Formation of Major Land-forms and Drainage in East Africa

Introduction

In these relief regions, there are different physical features such as mountains, hills, valleys, outcrop rocks and flat plains. You also learnt that these features are called landforms. In this chapter, you are going to explore how these and other landforms were formed; and how the landforms are related to the rocks. You will also learn how landforms
influence drainage and the lives of people.

Rocks and Landforms

Activity 8.1
In groups, choose one of these landforms, i.e. a volcanic mountain or rift valley, and:
1. explain how it was formed.
2. describe the nature of rocks making up the landform.
3. explain how the landform may affect drainage, climate, soils and human activities in the area.
4. share what you have written down with the rest of the class.

Activity 8.2
As a whole class, conduct a debate on the motion: “There are more problems than benefits in living on a volcanic mountain”. In Activity 8.1, you have realised that the chosen landform is made up of rock material. All landforms of the earth are made up of rocks. What do you understand by a rock?

Rocks are of different types and sizes. In terms of size, they may range from very tiny particles, such as sand grains, through small stones to very large boulders. Every rock is made up of several minerals and if you break it up and examine it scientifically, you can identify each mineral. Some of the minerals found in the rocks are useful to people. You will learn more about this in your Chemistry lessons in Term Three.

Activity 8.3
In pairs:
1. Copy the table below in your notebooks, study it and fill in the missing information in the right hand column.

2. Swap the filled table with another pair, comment on each other’s work and make the necessary corrections.
3. Think of how life in your community would be if there were no minerals on earth.

When you read the above table, you realise that most of the things we use in our daily lives are made out of minerals. Farm tools used to produce the food we eat, domestic utensils such as saucepans, plates and cups, detergents such as powder soap, and vehicles used for transport are all made from minerals. Our country and many other countries around the world also get large amounts of money through selling minerals and mineral products. Generally, without minerals, all humankind would find life very difficult. To understand this better, look at Figure 8.2.

Types of rocks

All rocks are not similar in colour, hardness, texture and ability to hold water. This is because they were formed by different processes and under different conditions. In geography, we divide rocks into three main types depending on how they were formed. The first main type of rock is formed when very hot molten rock from deep inside the earth rises to the surface of the earth. Imagine rock material similar to hot porridge boiling and rising through the solid rocks. When the molten rock reaches the surface, it cools and forms a new rock called igneous rock or fire-formed rock. You will learn more about this process later in this chapter.

Igneous rocks formed on the surface of the earth are called extrusive igneous rocks. Sometimes molten rock from the interior may fail to rise up to the surface of the earth and instead cools inside the local rocks. When this happens, intrusive igneous rocks are formed.

Activity 8.4
1. Carry out a library or internet search about the formation of igneous rocks, their types and examples.
2. Draw diagrams to illustrate how each type is formed.
3. Construct a table to summarise examples of each type of igneous rock.
4. Suggest ways in which igneous rocks may benefit people living in areas where they are found and your country.

The second main type of rock is formed when the old rocks of the earth are broken down by weathering, or eroded by running water, wind or moving ice. The smaller rock pieces transported are called sediments. When sediments are deposited in other places they accumulate, harden and form new rocks called sedimentary rocks. Sedimentary rocks formed this way are called mechanically-formed sedimentary rocks. Most sedimentary rocks are of this type. Examples are sandstone, shale and loess.

Activity 8.5
1. Carry out a library or internet search for diagrams showing the formation of different types of sedimentary rocks.
2. Draw the diagrams in your notebook and make a brief description of how each type is formed.

Some sedimentary rocks are formed when the remains of dead plants and animals get deposited together in large amounts. These are said to be organically formed sedimentary rocks. Coal, coral limestone, marble and peat are examples. Do you know that mineral oil or petroleum in the Albert Rift Valley in western Uganda has been formed in this type of sedimentary rock?

Other sedimentary rocks are formed in very hot and dry areas experiencing a lot of evaporation. For example, rock salt is a rock formed when salty water evaporates and leaves behind salt crystals. When crystals accumulate, they harden and form a hard rock. This is called a chemically-formed sedimentary rock. Other examples of chemically formed sedimentary rock are gypsum and dolomite.

Activity 8.6
In groups:
1. Discuss and suggest areas in East Africa where each type of sedimentary rock is likely to be found. Give reasons to support the areas you have suggested in each case.
2. Explain how each type of sedimentary rock may affect human activities in the area where it occurs.

