.

    Illuviation    : This is the process of accumulation of clay, aluminum and iron 
    usually from A and E horizons to B horizons.

    Leaching:     Leaching is the removal of soluble material in solution.  It is the 
    process by which water removes leached materials (organic and inorganic) in 
    solution from the upper horizon to the underlying horizon. It operates vertically 
    but not sideways. 

    Laterization:     Laterization is leaching of soils in warm and humid climates. It is 
    a process that occurs after the soluble mineral substances have been leached. 
    After leaching, the insoluble mineral compounds derived from the parent rock 
    remain on top, hence forming lateritic soils that are stony.

    Calcification:     This is the process in which calcium carbonates accumulates in 
    the ‘B’ horizon; particularly characteristic of low rainfall areas such as arid and 

    semi-arid climates.

     Application activity  6.1
     1. Soil forms continuously, but slowly, from the gradual breakdown of 
    rocks through weathering:
     a. Explain how organisms contribute to the formation of soil 
    b. Describe any three other processes leading to the soil formation 
    2. With reference to the knowledge and skills you have acquired in this 
    unit, discuss the difference between zonal soils, azonal soils and 
    intrazonal soils.
     3. Based on the level of weathering, describe the soils orders according 

    to American soil classification  

    6.3. Soil erosion: causes, effects, appropriate soil 
    management and the conservation measures and importance of soil 
    Learning activity  6.3
     1. What does soil erosion means?
     2. Identify major causes of soil erosion 

    3. Discuss on the effects of the soil erosion

     6.3.1. Cause of soil erosion 
    The predominant causes of soil erosion are either related to naturally occurring 
    events or influenced by the presence of human activity. If we want to prevent 
    soil from going away, we need to understand different factors contributing to 
    the soil erosion.  Some of the major causes of soil erosion include:
    – Overgrazing also causes excessive loss of water from the soil causing 
    it to become loose and fine grained and easily eroded. 
    – Rainfall: In a particular heavy rain result to excessive soil erosion and 
    thus poorly aerated
    – Drought: A long dry weather deprives the soil of moisture which holds 
    the soil together causing particles to loosen making it to be easily 
    brown by wind. 
    – Some human works in relation with excavation activities such as 
    quarrying, open-cast mining, building of estates and road construction 
    which loosen and expose the soil to erosion agents.
    – Slope of the landscape: The physical characteristics of the land can 
    contribute to the soil erosion. For example, steep slopes accelerate 
    soil erosion while gentle slopes experience less erosion, places with 
    rugged terrain experience gulley erosion while hilly and steep areas 
    experience rill and gulley erosion.
    – Poor cultivation techniques such as pulling hoe along the surface 
    when removing weeds which loosens the soil and when it rains it’s 
    washed away,  ploughing of land down slope which accelerates 
    soil erosion, cultivation of steep slopes and along river banks which 
    encourages soil erosion, burning which destroys vegetation covering 

    the soil exposing it to erosion agents etc.  

    6.3.2. Effects of soil erosion 
    Some of the greatest effects of soil erosion include:
    – Loss of topsoil: Soil erosion lowers the agricultural productivity of 
    land when fertile top soil is eroded.
    – Desertification: Soil erosion contributes to desertification when top 
    soil is eroded leaving bare ground destroying vegetation. 
    – Water pollution: Serious soil erosion is responsible to water pollution 
    when agro-chemicals and other chemicals are carried to rivers, lakes 
    or oceans.
    – Flooding: Another effect of soil erosion is that it contributes to flooding 
    by blocking river channels causing them to burst their banks during the 
    rainy season flooding the adjacent areas.
    – Alteration of the landscape: Soil erosion can cause significant 
    alteration to the natural shape of the land. For example, it can make 
    huge valleys to occur on plain lands.
    – Reduced organic and fertile matter: Removing topsoil that is heavy 
    with organic matter will reduce the ability for the land to regenerate new 

    flora or crops.

    – Eye and respiratory problems: Soil erosion especially one caused 
    by wind can cause eye and respiratory problems. The latter can happen 
    when people inhale the dust and soil particles being carried away by 
    the wind into their lungs. Eye problems can also occur when the dust 
    particles from wind erosion enter into the eyes.
    – Water siltation:     Persistent soil erosion causes siltation of water 
    reservoirs reducing their utility. For example,  H.E.P. generation, 
    navigation and fishing 
    – Destruction of properties:     It may cause collapsing of structures such 
    as buildings and bridges when soil around them is eroded weakening 

    their foundation.

    6.3.3. Appropriate soil management and the conservation 
    measures
    Generally, when it comes to finding solutions for soil erosion, the most useful 
    techniques found tend to be those that highlight reinforcing the structure of the 
    soil, and reducing processes that affect it.
    – Careful tilling: Due to the activity of preparing land for growing that 
    involves break up the structure of the soil, doing less tilling with fewer 
    passes will preserve more of the crucial topsoil 
    – Crop Rotation: If farmers want to keep their land happy and healthy, 
    they are strongly advised to apply crop rotation.  Growing crops which 
    require different nutrients on the same piece of land on rotational basis 
    to prevent exhaustion of particular mineral nutrients from the soil.   
    – Mixed farming: This involves growing crops and keeping animals on 
    the same farm. Consequently, manure from animals is used to enrich 
    the soil with minerals and improve its structure. 
    – Increased knowledge: another  major factor for preventing soil 
    erosion is education more and more people who work with the land on 
    why it is a concern, and what they can do to help reduce it.
    – Contour Ploughing: Ploughing across the slope rather than down 
    the slope. This practice helps to trap water on horizontal furrows thus 
    preventing excessive soil removal. 
    – Terracing: Through dividing the slope into a series of wide steps, crops 
    can be grown on them. This helps to trap the soil from being carried 
    away by running water and also traps water allowing it to gradually 
    infiltrate into the soil.  
    – Afforestation and reafforestation: Vegetation play a big role in 
    preventing soil erosion: 
    • Leaves reduce the force of rain drops preventing soil particles from 
    being removed.
     • Plants  protect the soil, more dense plant cover yields less damage 
    from erosion.
     • Vegetation increases the rate of infiltration of rain water into the soil 
    thus reducing runoff. 
    • Roots bind the soil particles together. 
    • Decayed vegetation provides humus which binds the soil particles 
    together. 
    – Planting wind breakers: Planting hedges or trees around plots in 
    large fields acts as wind breakers and also trap soil being carried by 
    water.  
    – Regulating livestock numbers: Matching the number of animals 
    kept to the carrying capacity of land.  
    – Paddocking: Overgrazing can also be prevented by paddocking which 
    ensures there is always pasture for animals and no area is overgrazed.
    – Constructing Gabions:  Construction of wire mesh boxes which are 
    filled with soil.  This allows water to pass through but trap the soil then 
    vegetation gradually grows on the trapped soil. 
    – Planting Cover Crops: Planting crops which cover the soil properly 
    and holds the soil in place e.g. sweet potato vines.  
    – Mulching: This practice consist of covering the soil with crop residues.  
    • It helps reducing the impact of rain drops on the soil.  
    • Decays enriching soil with nutrients. 
    • Reduce the rate of moisture evaporation from the soil.
     
    6.3.4. Economic importance of the soil
    – Soil provides physical support for the rooting system of plants and 
    protects root system from damage.  
    – It is a conducive habitat for burrowing animals and bacteria necessary 
    for breakdown of organic matter into humus. 
    – Soil acts as a medium through which nutrients and air are made available 
    to plants.  
    – It provides mineral elements to plants e.g. nitrogen, calcium, phosphates, etc.  
    – Serve as a construction material for building and other infrastructure. 
    Example, clay is used for making bricks and tiles. 
    – Clay soil is used in ceramics such as making pots.  
    – Source of minerals especially to expectant mothers. 
    – Soil contains valuable mineral elements such as alluvial gold.  
    – Soil supports plant life which is a source of food for people and animals 
    especially herbivores.  Soils are used for medicinal purposes e.g. clay 

    is mixed with some herbs for medical purpose in some communities.

    Application activity 6.3  

    Study carefully this photograph and answer the questions that follow

     1. Suggest what could be the cause of  the colored  river
     2. Examine the effects of soil erosion
     3. If you had a chance to become a chairperson in charge of environmental 

    conservation, what would you suggest to handle the above cases?

    Skills Lab
    Identify any area affected by soil erosion and explain to the local people 

    what should be   done to slow down the washing away of soil. 

    End unit assessment
     1. With reference to your knowledge and skills, show difference 
    between three categories of the soil in the world.
     2. Explain how soil erosion is one of the major problem challenging 
    agriculture.
     3. Soil is one of the amazing products of nature and without which 
    there would be no life. Justify 
    4. Most farmers in the northern province of Rwanda use terracing as a 
    measure of soil conservation. 
    a. Explain why terracing is mostly used in this area.
     b. Describe other soil conservation techniques used in your   area.
     c. Show how these techniques are helpful to environmental 

    sustainability.

  • UNIT 7: CLIMATE CHANGE

    Key Unit competence
    By the end of this unit, I should be able to discuss the climate change and 
    its impact on Rwanda and the other countries
     
    Introductory activity
     1) Using internet research  and other geographical materials make a 
    research to establish relationship  between the following concepts: 
    i) Climate change and global warming
    ii) Green house phenomena and desertification 
    2) Basing on the knowledge acquired from the question 1, assess the 
    consequences of climate change on Rwanda.
    3) Which area of Rwanda is likely to experience the desertification? 
    Give reasons supporting your answer.

    7.1. Climate change: definition, causes and effects  
    Learning activity  7.1
    Study carefully the below photographs and answer the questions 
    that follow:

    1) What does climate change mean? 
    2) Explain how industries contribute to the climate change? 
    3) Describe the effects of climate change
     
    7.1.1. Definition of climate change
     Climate change refers to the long-term changes in average conditions and 
    characteristics of earth’s lower surface atmosphere resulting either from natural 
    variability or human activities that change atmospheric conditions of a region or 
    location. It is also defined as a long term change of climatic elements such as 
    temperature, rainfall, wind speed and direction, sunshine, atmospheric humidity, 
    atmospheric pressure, cloud cover over a given region of earth’s lower surface 
    atmosphere or globally. 

    7.1.2. Causes of climate change
    The causes of climate change are classified into natural causes and man - made 
    causes
    i) Natural causes of climate change 
    Natural causes of climate change include:

    Variations in the earth’s orbital characteristics
    The more elliptical orbit makes the earth to be once year in closest position to 
    the sun (Perihelion: 147 500 000 km) or in farthest position to the sun (Aphelion: 
    152 500 000 km). At the Aphelion, the earth receives the least solar energy 
    while the maximum is received at the Perihelion. 

    Volcanic eruptions
     Volcanic activity affects the climate. World temperatures are lowered after a 
    series of volcanic eruptions. This is due to the increase in dust particles in the 
    lower atmosphere which will absorb and scatter more of the incoming radiation. 
    Sulphur dioxide gas is given off during some of the eruptions. This gas remains 
    in the atmosphere for as long as three years and it reacts with water vapor 
    and forms a bright layer in atmosphere. This layer reduces the amount of solar 
    radiation reaching the earth surface by reflecting some back to universe.

    Variations in solar output 
    Sunspot activity which occurs in cycles, may significantly affect our climate. 
    Times of high annual temperatures on earth appear to correspond to periods 
    of maximum sunspot activity.  The results found from satellites measurements 
    showed a decrease of 0.1% of the total solar energy coming to the earth in the 
    early 1980s. This value was obtained over a period of 18 months. It is predicted 
    that the increase in solar output of 1% per century will contribute to the increase 
    of the global average temperatures by between 0.50C and 10C. 