The third main type of rock is formed when igneous or sedimentary rocks get changed into new rocks. This change may take place when the areas where these rocks are found experience too much heat or pressure. The heat may come from molten rock which rises from the interior of the earth at temperatures of 1,000 degrees C or even higher.

The new rocks formed this way are called metamorphic rocks. For example, marble is a metamorphic rock formed when limestone or dolomite gets changed by heat or pressure. While slate, used for making nice finishes on modern houses, is formed from clay.

Each type of rock influences the formation of landforms in a different way. This is because each type has different characteristics, such as hardness and the ability to absorb water.

Activity 8.7
1. Copy the map in Figure 7.1 into your notebook and on it shade the type of rock likely to be found in each relief region. Give reasons to explain the distribution of rocks you have shown on the map.
2. Suggest which type of rock is:
i) hardest
ii) softest
3. How might the land-forms in each region have been influenced by the type of rock present?

Formation of land-forms in East Africa

The land-forms of the earth are of two major types. The first type is that of landforms that are formed by the rocks themselves. These are called structural land-forms. The second type is made up of landforms formed when some processes, such as erosion, work upon rocks and change them into new landforms.

The forces which shape the earth into landforms
Activity 8.8
Using your knowledge about the relief regions of East Africa, in groups do the following tasks:
1. Discuss how the rift valley and highlands of East Africa were formed.
2. Suggest where the forces that led to the formation of those landforms originated.
3. What could have caused those forces?
4. Write a report of what you have discussed and share it with other groups through a class discussion.

You have probably suggested that the rift valley and some highlands were formed as a result of faulting while other highlands were formed as a result of volcanic eruptions. Also, you could have suggested that both faulting and volcanic eruptions were caused by forces which originated in the interior of the earth. What is not very easy to understand is what caused those forces.

To understand why earthquakes, faulting, volcanic eruptions and folding take place, you need to learn about the nature of the earth.

Activity 8.9
In pairs or individually, use Figure 8.5 to do the following:
1. Count the plates shown on the map.
2. Make a list of all the tectonic plates.
3. What do the arrows at the boundaries of tectonic plates indicate?

We cannot get information about the interior of the earth by drilling or mining. The deepest mine in the world is 4.0 km below the surface, and from the surface to the centre of the earth it is more than 6,000 km. By studying earthquakes geologists come up with information about the interior of the earth.

Geologists believe that the solid outer layer of the earth, which is about 100 km, rests over a thicker layer of semi-molten rock. As you have learnt in Activity 8.8, the outer layer is made up of solid rocks which can be acted upon by different processes to form landforms. The landforms of East Africa are of two types namely, structural landforms and erosional landforms.

Earthquakes and land forms formed by faulting

Have you ever felt an earthquake or heard of an area where it occurred?
An earthquake is a sudden shaking of the ground caused by the passage of waves through the earth’s rocks. Such waves are called seismic waves. Seismic waves are produced when some form of energy stored in the earth’s crust is suddenly released within some limited region. Earthquakes most commonly occur in areas with rocks having lines of weakness called fault lines or simply faults.

Activity 8.10
Using your knowledge of the relief regions of East Africa:
1. Suggest areas in East Africa where earthquakes are most likely to occur.
2. Explain why you think the areas which you have suggested are likely to experience earthquakes.
3. Look at Figure 8.6 and explain how earthquakes affect people and their property.

The energy released into the rocks of the earth can lead to large scale cracking or fracturing up to the surface of the earth. When this happens, we say that faulting has taken place. For example, in Figure 8.5 (b), two fault lines have developed in the road. What do you think is likely to happen in the area if the ground is pulled apart along the fault lines?

In the same way, when the rocks of the earth experience faulting, there is movement and displacement of land blocks along the fault lines. This leads to the formation of rift valleys and Block Mountains or horsts.

Activity 8.11
In pairs or groups, study Figure 8.7 and do the following:
1. Copy the diagrams into your notebooks and give them labels 1-3 to identify the stages in the formation of faulted
landforms.
2. Explain the process going on at each stage and how it affects the land block.
3. On the diagram in stage 3, show the rift valley and Block mountains.
4. Suggest how the rift valley and Block Mountains may affect the lives of people in areas where they are found.