    Variation of aerosols in atmosphere
     Aerosols like solid particles of varying sizes and liquid droplets which include: 
    ploughed soil cover, deserts, rocks, salt particles from seas and oceans; 
    meteoric particles, organic matter, such as bacteria, seeds, spores and pollen. 
    These particles help in selective scattering of shortwave electro-magnetic solar 
    radiation which adds varied color of red and orange at sunrise and sunset. Some 
    of the aerosols, mainly water droplets, absorb certain amount of solar radiation 
    while some amount of radiant solar energy is reflected back to the space. The 
    high concentrations of aerosols in atmosphere decrease the temperatures to 
    reach the earth surface.

    Sunspots
     Sunspots, defined as dark areas within photosphere of the sun and surrounded 
    by chromosphere, are created in the solar surface (photosphere) due to periodic 
    disturbances and explosions. These dark areas are cool areas because they are 
    characterized by 1, 5000C less temperature than remain part of photosphere. 
    The increase or decrease in number of sunspot is completed in a cycle of 
    11 years. It is believed that the energy radiated from the sun increases when 
    the number of sunspots increases and consequently the amount of insolation 
    received at the earth’s surface also increases.
     
    ii) Human causes of climate change
     Human activities have been the mostly responsible for atmospheric alterations. 
    Human activities participate highly in atmospheric pollution leading to the 
    change in composition of atmosphere. 
    The atmosphere is polluted by human activities in the following ways:

     Variations of carbon dioxide in atmosphere
     Carbon dioxide (CO2 ) is an important heat-trapping (greenhouse) gas. It 
    is released through human activities such as burning fossil fuels and gases 
    released from industries, as well as natural processes such as respiration and 
    volcanic eruptions. There is a positive relationship between the concentration of 
    carbon dioxide in atmosphere and the global temperatures: high concentrations 
    of carbon dioxide result to the rise of temperature on the earth surface while low 
    concentrations of carbon dioxide result to the lower temperatures. 

    Forest and grassland fire
     It increases the concentration of carbon dioxide in atmosphere resulting from 
    the burn of trees and grassland which are cut and put under fire for different 
    purposes.
     
    Deforestation and land use changes
     When people clear large areas of forests and grasslands for cooking or 
    construction, they reduce the main disposal system for carbon dioxide from 
    atmosphere by photosynthesis, which leading to the increase of carbon dioxide, 
    and eventually to the increase of temperature on the earth surface.
     
    Industrial developments
     Gases like methane, nitrous oxide, chlorine, bromine and fluorine are added into 
    the atmosphere through industrial activities.

    Industrial waste and landfills
     Industries which are involved in cement production, fertilizers, coal mining 
    activities, oil extraction produce harmful greenhouse gases. Also, landfills filled 
    with garbage produce carbon dioxide and methane gas contributing significantly 
    to greenhouse effect.
     
    Urbanization
    The buildings of cities increase the reflection and decrease the absorption of 
    solar radiation which would change the temperatures on the earth surfaces. The 
    urban activities participate also in increasing the concentrations of greenhouse 
    gases in atmosphere leading to the rise in temperature.
     
    Increase in Population
     It is obvious that this last two decades the people have been huge increase in 
    the population. Now, this has resulted in increased demand for food, cloth and 
    shelter. New manufacturing hubs have come up cities and towns that release 
    some harmful gases into the atmosphere which increases the greenhouse 
    effect. So, more people means more usage of fossil fuels which in turn has 
    aggravated the problem.
     
    Farming
     Nitrous oxide is one the greenhouse gas that is used in fertilizer and contributes 
    to greenhouse effect which in turn leads to global warming.
     
    7.1.3. Effects of climate change in the world (global, Africa, Rwanda)
     i) Effects of climate change in the world
    The following are the effects of climate change in different parts of the world:
    Increase in the amount of rainfall: A rise in global temperatures could lead 
    to an increase of evapotranspiration. This could eventually lead to the rise in 
    amount of rainfall.
     
    Melting of glaciers:     A rise of temperature leads to the melting of glaciers in 
    polar and mountainous regions resulting into flooding. This would cause the 
    levels of the sea to rise by 20 cm by the year 2030.

    Rise in the sea and ocean levels:     The increase in the amount of rainfall and 
    melting of glaciers leads to the increase of the sea and ocean levels destroying 

    both human and physical features at the coast.
    Increases in intensity of extreme weather: Climate change increases 
    events such as heat waves, tornadoes and hurricanes.

     The prolonged severe droughts: Some regions may experience prolonged 
    droughts caused by reduction in rainfall, which may result in aridity.

     Depletion of ozone layer: High amount of harmful ultraviolet radiation 
    increases the cases of animal and human diseases such as cancers, blindness 
    and other eye diseases.

    Occurrence of acid rain:     Acid rain is harmful to animal and human being.

    Lower crop and timber yields:     Since ultraviolet radiation slows down many 
    aspects of plant growth such as photosynthesis and germination in many plants 
    leading to low production.

    R    eduction of plankton growth:     As temperature goes beyond coral reefs 
    living standard, fish breeding and feeding patterns are disrupted.

    Decrease of agricultural production    : In some regions, the rainfall may 
    decrease, or agriculture seasons be disrupted because of climate change. 
    Some regions became drier and make soil infertile for crop production.

    City environments becoming warmer:     The increase of carbon dioxide 
    makes the temperatures to increase most in urban areas.

     Water use and long-term planning: A wetter or drier climate can affect 
    water resources planning. Water reservoirs, dams, and hydroelectric projects 
    might become useless in coming years.
     
    Spread of vector-borne diseases: Because of high temperature there can 
    be an increased range of insects.

    Acidification of oceans:     This can create a reduction in plankton, coral reefs 
    and a drop-in fishing yield.
     ii) Effects of climate change in Africa
     The following are some of the facts showing the climate change and variability 
    in Africa:
    – Melting of glaciers on the top of the highest African mountains such as 
    Kilimanjaro, Rwenzori, Kenya and Karisimbi;
    – Warming in African tropical forests has been evaluated at 0.29 °C for 
    the past 10 years and 0.1 °C to 0.3 °C in South Africa, while it ranged 
    between 0.2 °C and 0.3 °C in the Nile Basin countries;
    – Decreasing trends in temperatures; in eastern Africa, the situation has 
    been complex because they have been observed over the regions 
    close to the coast or major inland lakes and increasing in the rest of 
    the region;
    – The gradual heating, between 1961 and 2000, over the continent 
    meant more warm spells (days) and fewer cold days across Africa. 
    An increase in temperature in Sahara desert has led to the decline in 
    volume of water in Lake Chad;
    – Fluctuations of precipitation; the extent of variability is complicated and 
    exhibits more spatial and temporal fluctuations across the continent;
    – The decrease in rainfall has been registered in West Africa (between 4 ° 
    and 20 °North; 20 °West and 40 °East), by up to 20% to 40% for the 
    periods 1931-1960 and 1968-1990 respectively. A similar decline in 
    mean annual rainfall has also been observed in the tropical rain-forest 
    zone. A reduction of around 4% in West Africa, 3% in North Congo 
    and 2% in South Congo for the period 1960-1998.;
    – Increases in rainfall have been registered in different parts of southern 
    Africa (e.g., Angola, Namibia, Mozambique, Malawi, and Zambia);
    – Increase in the desertification in south of the Sahara desert;
    – Links have also been identified between the warm Mediterranean Sea 
    and abundant rain fall over the surrounding regions.
     
    iii) Effects of climate change in Rwanda
     Rwanda experiences some rainfall events that cause unexpected flooding and 
    catastrophic events such as landslides etc. These extreme events are attributed 
    to climate change. The figure below represents some effects of extreme rainfall 
    events of climate change in Rwanda.
     
    The following are effects of climate change in Rwanda:
    – Significant increase in precipitations at a rate of between 2 and 6.5 mm 
    per year over the Congo-Nile crest and the northern highlands for the 
    period of 1935–1992.
    – Floods that occurred in May 2002 caused the death of 108 persons in 
    North western regions while the one occurred in 2007 have resulted to 
    displacement of more than 456 families and destruction of hundreds of 
    hectares of crops in Bigogwe sector in Nyabihu District;
    – During September 2008 heavy rainfall accompanied by winds affected 
    8 of the 12 sectors of Rubavu district and provoked the displacement 
    of more than 500 families, caused the destruction of about 2,000 
    hectares of crops and many other infrastructures;
    – Floods reported in September 2012 in Nyabihu, Rubavu, Bugesera 
    and Kirehe districts whereby more than 1000 families were displaced 
    and their crops submerged completely;
    – The landslides and floods caused by heavy rainfall are regulary observed 
    mainly in north- western parts of Rwanda (Rulindo, Gakenke, Musanze, 
    Nyabihu and Rubavu districts). For instance, the floods which occurred 
    on 2nd and 3rd April, 2016 caused the death of 12 people, with 19 
    injured and destruction of 196 houses across the country. The floods 
    which took place in Musanze district on 20th April 2016 caused the 
    destruction of 64 houses and many hectares of crops and cattle;
    – The significant increase in mean annual temperatures of between 0.036 
    and 0.066 °C per year for the period of 1961-1991;
    – Since 1902, a number of famines following prolonged droughts 
    episodes have been registered in Rwanda notably in eastern and 
    south-eastern regions;
    – More occurrences of lightning combined with the thunderstorms in 
    2013 caused 12 deaths in Karongi, 12 in Rubavu, 4 in Rusizi and 5 
    death in Rutsiro districts, respectively. The same districts suffered from 
    the same extreme weather events which were reported to cause 15 
    deaths in 2015 (January-October) with 30 people injured.

    Application activity 7.1
     1) Identify the areas of Africa that are susceptible to face the climate 
    change challenges? 
    2) Describe the effects of climate change in Eastern and Western 
    provinces of Rwanda.
     
    7.2. Global warming and the green house phenomena 
    (definition, causes and the effects)
    Learning activity  7.2
     1) Use different resources to find the meaning of the following:
     i) Global warming
     ii) Green house phenomena
    2) Explain the reasons of practicing greenhouse farming.

     7.2.1. Definitions of global warming and greenhouse 
    phenomena
    These two phenomena of global warming and greenhouse are related but are 
    different.   

    i) Global warming
     Global warming refers to the gradual rise in world temperatures. This is a gradual 
    increase in the average temperature of the earth›s atmosphere and oceans due 
    to increase in the amount of carbon dioxide. The increase in the amount of 
    carbon dioxide leads to greenhouse effect. It is a change that is believed to be 
    permanently changing the earth›s climate. An increase in greenhouse gases 
    increases the greenhouse effect which in turn increases the global warming. 
    In the last 100 years, the mean surface temperature on earth has increased by 
    0.5 °C.  

    i) Greenhouse effect 
    The greenhouse effect is a phenomenon in which the atmosphere of a planet 
    traps radiation emitted by sun. It is caused by gases such as carbon dioxide, 
    water vapor, and methane that allow incoming solar radiation to pass through 
    but retain heat radiated back from the planet’s surface. 

     7.2.2. Causes of global warming and green house 
    phenomena
    The following are the causes of global warming and green house phenomena:
     
    1) Human factors 
    Human activities produce various gases ejected in the atmosphere that are 
    responsible for the global warming. These activities are destroying earth at fast 
    rate: the emission of carbon dioxide from industries and vehicles, the burning 
    of fossil fuels, cutting of trees and forests to build some new buildings and new 
    malls, dumping of trash everywhere and not even recycling it, excessive use of 
    the plastics and smoke from factories. All the activities performed by human 
    beings are the major factors for gases that pollute the air and warm up the earth. 
    These may contribute to the destruction of the ecological balance of the nature 
    leading to the global warming.

    Burning of fossil fuels
     Fossil fuels are burnt on day-to-day basis. This activity produces large 
    percentage of gases such as carbon, petroleum, coal and many other different 
    gases which are emitted in earth’s atmosphere. Carbon dioxide being one of 
    gases with greenhouse effect is provided in excess in our atmosphere in far 
    greater quantity in comparison with other gases produced by human activities.

    Use of chemical fertilizers
    The use of the artificial chemicals for crops has become one reason for the global 
    warming. These chemicals are dangerous to the earth as well as to the human 
    beings. These fertilizers are rich in the nitrogen oxide which is more dangerous 
    than the carbon dioxide. Those oxides of  nitrogen destroy ozone layer even 
    faster than other greenhouse gas and hence let harmful ultraviolet rays enter 
    atmosphere thus making earth warm and leading to the global warming.

     Industrial advancement
     More and more different industries and factories are set up in modern world to 
    meet needs of the human beings. These factories need large amount of fuels like 
    some coal, petroleum for power generation and electricity required by machines 
    to work. Burning of these fuels also releases large amount of the carbon dioxide 
    which absorbs harmful radiations from sun making it warm, hence increasing 
    global warming.  

    Deforestation
    The mass removal of trees, called deforestation, also affects the amount of 
    carbon dioxide in our atmosphere. Forests around the world are being cleared 
    for cultivation, mining, building, roads building, grazing cattle, etc. As they grow, 
    trees take in carbon dioxide. When trees are removed, the carbon dioxide that 
    they could have removed from the atmosphere is left. Cut-down trees are often 
    burned. Burning produces more carbon dioxide.  If the trees are cut, plants will 
    not be able to produce oxygen and concentration of the carbon dioxide will 
    increase. Increase of the carbon dioxide in air is very harmful for the human beings 
    and also disturbs water cycle and hence total imbalance of our ecosystem. So 
    being one of greenhouse gases it will lead to the global warming.

    Air pollution
    The harmful gases emitted from vehicles and the factories and greenhouse gases 
    cause some pollution in the air and these gases get captured in atmosphere. 
    The smoke gather up in atmosphere forming some clouds full of harmful gases 
    which later fall as the acid rain which destroys plants. Plants provide us with 
    oxygen and if they die level of carbon dioxide will increase in atmosphere which 
    is known as a harmful gas. These gases emit heat which increases temperature 
    of earth, hence causing global warming.
     
    2) Physical factors 
    Volcanic eruptions
    Volcanic eruptions are also among the causes of global warming. These eruptions 
    contain the dust particles and gases like the sulfur dioxide which stays in the 
    atmosphere for years and blocks the sunlight from reaching surface of earth 
    making it somewhat cool. These dust particles affect balance of atmosphere 
    and become contributing factor of the global warming. 

    Depletion of ozone layer
     Depletion of ozone layers is an important factor that causes global warming. 
    The ozone layer is known as the layer outside the atmosphere which protects 
    surface of the earth from harmful ultra-violet and the infrared radiations causing 
    some dangerous diseases like the skin cancer. Ozone layer depletion is one 
    of causes of the global warming; entering of the harmful gases which helps 
    in heating up the earth but other greenhouse gases like the carbon dioxide 
    and methane that helps in heating up and tears up ozone layer making a hole 
    called “Ozone hole”. So, ozone layer depletes due to these gases which allow 
    ultra violet radiations to enter the earth’s atmosphere making it more warm than 
    normal and also affects temperature leading to the global warming. 

    7.2.3. Impact of greenhouse process on global warming  
    Greenhouse effect is a process in which the atmosphere of the earth traps 
    some of the heat coming from the sun and fails to radiate, making earth warming. 
    This is due to the burning fuels, cutting of trees, concentration of the heat on 
    earth is increased to some abnormal levels making the greenhouse effect as 
    one of the major causes of the global warming. Carbon dioxide, nitrous oxide 
    and methane are the greenhouse gases which help to keep earth warm. It is 
    natural phenomenon that takes place with adequate concentrations of some 
    greenhouse gases. When concentration of these gases rises then they disturb 
    climatic conditions, thus making earth warmer. These gases are not able to 
    escape and that causes the worldwide increase in temperatures. So balance of 
    the carbon dioxide and some other gases should be maintained so that it does 
    not become major reason for the global warming.

    A    pplication activity 7.2
     1) Explain why causes of climate change and green house differ in rural 
    and urban areas. 
    2) Among the effects of climate mentioned above, which ones do you 
    observe in your local environment?
     3) Referring to the greenhouse phenomenon, describe the advantages 
    and disadvantages of the farming practiced in greenhouse.
     
    7.3.  Adaptation measures and mitigation for the climate change  
    Learning activity  7.3
    In your local environment, identify any evidence of climate change and 
    propose sustainable strategies to deal with it.
     
    7.3.1. Adaptation measures for climate change 
    Adaptation for climate change refers to measures and strategies taken to 
    cope with climate change and variability. These measures vary from one domain 
    to the other like agriculture, livestock keeping, tourism, public health and 
    water management; from one climatic region to the other as dry, wet, hilly, flat, 
    depression, mountains, floodplains; from season to season as in dry and wet 
    seasons; and across diverse actors as private, public, national, international, 
    NGOs, local communities. Hence, adaptation measures are many and are not 
    homogeneous. Some of them are briefly described below: 

    Maintaining current ecosystems wherever possible:     This implies 
    strengthening, extending and in some cases refining global protected area 
    networks to focus on maintaining large blocks of intact habitat with a particular 
    emphasis on climate change.

    Agro-forestry:     This is a land-use system that incorporates trees in food crop 
    fields. In other words, it is a combination of agriculture and forestry for more 
    diverse, profitable, productive and sustainable land use. 

    Progressive and radical terracing    : This is used to reduce runoff, soil 
    erosion and landslides. At the same time, terracing helps to improve soil 
    quality and moisture retention, especially in steep areas.

    Soil fertility conservation:     Practices like the use of manure, mulching, 
    planting of leguminous crops help to improve soil fertility by increasing the 
    micro-organism composition in the soil.

    Seed and grain storage    : This involves collecting seeds and grains from 
    farmers at post-harvesting season and releasing them within the timely agreed 
    periods. 

    The use of pesticides:     It is a wide range use of compounds such as 
    insecticides, fungicides, herbicides, rodenticides, molluscicides, nematicides, 
    plant growth regulators and others to control pests, insects, fungi, weeds, 
    bacteria, rodents, all of which are harmful to crops. 

    Ecological pest management:     This is the use of natural enemy dynamics 
    or environmental positioning (e.g. crop shading) to eliminate or reduce the 
    presence of pests.

    The use of improved seeds and species:     This is vital to improve crop 
    productivity. 

    Crop varieties and diversification: This measure consists of integration of 
    different varieties of crops and hybrids of a particular crop. Multiple cropping 
    aids in replenishing the soil and maintaining its fertility by ensuring that there 
    is a constant balance of nutrients by decreasing dependence and saturation 
    of any one product.

    Land use consolidation programmes:     This encourage farmers with 
    adjacent lands to grow the same crop. This facilitates the provision of inputs 
    (e.g. seeds and fertilizers), post-harvest activities (e.g. driers, seed and grain 
    storage facilities) and safer and faster transport of agricultural products. 

    Rain water harvesting:     It is the practice of collecting and storing rainwater 
    from rooftops, land surfaces or rock catchment areas for different use.

     Irrigation like drip irrigation is a practice based on the constant application 
    of specific and controlled quantity of water to the crops. The system uses 
    pipes, valves and small drippers or emitters that transport water from the 
    sources (i.e. wells, tanks and reservoirs) to the root area and applying it in 
    controlled quantities and pressure specifications while Sprinkler irrigation 
    involves spraying the crops with water using sprinklers in a manner that 
    resembles rainfall.

    Wastewater use    : It forms a reliable source for crop irrigation and a positive 
    way to dispose of sewage water. Whereas wastewater contains a lot of 
    nutrients on the one hand, it carries pollutants like micro and macro organic 
    and inorganic matters that potentially pose hazards to human health, the 
    environment, crops and soils, on the other.

    Biotechnology of crops    : It involves the practical application of biological 
    organisms, or their sub-cellular components in agriculture and livestock. The 
    techniques currently in use include tissue culture, conventional breeding, 
    molecular marker-assisted breeding and genetic engineering. 

    Barrier crops:     These are crops that are used as a cultural control strategy 
    for reducing the spread of pests and diseases to the most vulnerable crops. 
    These crops provide benefits over “hard infrastructure” in a number of ways: 
    first, they offer a natural form of protection; second, they contribute to the 
    biodiversity and often soil improvement; third, they can provide an added 
    source of food provisions or income and, finally; they can play a determinant 
    role in soil erosion reduction. 

    Integration of meteorological information in agriculture:     It is used to 
    develop early warning systems, crop monitoring and disaster management. 

    Training farmers: By offering short courses, seminars and group discussions 
    on the impacts of climate changes and on various ways of adaptation.

    Facilitating the farmers: this consists of facilitating farmers to access 
    capital that they need to purchase seeds, installation of tube wells, drilling 
    of pumping sets, chemical fertilizers, plant protection chemicals, tractors, 
    harvesters, threshers and other accessories.
     
    Development of infrastructure: This concerns the improvement of 
    transport networks, electricity and marketing facilities which use to be 
    affected by climate change phenomena to promote a sustainable livelihood 
    of population.

    Development of agricultural institutions:     The institutions such as 
    universities provide experts and researchers who offer critical services 
    like assessment, promotion of agricultural and livestock innovations and 
    dissemination of research findings to agronomists and farmers at all levels.
     
    7.3.2. Measures for mitigating the climate change
     Mitigation measures for climate change consist of actions to limit the magnitude 
    and the rate of long-term climate change. Climate change mitigation generally 
    involves reductions in human (anthropogenic) emissions of greenhouse gases. 
    Anthropogenic greenhouse gases include carbon dioxide (CO2), methane 
    (CH4), Nitrous oxide (N2O) and a group of gases referred to as halocarbons. 

    The following are mitigation measures for climate change:
     Storing and reducing carbon dioxide: Carbon dioxide can be captured and 
    stored, but also it can be reduced. Carbon dioxide Capture and Storage (CCS) 
    is a process consisting of the separation of CO2 from industrial and energy 
    related sources, transport to a storage location and long-term isolation from the 
    atmosphere. Conserving electricity is one strategy to reduce CO2. 
    When we conserve electricity, we reduce the amount of fossil fuel that must be burnt. One 
    way to save fuel is to change daily activities that rely on energy from burning 
    fuel.

    Use of energy that reduces the atmospheric pollution:     The use of 
    renewable energy supply technologies, particularly solar, wind, geothermal and 
    biomass are recommended to reduce the atmospheric pollution. Renewable 
    energy systems such as hydro-electricity can contribute as well to the security 
    of energy supply and protection of the environment.

    Reduction of the energy use in buildings:     Cooling energy use in buildings 
    can be reduced by different measures, for example reducing the cooling load by 
    building shape and orientation. 