The processes which you have described in the previous activity are similar to those that led to the formation of the East African Rift Valley and the Ruwenzori Mountains. The steep walls on both sides of the rift valley are called fault scarps or escarpments.

Figure 8.7: Formation of landforms by faulting
Within the rift valley, another phase of faulting can take place. This is called secondary faulting and it may lead to the formation of a secondary depression. When the secondary depression gets filled with water, it becomes a rift valley lake. See Figure 8.8.

Lake Albert in western Uganda and Lake Tanganyika are examples of such lakes. Think of other examples of rift valley lakes in East Africa.

Vulcanicity

Activity 8.12
In groups,
1. Think of the volcanic features you know in Uganda, East Africa, and other parts of the world.
2. Make a list those features.
3. Swap your list with another group and see the features you have in common.
4. Discuss and agree on what is meant by vulcanicity and volcanicity.

Imagine you are cooking porridge in a closed pot or kettle. What do you think will happen to the cooking porridge if you heat it at high temperature? You have probably thought of the porridge trying to escape by forcing the pot cover open or through the spout of the kettle. In the same way, the solid layer of the earth on which we live rests upon a
porridge-like inner layer. The inner layer is made up of molten rock material called magma. Apart from molten rock material, magma can also contain ash, cinders, and gases. To understand this, look at Figure 8.9.

Magma exists in a molten state because of the extremely hot temperature in the interior of the earth. When magma gets overheated, the pressure inside the inner layer rises and forces the molten rock to rise like boiling porridge. The cracks or openings through which magma is forced out of the interior of the earth are called volcanic vents. The process by which magma escapes from the interior of the earth is called a volcanic eruption. When magma reaches the surface of the earth, it cools, becomes solid and forms a rock called lava. All physical features formed from lava are called volcanic features.

Features of a volcano

Activity 8.13
In pairs,
1. Carry out a library or internet search about the features of a volcano.
2. Copy Figure 8.9 (b) into your notebooks and using the information you have got from the search, name the features labelled 1-6.
When volcanic eruptions occur, they do not lead to the formation of volcanic mountains only but also create other landforms. Volcanic eruptions also lead to the formation of lakes.

Activity 8.14
In groups,
1. Carry out a library or internet search on landforms resulting from volcanic eruptions.
2. Make a list of the major landforms and draw diagrams to explain how each landform is formed.
3. Find out the types of lakes formed due to volcanic eruption; and draw diagrams to explain how each type is formed.
4. Suggest how the volcanic landforms and drainage features might be useful to people.
5. Suggest the disadvantages associated with volcanic landforms and lakes.

Warping

In Chapter Seven, you learnt that much of East Africa is a plateau. What do you understand by a plateau? The East African plateau has been affected by many processes which have created depressions and hills. Some of the
large depressions have been filled with water to form lakes.

Activity 8.15
In groups,
1. Choose two people to hold a sheet of newspaper or a piece of cloth by its corners.
2. Let them slowly lift the extreme ends of the paper or cloth as the rest of you observe and note down what happens.
3. Write down what you have observed and draw a diagram to illustrate your observation.
4. Share what you have written and the diagram you have drawn with other groups through a whole class discussion.

What you have observed is called sagging. In the same way, when certain parts of the East African plateau were acted upon by forces which originated in the interior several million years ago and pushed the eastern and western sides of the plateau upwards, the central part sagged in. That process is called down warping. It led to the formation of very large depressions. These depressions are now occupied by lakes.

Activity 8.16
In pairs,
1. Open the atlas and look at the map of East Africa showing physical features.
2. Suggest lakes in East Africa which were formed as a result of down warping. Give reasons to support your suggestion.
3. Draw a sketch map of East Africa showing the down warped lakes and other drainage features connected to them.

Chapter Summary

In this chapter, you have learnt that:
1. all the landforms of the earth are made up of rocks.
2. every rock is made up of several minerals, some of which are useful to us.
3. the rocks of the earth can be divided into three types depending on how they were formed.
4. East Africa has different landforms because the region has been affected by several physical processes, including earth movements.
5. structural landforms were formed by processes originating in the movement of tectonic plates.
6. landforms have a strong influence on the drainage of an area.
7. rocks, landforms and drainage influence the way people live, especially the economic and leisure activities they carry out.