    Land-use management:     Forest land, cropland, grassland, wetlands, 
    settlements have to be well managed by fighting against any threaten to them. 
    Changes in land use may result in net changes in carbon stocks and in different 
    impacts on water resources. 

    Crop land management:     The use of agricultural practices which promote 
    the conservation of water, and its quality. There is a need for improved crop 
    and grazing land management to increase soil carbon storage; restoration of 
    cultivated peaty soils and degraded lands.
     
    Afforestation and reforestation: The increase of number of trees helps to 
    capture the CO2 and decreases the flow of water from catchments.

    Solid waste management and waste water treatment:     Controlled landfill 
    (with or without gas recovery and utilization) controls and reduces greenhouse 
    gas (GHG) emissions but may have negative impacts on water quality in the 
    case of improperly managed sites.

    Application activity 7.3
     1) If you were the Director General of REMA, demonstrate the adaption 
    measures to climate change in Rwanda.
     2) Suppose that you are a manager of a big industrial complex, describe 
    the strategies to mitigate climate change.
     3) Explain the process by which the use of refrigerator contributes to 
    climate change.
     
    7    .4. Desertification (definition, causes, effects)
     Learning activity  7.4
     Study the photograph below and answer the questions that follow:
     1. Explain what happen in the area shown in the photograph.
     2. Explain how climate change contributes to desertification.
     3. Referring to the figure below, describe the challenges that face 
    people living in desert areas.

     
    7.4.1. Definition of desertification
     Generally, desertification is described as the turning of the land into desert. It 
    is the process by which the land undergoes degradation from which a relatively 
    dry land region becomes increasingly arid, typically losing its bodies of water as 
    well as vegetation and wildlife. Desertification is caused by a variety of physical 
    factors, mainly the climate change and human activities.

    7.4.2. Causes of desertification
     Desertification is caused by a combination of factors that change over 
    time and vary with location. These include the following: 
    Less rainfall (total amount) and increased drought (frequency and 
    intensity) lead to drought of rivers and water bodies and decrease in protective 
    vegetation cover. 

    Global warming:     It causes higher temperatures and increased 
    evapotranspiration. This reduces condensation and leads to shortage of 
    rainfall.

    Population growth:     The effect of this is the over-cultivation which reduces 
    soil fertility and leaves the soil exposed to erosion.   

    Deforestation:     An increased demand for cultivation land, wood for cooking, 
    heating, building, increases the risk of soil erosion. 

    Poor crop cultivation practices    : Some farmers do not know how to use 
    the land efficiently. Farmers may essentially strip the land of everything that 
    it has before moving on to another plot of land. By stripping the soil of its 
    nutrients, desertification becomes more and more of a reality for the area that 
    is being used for farming.

    Urbanization and other types of land development:     Development can 
    cause people to go through and kill the plant life. It can also cause issues 
    with the soil due to chemicals and other things that may harm the ground. As 
    areas become more urbanized, there are less places for plants to grow. This 
    can contribute to the process of desertification.

    Soil erosion:     The losses of the top soils and vegetation leads to the 
    desertification.

    Climate Change    : Climate change plays an important role in desertification. 
    As the days get warmer and periods of drought become more frequent, 
    desertification becomes more and more eminent. Unless climate change is 
    slowed down, huge areas of land will become desert; some of those areas 
    may even become uninhabitable as time goes on.

    Over exploitation of the land of resources:     If an area of land has natural 
    resources like, oil, or minerals, people will come in and mine it or take it 
    out. The removal of resources is usually associated with the striping of the 
    soil and depletion of nutrients. Consequently, plants are died and from there 

    starts the process toward becoming a desert biome as time goes on.

    Natural disasters: There are some cases where the land gets damaged 
    because of natural disasters, such as natural fires, drought, floods, and 

    earthquakes.

    Rise of salinity: In the soil which causes the vegetation to be stunted.
    Overgrazing: If there are too many animals that are overgrazing in certain 
    spots, it is difficult for the plants to grow back. Biomes are affected and lose 

    their original vegetation.

     7.4.3. Effects of desertification
     The following are the major effects of desertification:
     Farming becomes unproductive: If an area becomes a desert, it’s almost 
    impossible to grow substantial crops there without special technologies. 
    This can cost a lot of money to try and do so as many farmers will have to sell 
    their land and leave the desert areas.

    Hunger (famine):     Without farms in these areas, the food that those farms 
    produce will become much scarcer. The people who live in those local areas 
    will be a lot more likely to try and deal with hunger problems. Animals will also 
    go hungry due to food shortage.

    Flooding:     Without the plant life in an area, flooding is much more eminent. 
    Some huge rivers cross deserts which experience a lot of flooding because 
    there is nothing to stop the water from gathering and going all over the place. 

    Poor water quality:     If an area becomes a desert, the water quality is going 
    to become a lot worse than it would have been otherwise. This is because 
    the plant life plays a significant role in keeping the water clean and clear.

    Overpopulation of the new areas    : When areas start to become desert, 
    animals and people will go to other areas where they can actually thrive. This 
    causes overcrowding and overpopulation, which will, in the long run, end up 
    continuing the cycle of desertification that started this whole thing anyway.

    Poverty:     All of the issues that are described above (related to the problems 
    of desertification) can lead to poverty if it is not kept in control. Without food 
    and water, it becomes harder for people to thrive, and they take a lot of time 
    to try and get the things that they need for their subsistence.

    Acceleration of desertification    : The increased frequency and severity of 
    droughts resulting from projected climate change is likely to further accelerate 
    desertification.

    Involuntary migration:     Rural population affected by the effects of climate 
    change, especially the drought or aridity migrate towards different areas. This 
    may also lead to rural exodus. 

    Shortage of drinking water and water to use for other purposes: 
    This is where overpopulation causes pressure to exploit dry lands for farming. 
    These marginally productive regions are overgrazed, the land is exhausted, 
    and groundwater is over drafted.
     
    Application activity 7.4 

    Observe carefully the picture below and answer the questions that follow:

     i) Referring to the factors of desertification discussed above, describe 
    the causes of the above phenomenon.
    ii) Explain the effects of drought to the people living in such area.
     iii) Considering the physical conditions of Rwanda, suggest the districts 
    in which the above phenomenon is likely to happen and the strategies 

    to limit this problem.

    Skills Lab
     Provide specific examples and analyze how human activities affect climate change.

    End unit assessment
     1. Compare the factors that can cause the climate change in China 
    and Rwanda.
     2. Explain the causes of climate change in developed and developing 
    countries.
     3. The World needs to develop at high rate with its industrialization 
    processes which is among the most causes of greenhouse effects. 
    Suggest the mitigation measures for climate change in this regard. 
    4. The world is facing the problem of climate change and this is 
    substantially leading to the problem of desertification.
     a. Indicate the most affected areas by that problem? 
    b. Suggest the sustainable strategies to address the problem of 

    desertification.

  • UNIT 8: GLOBAL DRAINAGE SYSTEMS

    Key Unit competence
    The student-teachers should be able to investigate the economic importance 

    of the global drainage systems and the reasons for their conservation

     Introductory activity
     1. Do research using the internet and other geographical resources 
    to explain the following drainage terms: Drainage system, river 
    discharge, river velocity, catchment area, river divide and river basin
     2. Explain the processes of river erosion, river transportation and river 
    deposition.
     3. Explain  the importance of drainage systems

     4. Discuss why there is need to conserve drainage systems

     8.1. River system
     Learning activity  8.1
    1. Do research and explain the types of rivers and the river profiles. 

    2. What do you understand by the concept of a river profile?

     8.1.1. Definition of a river and the associated terms 
    A river is a large natural stream of fresh water flowing along a definite course, 
    usually into the sea, being fed by tributary streams. The water originates from a 
    known source and empties into a sea, lake or another river. The river flows along 

    a channel, whose water volumes increases as the river goes downstream. 

     The following terms are used in describing a river channel
    Discharge: is the amount of water originating from precipitation which reaches 
    the channel by surface runoff, through flow and base flow. Discharge is, 
    therefore, the water not stored in the drainage basin by interception, as surface 
    storage, surface moisture storage or groundwater storage or lost through the 
    evapotranspiration. 

    River Velocity:     Is the speed at which the water flows through the channel. It is 
    less at the sides and bed than at the center of a river. The velocity also depends 
    on the river’s gradient.

    A river Basin    : Is an area of land drained by a river and its tributaries. Its 
    boundary is marked by a ridge of high land beyond which any precipitation will 
    drain into adjacent basins. This boundary is called a watershed. 

    A river divide:     This is the crest of the upland or mountain from which the 
    streams flow down the slopes on both sides to their journey.

    River width:     This is the distance across the surface of a river from one bank to 
    another bank.

    River depth:     Is the vertical distance from the river surface down to its bed.
    River slope, also called river gradient is the angle between the horizon and 
    the river surface. 
    Catchment area is an area from which a river derives its water. This can be an 

    upland or mountain.

     8.1.2. Types of rivers
     There are different types of rivers. The following are the main ones:
     • Perennial River: This is a river with water flowing permanently in its 
    channel throughout the year. 
    • Intermittent River: This is a semi-permanent river which stops flowing 
    at some point in space and time. It stops to flow every year or at least 
    twice every five years. 
    • Ephemeral River: This is a seasonal river that flows only when there 

    is heavy rain or when snow has melted.

     8.1.3. The river system: The work of a river
     As a river moves from its source to its mouth, it performs the triple function 
    (three phases) of erosion, transportation and deposition. The following is the 
    work of a river:

    A. River erosion
     This involves the removal of different soils and rock particles of varying sizes 
    from the river’s bed and banks.  Erosional work of rivers depends on the channel 
    gradient, the volume of water, the river’s velocity, water discharge and the 
    sediment load (amount of eroded material). The river erosion is at its peak when 
    the river passes through a steep gradient where the speed of flow is great. The 
    river erodes its bed and channel in the following ways:
     • Hydraulic action: This is the process by which fast flowing water 
    enter into the cracks on the river bed and channel sides. The repeated 
    friction and pressure of water force cracks to widen and finally erode 
    weaker rocks.
     • Solution or corrosion: This is the removal of rocks like salt, limestone 
    etc. that are soluble in water. Such rocks dissolve in water and are 
    carried in solution form.
     • Abrasion or corrasion: This is the erosion of the river’s bed and 
    channel sides by the rolling action of materials or river load against rocks. 
    The heavier rocks transported in water rub and slid against the bed and 
    channel rocks eroding them as they are transported downstream.
     • Attrition: This is the erosion of the river’s load by the load itself. As 
    rock fragments moving as load are transported downstream, boulders 
    collide with other material and they are fragmented and gradually 
    reduced in size, and their shape changes from angular to rounded.
     
    B. River transportation and types of steam loads
     Rivers transport refers to the carrying away of eroded material downstream. 
    As represented on figure below, rivers transport their load in the following ways:
     • Solution: This is the downstream movements of soluble material like 
    salt, carbonates dissolved in water. 
    • Suspension: This is where the light particles of plants, soil and rocks 
    are carried away while floating or maintained within the turbulence flow of water.
     • Saltation: This occurs when the load carried by the river is transported 
    in a series of short jumps or hops. It involves the transportation of 
    particles which are not too heavy but cannot remain suspended in 
    water. Materials such as pebbles, sand and gravel are temporarily lifted 
    up by the river currents and then dropped back along the bed in a 
    hopping motion. Such movements are known as hydraulic lift.
     • Traction: This is where large and heavy materials are rolled, pushed 
    and dragged downstream by the force of moving water. Such materials 

    include rocks, pebbles and boulders.

    Transport of solid load in a stream: Clay and silt particles are carried in 
    suspension. Sand typically travels by suspension and saltation. The largest 
    (heaviest) particles move by traction.

    There are     three main types of stream load. 
    1. Mineral and chemical elements of rock material held in solution constitute 
    the dissolved or solution load. 
    2. Suspended load consists of the small clastic particles being moved in 
    suspension. 
    3. Bed load is constituted of larger particles that move in traction along the 

    streambed.

     C. River deposition
     This refers to the situation where a river fails to transport its load. The river, then 
    drops its load due to the reduction in its energy. The heavy load is selectively 
    deposited first, while the fine and lighter particles are deposited last. The material 

    deposited by a river is referred to as alluvium.

     8.1.4. The river profile and its characteristics 
    A river profile is a section through the river channel from its source to its mouth 
    or from one bank to another. There are two types of river profile: cross profile 
    and long profile.
    – Cross profile
     This is also known as the transverse section of a river. It is the shape a river 
    assumes from one river bank to the other. It develops as a result of down
    cutting and lateral cutting of the riverbed and banks by water currents. This 
    undercutting makes a section of a river valley, have different shapes and forms. 

    For example, in the upper valley, vertical erosion produces a steep “V”-shaped 

    valley. However, this depends on the rate of erosion and weathering taking 

    place on the valley sides.

     In the middle and lower stages, the river valley begins to become shallow and 

    wide due to increased lateral erosion. The valley assumes a “U” shape.

    – Longitudinal profile 
    This is the longitudinal section of a river. It contains a variety of erosional and 
    depositional features. Based on its distinctive characteristics, the long profile of 
    a river is divided into three stages (upper/youthful, middle/mature and lower/old 

    stages) known also as normal cycle of erosion

    The Course of a river presents three successive stages. They are represented 
    on figure bellow and described as follows: 
    • The youthful stage, referred to as upper stage of a river is found in 
    the mountains and hills where the river rises from its source. It has the 
    following characteristics:
    – The topography at this stage is steep and the river is usually fast 
    flowing in the upper course. 
    – The main river gradually deepens its valleys. 
    – Often waterfalls and rapids are also found in this course;
    – The main type of erosion is vertical. The valleys are narrow and deep. 
    – The features found in this stage include gorges, rapids and 
    waterfalls. 
    – There are lots of stones and boulders for the water to flow over. 
    The river starts as a stream in the upper course and flows through 

    V-Shaped valleys. 

     • Mature stage is known as valley stage. This middle course 
    corresponds to the mature stream and presents the following 
    characteristics: 
    – This is the stage between the upper and lower courses of the river;
    – The slope of the riverbed is reduced, and the speed of the water is 
    also reduced;
    – The main type of erosion is lateral and the river begins to widen its 
    channel. There is also some deposition of sediments;
    – More tributaries join the river, leading to a large volume of water;
    – The river begins to meander or follow a winding course;
    – The features found in this stage include cliffs, slip-off slopes and bluffs.
    • Old stage,  also known as the old stream, is the lower course 
    where the river becomes its widest and deepest. It has the following 
    characteristics:
    – The slope of the river is very gentle; therefore, the river flows slowly. 
    – The valley is shallow, wide and flat. 
    – Seasonal floods occur. 
    – There is a lot of deposition of sediment on its bed. 
    – The features found in this stage include ox-bow lakes, deltas, 

    floodplains etc.

    Application activity  8.1
     1. Explain the major work of a river.
     2. Describe the characteristics of a river that you observe in your local 
    environment and how that river affects the environment around.
     
    8.2. Formation of the major landforms associated with a     river profile
     Learning activity  8.2

     Observe the diagram below and answer the following questions.

    1) Name the landforms labeled  a, b, c, d and e;
     2) Apart from the features named above, what are other landforms 

    created by a river?

    8.2.1. Formation of landforms in youthful stage
     Youthful stage is the first stage of a river near its source. This stage is 
    characterized by a steep gradient, fast flowing water, vertical erosion etc. There 
    are several landforms that are created in this stage especially due to vertical 
    erosion and the nature of the gradient. The landforms like waterfalls and rapids, 

    potholes and plunge pools are the main landforms:

    i) Waterfalls and rapids
     Waterfall refers to movements of water or simply sudden descents of water 
    due to abrupt breaks in the longitudinal course of the river. Waterfalls are mostly 
    caused by variations in the relative resistance of rocks and topographic reliefs. 
    A waterfall, therefore, is a vertical drop of a big volume of water from a great 
    height along the profile of a river.

    Rapids     are alternate breaks along the river’s profile. Rapids are smaller than   
    waterfalls. Generally, they are found upstream from the main falls, and are also 

    found independently.

    ii) Potholes and Plunge pools
     These are kettle-like and cylinder-shaped depressions in the rocky beds of the 
    river valley. They are circular depressions cut at the bed of the river by fast 
    flowing water. They are formed due to saltation and traction movement of large 
    pebbles and boulders on resistant rocks. Plunge pools are formed when pot 
    holes are further widened and deepened by circular and fast movements of  water.

    iii) Interlocking spurs
     These are alternate bands of resistant rocks or hill sides formed when the river 
    attempts to avoid hard and resistant rocks on a steep gradient. The hard rocks 

    are not eroded hence, the river meanders between interlocking headlands.

     8.2.2. Formation of landforms in mature stage 
    A mature stage of the river is the middle stage of a river’s course where the 
    gradient is lower and where the river begins to flow slowly as it widens its 

    channel. 

    The following are the major landforms:

    i) River valleys    : The valleys carved out by the rivers are significant erosional 
    landforms. The shape and dimension of fluvial originated valleys change 
    with the advancement of the stages of fluvial cycle of erosion.  

    ii) Gorges and Canyons    : Are very deep and narrow valleys with steep 
    sides/slopes that are wall-like. They are formed when water falling over 
    the hard rock, undercuts the rock leaving it hanging. The hanging rocks 
    may cause water to retreat upstream leaving behind a narrow and deep 

    sided valley

    iii) Alluvial fans: These are fan-shaped deposits of coarse alluvium. They 
    are formed when a fast flowing river loses its velocity when it enters the 
    gentle slope. The river immediately deposits its load composed of course 
    materials especially rocks, boulders and bigger pebbles. The deposits 

    are laid in form of a fan, hence the name, “alluvial fan”.

     iv) River Benches: These are step-like flat surfaces on either side of the 
    lowest valley. The benches or terraces formed due to differential erosion 
    of alternate bands of hard and soft rock beds are called structural 
    benches or terraces because of lithological control in the rate of erosion 

    and consequent development of benches.

    v) River terraces: The narrow flat surfaces on either side of the valley floor 
    are called river terraces which represent the level of former valley floors 

    and the remnants of former (older) flood plains.

    8.2.3. Formation of landforms in old stage 
    The lower or old stage of river is the last stage where a river nears its destination. 
    This stage is characterized by large deposits along the river’s bed and channel. 
    The large deposition is a result of increased lateral erosion, very slow movement 
    of water and very wide river channel. In this stage the river drops its load due to 
    the reduction in its energy. The material deposited by a river is called alluvium. 
    River deposition results into the formation of the following features:
     
    i) River meanders
     River meanders are the bends of the rivers. The bends of sinuous rivers have 
    been named meanders on the basis of Meander River of Asia Minor (Turkey) 
    because it flows through numerous bends. Each bend of the meander belt 
    has two types of slopes of valley sides. One side is characterized by concave 
    slope while the other side of the meander belt is characterized by convex slope. 
    The convex or slip off slope receives deposition mostly of sands and gravels 
    and alluvium at other times. Therefore, the bank of maximum deposition is also 
    called a slip-off slope. The concave slope is a bank of maximum erosion or 

    undercutting. It is steeper than the slip-off slope.

     ii) An Ox-bow lake
     This is a horse-shoe lake formed due to stagnation of water in the abandoned 
    meander loop. Ox-bow lakes are formed when a river develops very pronounced 
    meanders in the flood plains. As erosion and deposition continues on the river’s 
    banks, the neck of the meander is cut off and the water flow straight by-passing 
    the old meander. The abandoned or cut off meander therefore becomes an ox

    bow lake.

    iii) Flood plain
     This is a very gentle low-lying plain of alluvial deposits on a floor of a river valley. 
    It is formed where a river flows in a meandering way. As a river swings back and 
    forth across the valley, it widens its valley floor. The valley becomes so broad 
    that the meanders swing freely without touching the valley sides. When the level 
    of water rises during the flood time, all the plain along the river valley becomes 

    flooded. The river then deposits its alluvium in the plain.

     iv) Levees
     These are raised river banks made up of alluvial deposits. Levees are formed 
    when a river deposits its load along its banks during flooding. Slightly coarse 
    materials are deposited on the banks, while finer alluvium is transported further 
    onto the flood plains. With time, accumulation of coarse material raises the 
    banks of the river to form levees. During the dry seasons when the river retreats 
    into its channel, deposition are left both on the river’s bank and on its bed. This 

    leads to the formation of raised river beds and banks.

    v) Deferred tributaries
     These are small tributary rivers that flow alongside the main river. They are 
    formed when raised levees stop tributaries from joining the main stream. As a 
    result, such tributaries, flow parallel to the main river until they encounter a break 
    in the river bank where they now can join the main stream. They are thus referred 
    to as deferred tributaries or Yazoo streams. The point at which they join the main 
    stream is referred to as a deferred confluence. The tributary flows to the main 

    channel and finally break through levees and join the main channel.

    vi) Braided channel
     This is a wide and shallow channel where a river breaks into a series of 
    interconnecting distributaries separated by sandbanks and islands of alluvium. It 
    is formed in the middle or old stage of a river where the valley is wide and gently 
    sloping. The river carrying a large load flows at a low velocity, fails to transport 
    its load and finally deposits its load on the bed. Gradually, the river bed is raised 

    and the deposits divide the flow of water into small tributaries and distributaries.

    vii) Delta
     A delta is a low-lying swampy plain of alluvium at the mouth of a river. A delta 
    forms when a river fails to carry its entire load into the sea or mouth but deposits 
    these into its mouth. The deposits divide the river’s mouth into tributaries and 
    sub tributaries. The deposits gradually become colonized by various types of 
    plants and form a triangular shaped mouth of a river. This is called delta. The 
    river splits up into several separate channels in much the same way as river 
    braids. Deltas are classified into three categories depending on the shape and 
    growth where there are growing deltas and blocked deltas. They include the 
    following:
     • Estuarine deltas,
     • Arcuate deltas, 
    • Bird’s foot deltas.
    – Estuarine delta:     This is a submerged mouth of a river. It is a delta formed 
    from materials deposited in the submerged mouth of a river. This takes the 
    shape of the estuary. Examples are the Zambezi Estuary in Mozambique, 
    and Volta Delta in Ghana.
    – Arcuate delta:     this is a triangular and convex shaped delta. It is formed 
    by a river with many distributaries transporting materials. It occurs where 
    off-shore currents are strong enough to round the seaward edge of the 
    delta. Examples are Sondu Delta in Kenya, Nile Delta in Egypt and Amazon 
    Delta in Brazil.
    – Bird’s foot delta    : This is a delta that looks like the claws of a bird’s foot. 
    It is also known as digitate delta. It is formed when a river transporting 
    large load of mainly fine material enters into water that has low energy wave. 
    The distributaries extend from the shore into the open water. Examples are 

    Omo River Delta on Lake Turkana and Mississippi Delta in the USA.

    Application activity  8.2
     1. Visit the nearest rivers and do the following:
     i) Identify the landforms formed along a river.
     ii) Explain the importance of the above landforms to the local people.
     2. Describe the relationship between landforms in the lower stage of a 

    river and human activities


         8.3. River capture, river rejuvenation, superimposed and 
    antecedent drainage and impact of rivers
     Learning activity  8.3
     1. Make a research and establish the effects of the river capture and 
    river rejuvenation.
     2. Identify how superimposed and antecedent drainage are formed. 

    3. Discuss the importance of rivers.


         8.3.1. River capture
     A) Definition of river capture
     River capture refers to the diversion of headwaters of a weaker river system into 
    a system of the stronger neighboring river. It is also referred to as river piracy. 
    The point of capture is known as “elbow of capture”. This point is usually found 
    near the dry valley or misfit stream. A misfit stream is the river whose water 
    has been beheaded or diverted into another stream. It contains very little or no 
    water at all and is not therefore fit to be in that river. This is why it is called misfit 
    stream. Beyond the misfit stream is a valley that no longer contains water. It is 

    only covered by old alluvial deposit. This is called a dry valley. 


    B) Features of river capture
    There are four major features of river capture: elbow of capture, cols or wind 

    gaps, misfit or under fit streams and dry valleys.

    C) Causes of river capture
     A river capture can be caused by headward erosion, lateral erosion, or 
    coalescence of meanders. The following are the causes of river capture:
     • The presence of a river with a larger volume of water compared to its 
    neighbour (the weaker river). The stronger river erodes its valley faster 
    by vertical erosion compared to its neighbour.
     • The presence of soft  and easily eroded rocks in the valley of a stronger river
     • Earth movements like faulting, folding, warping and volcanicity on the 
    valley of a stronger river can also cause river capture
     • Change in base level as a result of river rejuvenation. A fall or rise in a 
    river’s base level can cause river capture
     For river capture to take place, the following conditions are necessary:
     • There must be a powerful river or pirate stream and a misfit stream 
    flowing adjacent or parallel to each other.
     • The pirate river must be flowing over a much steeper valley than the 
    misfit or beheaded stream
    • The pirate river must be having more active head ward erosion compared 
    to its neighbouring river
     • The pirate river must be flowing over easily eroded rocks compared to 
    those of its neighbour
     
    D) Effects of river capture
     The following are the effects of river capture (after the occurrence of river 
    capture):
     • The volume of water in the pirate stream increases;
     • The capturing/beheading river becomes bigger and more stronger than 
    it was before capture;
     • The beheaded stream having lost its waters contains very little water 
    and almost dries off (a misfit river);
     • The pirate river develops an elbow of capture. This denotes a sharp 
    change in the direction of a river course (at the point of capture);
     • The valley of the beheaded stream below the point of capture becomes 
    dry and hence the name, “wind gap”;
     • Incision of the pirate river near point of capture. This valley becomes 
    wider due to increased vertical erosion (head ward erosion).
     
    8.3.2. River rejuvenation
     A) Definition of river rejuvenatio    n
     River rejuvenation is the renewed erosive activity of a river. It is an acceleration 
    of erosive power of the fluvial process of rivers. Rejuvenation length is the period 
    of the cycle of erosion. For example, if the cycle of erosion is passing through 
    senile stage (old stage) characterized by gentle channel gradient, sluggish river 
    flow and broad and shallow alluvial valleys, after rejuvenation (caused either 
    due to substantial fall in sea level or due to uplift of landmass) the cycle is 
    interrupted and is driven back to juvenile (youth) stage characterized by steep 
    channel gradient and accelerated valley incision. 
    There are three types of rejuvenation as follows:
    i) Dynamic rejuvenation: It is mainly caused by uplifting in the landmass, 
    tilting of land area and lowering of the outlet.
     ii) Eustatic rejuvenation: This occurs because of changes in sea level 
    due to diastrophic events (subsidence of sea floor or rise of coastal land) 
    and glaciations causing fall in sea level.
     iii) Static rejuvenation: Its main causes are decrease in the river load, 
    increase in the volume of water and consequent stream discharge due 
    to increased rainfall, increase in water volume of the main river due to 
    river capture.
     
    B)  Causes of river rejuvenation
     River rejuvenation is caused by the following:
     • A fall in base level or fall in the level of the sea.
     • Earth movements involving uplift, down faulting 
    • River capture which may cause an increase in the volume of water (river discharge)

     • Change in rock resistance

    C)  Effects of river rejuvenation on the landscape
     River rejuvenation produces several features as follows:
     • Knick point: This is a break of slope in the long profile of a river valley. 
    It indicates the point where rejuvenation started. Knick points are 
    associated with rapids and water falls.
     • Paired terraces: These are steps or bench-like river valleys on both 
    sides of a rejuvenated valley. They are marked by old alluvial deposits 
    laid down before river capture occurred. It is therefore a part of the 
    former flood plain valley that is above the present river level.
     • Incised meanders: An incised meander is a curved bend of a river 
    that has been incised or cut into the land surface so that a river now 
    winds between steep valley walls. Incised meanders develop from an 
    already meandering river.
     • Ingrown meanders: These are incised meanders with asymmetrical 
    steep valley sides. They develop on resistant rocks and where the base 
    level falls gradually and the meander shifts gradually and laterally
     • Valley within a valley: This is also referred to as a rejuvenation gorge. 
    These are steps at the opposite sides of a rejuvenated valley. They form 
    where rejuvenation was very rapid with a large fall in base level. The 
    river flows in a deep channel within paired terraces that were once the 

    remains of the flood plain. 

    8.3.3. Superimposed and antecedent drainage
     A)  An antecedent drainage
     This is a drainage made of streams that maintain their original course and 
    pattern despite the changes in underlying rock topography.  Antecedence is 
    when the drainage pattern developed before such structural movements as the 
    uplift or folding of the land, and where vertical erosion by the river was able to 
    keep pace with the later uplift. A stream with a dendritic drainage pattern for 
    instance, can be subject to slow tectonic uplift. However, as the uplift occurs, 
    the stream erodes through the rising ridge to form a steep-walled gorge. The 
    stream thus keeps its dendritic pattern even though it flows over a landscape 

    that will normally produce a trellised drainage pattern. 

    B)  A superimposed drainage
     This kind of drainage pattern seems to have no relationship to the present-day 
    surface rocks. Superimposed pattern is a drainage that formed over horizontal 
    beds that overly folded and faulted rock with varying resistance. The stream 
    erodes through the underlying horizontal beds, and retains its course and 
    pattern despite changes in the underlying rock. The stream erodes a gorge in 

    the resistant bed and continues its flow as before.

    8.3.4. Impact of rivers
     Rivers play an important role both to human beings and the surrounding 
    environments. Rivers can also negatively affect people and the surrounding 

    environments. 

    A) Positive impacts of rivers
     The rivers and riverine landforms present the following advantages for humans:
    – Rivers provide water for various uses such as domestic, industrial uses, 
    drinking by animals;
    – Navigable rivers provide natural route-ways used for transportation;
    – Rivers provide water for irrigation especially in areas of low rainfall.  This 
    promotes agriculture, hence increasing food production;
    – Waterfalls provide natural sites for the production of hydroelectric 
    power. Examples are: waterfall between lakes Burera and Ruhondo, 
    River Rusizi in Rwanda, River Tana in Kenya, River Volta in Ghana, water 
    falls along River Nile, etc;
    – River Ria, estuaries and deltas are deep and sheltered, hence they 
    promote the development of ports like Alexandria on the Nile delta;
    – Building materials such as sand, gravel and pebbles are obtained from 
    river beds and valleys;
    – Some rivers have spectacular features such as waterfalls, gorges and 
    canyons which attract the tourists. For example, Rusumo falls on river 
    Akagera in Rwanda;
    – Alluvial deposits in some river valleys are a source of valuable minerals 
    such as alluvial gold for example in Miyove valleys in Northern Province 
    of Rwanda;
    – Building materials such as sand, gravel and pebbles are obtained from 
    riverbeds and valleys;
    – Flood plains and deltas contain fertile alluvial soils which have been 
    exploited for agriculture. Example is the Nyabarongo river valley, Nile 
    valley in Egypt etc;
    – The livestock activities are mostly developed near water bodies where 

    drinking and green vegetation water is available throughout the year.

    B) Negative effects
     The following are some of disadvantages of rivers and riverine landforms that 
    influence negatively humans:
    – Some large rivers form barriers to communication between communities 
    of the same culture;
    – During flooding some rivers cause destruction of property and loss of 
    human life;
    – Some river water may act as a medium for the spread of water borne 
    diseases, for example, Malaria, Bilharzia;
    – Some rivers host dangerous animals such as crocodiles and 
    hippopotamuses. These at times attack human beings and destroy crops.
     
    Application activity  8.3
     1. Using your knowledge and skills acquired in this unit, explain the 
    factors that favour river capture.
     2. Examine the difference between river capture and river rejuvenation.
     3. Analyze the impact of rivers to the development of the country.
     
    8.4. Lakes, Seas and Oceans
     Learning activity  8.4
     1. Identify any 5 lakes found in Rwanda.
     2. Use internet and other geographical resources to research on types 

    of lakes and their mode of formation.

    8.4.1. Types of Lakes
     A lake is a large mass of water that occupies a basin or depression on the 
    surface of the earth. Lakes receive water from streams, overland flow, and ground 
    water, and so they form part of drainage systems.  Lakes may be permanent or 
    seasonal. This depends on the volume of water that gets in, and the amount of 
    water that is lost. The loss of water is through evaporation and river outlets. 
    Lakes are categorized according to their mode of formation. They are grouped 
    in various ways as follows:
    – Through earth movements (tectonic lakes)
    – Volcanic action (lava dammed and crater lakes)
    –Erosion (erosional lakes)
    – Deposition (depositional lakes)

    – Human activities (man-made lakes)

    8.4.2. Mode of formation of Lakes
     The lakes are differentiated on the basis of their mode of formation. The following 

    are the major modes of lakes’ formation.

    A) Lakes formed by earth movements
     Lakes caused by crustal warping: These are lakes that occupy a basin
    like depression. They were formed when water occupied down warped basins 
    immediately after crustal warping. These lakes are also called subsidence 
    Lakes. Examples are Lake Chad and Lake Victoria in Africa. In Rwanda, Lakes 

    like Muhazi, Mugesera, Cyohoha were also formed as a result of subsidence.

    Rift Valley Lakes: These are Lakes that occupy depressions within rift valleys. 
    They are usually deep, elongated, and have steep sides. They are located on 
    the floor of a rift valley. Examples are Lakes Kivu in Rwanda, Turkana in Kenya, 

    Tanganyika and Malawi in Tanzania.

    B) Lakes produced by glacial erosion and glacial deposition
     Cirque lake, also call a Tarn Lake is a Lake that forms in a glaciated highland. 
    Such lake occupies an armchair-like depression, called a cirque. During thawing 
    (melting of snow), water collects in circular depressions that were left behind 
    where large avalanches or boulders were uprooted by melt glaciers. 
    A cirque lake sometimes feeds a mountain river. Tarns occur on the sides of 

    Mount Kenya like Teleki Tarn and on Mt Rwenzori for example Stanley Lake.

    Trough Lake: This occupies an elongated hollow excavated by ice on the floor 
    of U-shaped valley. It is sometimes called a ribbon lake. Lake Michaelson, in the 

    Gorges Valley, near to Mount Kenya, is a trough lake.

    • Kettle Lakes: These are small lakes that are formed in depressions 
    in glaciated lowlands. They are formed when melt water occupy 
    depressions called kettle holes. 
    • Moraine dammed lakes: These are lakes that form in glaciated 
    lowlands when a moraine dams the flow of melt waters in glaciated 

    lowlands.

    C) Lakes produced by wind erosion
     These are lakes that form in desert depressions left behind where large masses 
    of sand dunes and pebbles have been removed. Wind deflation sometimes 
    produces extensive depressions which reach down to the water-table in arid 
    deserts. The lakes of these depressions are not always true lakes-they may 
    be nothing more than muddy swamps. The Quattara depression, in Egypt, is a 
    good example.

    More permanent desert lakes develop when an aquifer becomes exposed. 
    These lakes are called oases. Some desert lakes dry up because of excessive 
    evaporation and all that remains is a lake bed of salt. This is called a playa or a 
    Salt Lake.
     
    D) Lakes produced by river deposition 
    Ox-bow Lake: It is formed when a meander loop of a river on a flood plain is 

    cut off from the main river. The river Galma, in Nigeria, has several ox-bow lakes.

    Delta Lake: This Lake is formed by the deposition of alluvium by rivers turning 
    either a part of the sea into a lagoon, or part of a distributary into a lake. The 

    Etang de Vaccares is a delta lake. Delta lakes occur in the Nile Delta, in Egypt.


    Flood plain Lake: A levée sometimes prevent water from returning to the river, 
    thus causing a lake to form. There are several lakes of this type on the River 

    Congo.

    Boulder Clay Lake: Some boulder clay deposits contain depressions which 
    become the sites for lakes. There are lakes of this type in Northern Ireland.

    A. Lakes produced by marine deposition
    Lagoon: This is a lake formed by a sand bar or sand spit extending along a 
    coast and cutting off a coastal indentation hence forming a lagoon.  Sometimes 

    a barrier beach extends across the mouth of a river, producing a lagoon.


    B. Lakes produced by volcanicity: 
    1. Crater lakes or caldera lakes are formed in volcanic craters and calderas, 
    which fill up with precipitation more rapidly than they empty via evaporation, 
    groundwater discharge or combination of both. Crater (small volcanic 
    depression) and Caldera (large volcanic basin) There are several caldera 
    lakes in Africa: Lake Shala, in Ethiopia, Lake Ngorongoro in Tanzania, Lake 
    Toba, in Sumatra (Indonesia) is also a caldera lake. In Rwanda, the Crater 

    Lakes are also found on Mountains Bushokoro, Muhabura and others. 

     2. Lava-dammed lake: A flow of lava may sometimes block the flow of a river 
    valley which causes a lake to form. The Sea of Galilee, in the Jordan valley, 
    was formed by lava damming the flow of river Matiandrano. The lava dammed 
    lakes in Rwanda are Lakes Burera and Ruhondo in Burera district of Northern 

    Province.

    C.  Other types of lakes
    – Solution Lake: This sometimes develops in a limestone area when 
    rainwater has dissolved the rocks to form a cave, and when the floor of 
    this cave is near to the base of the limestone. Lake Scutari, in Yugoslavia, 
    is a solution.
    – Temporary Barrier Lake    : Such a lake forms when an avalanche, or 
    scree fall, or landslide blocks a river valley. A lake of this type is only 
    temporary.
    – Man-made lake    : This is often called a reservoir. It is deliberately 
    formed by building a dam across a narrow, steep-sided section of 
    a river valley, usually a gorge, or constructing a wider depression or 
    water dam to trap rain water in a valley for the purpose of storing water 
    for irrigation, wet rice cultivation or for developing hydroelectricity or 
    both. Such lakes in Rwanda are Cyabayaga in Nyagatare District and 
    Rugeramigozi in Muhanga District.
    – Lakes produced due to mass movement:     Movement of debris 
    down slope due to the influence of gravity may block a river valley. They 
    may be landslides, mudflows, avalanches or rock slides.
    – Lakes produced by alluvial deposits    : These are lakes formed 
    because of back ponding by rivers. Such lakes form in depressions 
    within river valleys. Examples of such lakes are; Rweru, Ihema, Hago 
    Rwanyakizinga etc. along the valley of river Akagera.
     
    8.4.3. Impact of lakes
     The usefulness of lakes to human society are briefly described below.
     • Source of fish: Lakes are habitats for different varieties of fish. This 
    has favoured the development of fishing and related industries. 
    • Source of minerals and natural gases: lakes such as Magadi in 
    Kenya, Natron in Tanzania and Katwe in Uganda are source of salt, Lake 
    Kivu in Rwanda contains natural gas. 
    • Tourism: Lakes provide beautiful sceneries and other activities which 
    attract tourists. This earns a country foreign exchange.
    • Cheap transport: Lakes form cheap natural waterways for goods 
    and passengers. 
    •Source of power: Some lakes have been harnessed for the generation 
    of hydroelectric power. For example, Lakes Burera and Ruhondo 
    generate power on Ntaruka hydroelectric power plant.  
    • Source of useful water: Lakes are sources of water for domestic and 
    industrial uses.
     • Source of drinking water for animals like cattle, sheep, goats, etc.
    • Source of building materials: Some lakes are source of building 
    and construction materials such as sand, pebbles, small rocks, water 
    used in construction, etc.
     • Regulating river flow: Some lakes help in controlling floods by 
    regulating the flow of rivers.
     • Modification of climate: Lakes are important factors controlling the 
    climate of the surrounding areas because they provide the moisture. 
    The lakes also modify the climate of the adjacent areas.
     •  Source of rivers: Some lakes are sources of rivers. They act as 
    reservoirs and stores of water to rivers. For example, Lake Kivu is a 
    source of river Rusizi, Lake Muhazi is source of Nyabugogo River, etc.
     
    8.4.4. Distribution of seas and Oceans
     A) Distribution of Sea    s
     A sea is a very large mass of saline water that occupies a very huge depression. 
    Seas occupy large basins on the continental margins. Lakes are smaller than 
    seas but seas are also smaller than oceans. Seas are of two types namely: 
    • Inland seas. These are shallow seas over part of a continent. They are 
    connected to oceans by straits
     • Marginal Sea. This is a sea partially enclosed by islands, archipelagos, 
    or peninsulas, adjacent to or widely open to the open ocean at the 
    surface, and/or bounded by submarine ridges on the sea floor.


    B) Distribution of oceans
     An ocean is a large mass of saline water. Oceans occupy basins between 
    continents. There are five oceans in the world. These are as follows: 
    • Southern (Antarctic) Ocean: with an area of 20 million kilometers square
     • Arctic Ocean: with an area of 14 million kilometers square
     • Indian Ocean: with an area of 68.5 million kilometers square
     • Atlantic Ocean: with an area of 76 million kilometers square

     • Pacific Ocean: with an area of 155 million kilometers square

     Application activity  8.4
     1. Draw a sketch map of Rwanda and on it indicate the types of Lakes.

     2. Explain their mode of formation.

     Skills Lab
     Water pollution is a result of human activities. Give advice on how to 
    prevent it. 

    End unit assessment
     1. Some ocean currents originate from warm regions and others from 
    cold regions. Describe the relationship between ocean currents and 
    the atmospheric circulation.
     2. Conduct your own research to describe the major ocean management 
    projects in the world.
     3. Discuss the economic advantages of drainage in Rwanda, and in the world.
     4. Explain the strategies to mitigate natural hazards associated with 

    drainage system.

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    Active continental margins: Continental margins that coincide with 
    tectonically active plate. Asthenosphere: The plastic like, soft layer below the 
    lithosphere in earth’s mantle, beneath the rigid lithosphere.
    Adaptation measures for climate change: Measures and strategies taken 
    to adapt to climate change and its variability.
    Aerosols: Suspended Particulate Matter (SPM) in the atmosphere including 
    solid particles of varying sizes and liquid droplets are collectively called aerosols 
    which include: ploughed soil cover, deserts, rocks, salt particles from seas and 
    oceans; meteoric particles, organic matter: bacteria, seeds, spores and pollen.
    Andesite: Extrusive igneous rock of diorite composition, dominated by 
    plagioclase feldspar; the extrusive equivalent of diorite.
    Antecedent drainage: A part of a river slope and the surrounding area uplifts 
    but the river maintains its original slope.
     Basalt: Extrusive igneous rock of gabbro composition; occurs as lava.
     Bleach coral reefs: These are white coral reefs after expelling the algae 
    (zooxanthellae) 
    Clay minerals: class of minerals produced by alteration of silicate minerals, 
    having plastic properties when moist. 
    Clay: sediment particles smaller than 0.004 mm in diameter.
    Climate change mitigation: Involves reductions in human (anthropogenic) 
    emissions of greenhouse gases. 
    Climate variability: It is variations of atmospheric conditions at a specific 
    location or globally in short term. 
    Coal: Rock consisting of hydrocarbon compounds, formed of compacted, 
    lithified, and altered accumulations of plant remains (peat).
    Collision: Process where two continental crust collide and, as neither can sink, 
    are forced up into fold mountains.
    Compression (tectonic): Squeezing together, as horizontal compression of 
    crustal layers by tectonic processes. 
    Conglomerate: A sedimentary rock composed of pebbles in a matrix of finer 
    rock particles.
    Continental crust: Crust of the continents, of felsic composition in the upper 
    part; thicker and less dense than oceanic crust. 
    Continental drift: Hypothesis proposed by Alfred Wegener, which states that 
    continents have moved horizontally around the globe, over time, to reach their 
    current location.  
    Continental lithosphere: Lithosphere bearing continental crust of felsic 
    igneous rock.
    Continental margins tectonic: Marginal belt of continental crust and 
    lithosphere that is in contact with oceanic crust and lithosphere, with or without 
    an active plate boundary being present at the contact.
    Continental margins: A zone which combines both the continental shelf and 
    the continental slope and is distinct from the deep-sea floor.
    Control Gate: A facility used to control over the water travelling in penstock.
    Convection current: The driving forces of plate tectonics in which hot, plastic
    like material from the mantle rises to the lithosphere, moves horizontally, cools, 
    and sinks back to the mantle.
    Convergent boundary: In plate tectonics, the boundary between two plates 
    that are converging, or moving toward each other.
    Coral reef: Skeletons of very small sea creatures.
    Coral: A marine polyp capable of secreting calcium carbonate to build an 
    external skeleton.
    Coriolis force: Deflecting motion caused by the rotation of the earth which 
    makes a body or current moving across its surface to be deflected to the right 
    in the north hemisphere, and to the left in the south hemisphere. 
    Crane: A type of machine, generally equipped with a hoist rope, wire ropes or 
    chains, and sheaves that is used both to lift and lower the gates which regulate 
    intake gates or water flow from reservoir through the tunnel of a dam.
     Crust: Outermost solid layer of the earth, composed largely of silicate materials
     Dam: a barrier constructed across a river to hold back water and raise its level, 
    forming a reservoir used to generate electricity or for domestic, irrigation or 
    industrial water supply. Some dams are built also to preventing the flow of water 
    or loose solid materials (such as soil or snow).
    Deposition: The laying down of material that has accumulated after having 
    been eroded and transported.
    Desertification: Land degradation in which a relatively dry land region 
    becomes increasingly arid, typically losing its water bodies as well as vegetation 
    and wildlife.
    Development: The process in which some economic sectors or activities 
    (e.g. agriculture, industry, technology, etc.) grow or change and become more 
    advanced
    Diorite: Intrusive igneous rock consisting dominantly of plagioclase feldspar 
    and pyroxene; a felsic igneous rock.
    Divergent boundary: In plate tectonics, the boundary between two plates that 
    are diverging, or moving away from each other.
    Dolomite: Carbonate mineral or sedimentary rock having the composition 
    calcium magnesium carbonate.
    Drainage pattern: A plan made by a river and its tributaries along the landform
    Dredging: Clear the bed of a harbour, river, or other area of water by scooping 
    out mud, weeds, and rubbish with a dredge”the dredging and deepening of the 
    canal”.  
    Dry farming: This is also called Dry land Farming. It is the cultivation of crops 
    without irrigation in regions of limited moisture, typically less than 20 inches (50 
    centimetres) of precipitation annually.     
    Earthquake: A trembling or shaking of the ground produced by the passage 
    of seismic waves.
     Ecosystem: Total living things in an area including ways they interact each 
    other in the environment
    Effluents: Liquid waste or sewage discharged into a river or the sea from 
    industries.
    Eustasy: any uniformly global change of sea level that may reflect a change in 
    the quantity of water in the ocean, or a change in the shape and capacity of the 
    ocean basins
    Extinction: the state or process of being or becoming extinct /disappearance, 
    vanishing.
    Extrusive igneous rock:  Rock produced by the solidification of lava or ejected 
    fragments of igneous rock (tephra).
    Feldspar: Group of silicate minerals consisting of silicate of aluminum and 
    one or more of the metals potassium sodium, or calcium (See also plagioclase 
    feldspar, potash feldspar) 
    Felsic igneous rock:  Igneous rock dominantly composed of felsic minerals.
     Felsic minerals (felsic mineral group):  Quartz and feldspars treated as a 
    mineral group of light color and relatively low density. (See also mafic minerals.)
     Flood control: Methods are used to reduce or prevent the detrimental effects 
    of flood waters.
     Gem: Also called Game stone is a valuable mineral highly prized because it is 
    rare and beautiful.  
    Gentle slopes: These are areas located in rolling countryside where slope is 
    between 5 and 15% and the pattern of rainfall distribution regularly results in 
    erosion events. They are very common in Mediterranean countries
    Glacier: It is a large mass of ice in motion.
    Gondwanaland: A supercontinent of the Permian period including much of the 
    regions that are now South America, Africa, Antarctica, Australia, New Zealand, 
    Madagascar, and peninsular India. 
    Granite:  Intrusive igneous rock consisting largely of quartz, potash feldspar 
    and plagioclase feldspar with minor amounts of biotite and hornblende; a felsic 
    igneous rock
    Gravity: The force by which objects are attracted to one another because of 
    their mass on the earth surface.
    Greenhouse effect: Is process in which atmosphere of earth trap some of 
    heat coming from sun, making Earth warm than usual. 
    Holomorphic soils: These are intrazonal soils which have developed in areas 
    where salts have accumulated at or near the surface.
    Hurricane: A type of tropical cyclone with sustained winds that exceed 74 mph 
    and accompanied by rain, thunder and lightning
    Hydromorphic soils: These are intrazonal soils developed in presence of 
    excess water.
    Ice cap: An area of permanent ice. 
    Intrusive igneous rock: Igneous rock body produced by solidification of 
    magma beneath the surface, surrounded by preexisting rock.
    Laurasia: A supercontinent of the Permian period, including much of the region 
    that is now North America and western Eurasia.
    Lava:  Magma emerging on the Earth’s solid surface, exposed to air or water.
    Levee: Also called embankment or flood bank or stop bank is an elongated 
    naturally occurring ridge. It is usually earthen and often parallel to the course of 
    a river in its floodplain or along low-lying coastlines.
    Lithosphere: The rigid, outermost rock layer of the earth, about 100 km thick, 
    composed of the crust and part of the mantle, lying above the asthenosphere.
    Mafic igneous rock: Igneous rock dominantly composed of mafic minerals.
     Mafic minerals (mafic mineral group): Minerals, largely silicate minerals, rich in 
    magnesium and iron, dark in color, and of relatively greater density.
    Magnetometer: A sensitive instrument that records magnetic data and is used 
    to study earth’s magnetic field.
    Marble: Variety of metamorphic rock derived from limestone or dolomite by 
    recrystallization under pressure.
    Metamorphic rock: Rock altered in physical structure and/or chemical 
    (mineral) composition by action of heat, pressure, shearing stress, or infusion of 
    elements, all taking place at substantial depth beneath the surface.
    Mid-oceanic ridge: One of three major divisions of the ocean basins, being 
    the central belt of submarine mountain topography with a characteristic axial rift. 
    Mineral: Is a naturally occurring chemical compound, usually of crystalline form 
    and abiogenic in origin (not produced by life processes). A mineral has one 
    specific chemical composition, whereas a rock can be an aggregate of different 
    minerals or mineraloids. The study of minerals is called mineralogy
    Oasis: A moist fertile place in the desert usually surrounding a well or spring
    Oceanic crust: Crust of basaltic composition beneath the ocean floors, 
    capping oceanic lithosphere.
    Oceanic lithosphere: Lithosphere bearing oceanic crust.
    Oceanic trench: Narrow, deep depression in the seafloor representing the line 
    of sub-duction of an oceanic lithospheric. 
    Ore: A mineral containing a useful substance, such as metal, that can be mined 
    at a profit.
    Ox-bow Lake: A horse shoe shaped lake form from a meander that is cut off 
    and abandoned by the main river.
    Pangaea (pan JEE uh): The name Alfred Wegener gave to the single large 
    landmass, made up of all continents, that he believed existed before it broke 
    apart to form the present continents.
    Parent rock: It is the material (rock) from which soil is formed.
    Passive continental margin: Continental margin lacking active plate 
    boundaries at the contact of continental crust with oceanic crust.
    Peridotite: Igneous rock consisting largely of olivine and pyroxene; an ultramafic 
    igneous rock occurring as a pluton, also thought to compose much of the upper 
    mantle.
    Petrology is the branch of geology that studies rocks and the conditions under 
    which they form. Petrology has three subdivisions: igneous, metamorphic, and 
    sedimentary petrology
    Plate tectonics: Theory that earth’s crust and upper mantle (lithosphere) are 
    broken into sections, called plates that slowly move around on the mantle.
    Prevailing wind: The direction of wind most frequently observed during a 
    given period.
    Pyroclastic materials: The fragmental rock products ejected by a volcanic 
    explosion having been broken by fire. 
    Quartzite:  Metamorphic rock consisting largely of the mineral quartz.
    Reservoir: Usually means an artificial lake, storage pond or impoundment 
    created using a dam or lock to store water. Reservoirs can be created by 
    controlling a stream that drains an existing body of water. 
    Rhyolite: Extrusive igneous rock of granite composition; it occurs as lava or 
    tephra.
    Ridge: An elongated area of relatively high altitude bordered by an increasingly 
    low altitude side.
    River capture: The diversion of waters of a weaker river into the system of a 
    stronger river.
    River profile: A section of a river from its source to its mouth.
    River rejuvenation: The renewed erosive activity of a river.
    River terraces: A portion of the former flood plain of a river now, abandoned 
    and left at a higher level as the stream down cuts its sides
    River: A mass of flowing water from a known source to a known destination
    Rock or stone is a natural substance, a solid aggregate of one or more minerals 
    or mineraloids.
     Run off: The proportion of rain water that reaches streams either by flowing 
    over ground. 
    Sandstone: Sedimentary rock consisting largely of mineral particles of sand 
    size.
    Schist: Foliated metamorphic rock in which mica flakes are typically found 
    oriented parallel with foliation surfaces.
    Sea: A body of salt water smaller than an ocean and generally in proximity to 
    continent.
    Seafloor spreading: The theory 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.
    Sediment: Finely divided mineral matter and organic matter derived directly or 
    indirectly from pre-existing rock and from life processes.)
     Sedimentary rock:  Rock formed from accumulation of sediment.
    Shale: Fissile, sedimentary rock of mud or clay composition, showing lamination.
    Siltation: It is the pollution of water suspended sediments dominated by clay 
    and silt. Siltation is most often caused by soil erosion. 
    Slate:  Compact, fine-grained variety of metamorphic rock, derived from shale, 
    showing well-developed cleavage.
    Slope: It is an inclined surface.
     Snow: precipitation in form of white ice crystals
     Soil: It is the thin layer of unconsolidated material covering the surface of the 
    earth that is able to support plant life. 
    Spreading plate boundary: Lithospheric plate boundary along which two 
    plates of oceanic lithosphere are undergoing separation, while at the same time, 
    new lithosphere is being formed by accretion.
    Steric effect: When some regions experienced sea level rise while others 
    experienced a fall, often with rates that are several times to the global mean rate.
    Subduction zone: In plate tectonics, the area where an ocean-floor plate 
    collides with a continental plate and the denser ocean plate sinks under the 
    less dense continental plate.  It is a boundary between two crustal plates along 
    which subduction is occurring and lithosphere is being consumed.
    Subduction: Descent of the down bent edge of a lithospheric plate into the 
    asthenosphere so as to pass beneath the edge of the adjoining plate.
    Superimposed drainage: A drainage pattern which exhibits a discordant 
    drainage: with the underlying rock structure because it is originally developed 
    on a cover of rocks that have now disappeared owing to denudation.
    Surface run off: The proportion of rain water that reaches streams either by 
    flowing over ground or by seeping through the soil.
    Syzygy: A term given to the situation when the earth, moon and sun are in 
    conjunction or opposition. i.e. when they are all in a straight line.
    Tectonic: Pertaining to the internal forces which deform the earth’s crust 
    thereby affecting the pattern of sedimentation or resultant landforms.
    Terra Rosa: It is a reddish clay-loam soil developed under a warm seasonally 
    dry climate on limestone. 
    Tethys Sea: inland sea from where the two blocks of landmasses separated 
    Tidal currents: A horizontal movement of sea water in response to the rise and 
    fall of the sea or ocean.
    Tide: The regular rise and fall of water level in the world’s oceans, resulting 
    from the gravitational attraction that is exerted upon the Earth by the sun and 
    the moon.
    Tornado: A violently rotating column of air that extends from a thunderstorm to 
    the ground and is often - although not always - visible as a funnel cloud.
    Transform fault: In plate tectonics, a boundary between two plates that are 
    sliding horizontally past one another.
    Transform plate boundary: Lithospheric plate boundary along which two 
    plates are in contact on a transform fault; the relative motion is that of a strike
    slip fault.
    Tsunami: Train of sea waves set off by an earthquake (or another seafloor 
    disturbance).
    Tuffaceous limestone: A sedimentary limestone that contains up to fifty 
    percent volcanic tuff these are ash and cinders.
    Ultramafic igneous rock: Igneous rock composed almost entirely of mafic 
    minerals, usually olivine or pyroxene group.
    Visibility: The longest distance that prominent object can be seen.
    Volcanism: General term for volcano building and related forms of extrusive 
    igneous activity. 
    Volcano: Conical, circular structure built by accumulation of lava flows and tephra.
     Wave: Is a deformation of water surface in the form of oscillatory movement 
    which manifests its self by an alternating rise and fall of that surface.
     Windblown area: This is an area which experiences a lot of wind as an agent 

    of erosion.


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