Geography and Environment

Introduction

The main external land forming processes are erosion, weathering and mass wasting. These processes shape the way the landscape looks like.

Activity 5.1

In pairs, use different geographical sources to define the following terms:

       a)  Soil erosion

       b)   Weathering

       c)  Mass wasting

5.1 Soil erosion

This refers to the removal of the topsoil by agents of erosion. Soil erosion takes place in two steps:

             • Breakdown of soil particles

             • Transportation of the broken particles

5.1.1 Agents of erosion

Agents of erosion are the things that transport the top loose soil from one place to another. The agents of erosion include:

    a)    Running water

    b)    Wind

    c)    Moving ice

    d)    Gravity

    e)    Animals

    f)     Humans

NOTE: All these factors are interdependent. This means that one factor may aid the other in causing soil erosion. Few are the cases when one factor can independently cause erosion.

a)  Running water: Flowing water on the surface of the earth can carry loose soil particles from one place to another. The amount of soil that can be carried depends on:

          • The volume of flowing water

          • Speed of water

          • Slope of the land

          • Nature of the land, that is, whether bare or with vegetation

b)   Wind: The strength of the wind determines how much erosion it can cause. This also depends on how long the wind blows, as well as how bare the land is.

c)  Moving ice: This is also called a glacier. The extent of erosion a glacier can cause depends on:

        •  The volume of the glacier

        •  Speed of the glacier

        •  Slope of the land

d)   Gravity: This is the force that pulls things towards the centre of the earth. It mainly causes erosion on slopes by accelerating the movement of agents such as water and ice. The larger the amount of soil to be carried the greater the effect of gravity.

e)  Animals: Animals, both large and small, cause erosion in many ways. Burrowing animals dig out the soft, loose soil which is then easily transported down slope. Larger animals on the other hand tramp on the vegetation leaving the land bare. Those with hooves loosen the bare soil making it easy to be carried by other agents such as glaciers, water and wind.

f)   Humans: Various human activities may directly cause soil erosion. This includes ploughing near river banks and down slope. Sometimes, people clear vegetation either for settlement or farming. This makes it easy for other agents of erosion to take effect.

Remember!

The effects of soil erosion can only be avoided if it is controlled in its initial stages. This is easier than trying to reverse the situation in its advanced stages. It is also cheaper and time saving.It is our responsibility to take care of the environment.

We can achieve this by planting trees and ensuring good farming practices. For the future generations to benefit from the environment, we need to conserve it.

5.1.2 Types of erosion

Soil erosion takes place through the following ways:

On bare land, rain drops have a big effect on the top, loose soil. The impact of the drops breaks and displaces the soil particles. This causes them to be thrown away from their original positions in a splash. This is what is called splash erosion.

Splash erosion is the first stage of the erosion process.

With continued rainfall in gently sloping areas, rain water spreads out over a large area. As it slowly moves, it carries the loose, top thin layer of soil. This process is referred to as sheet erosion. The effect of sheet erosion is not easily noticeable since it involves a thin layer of soil.

Wind erosion can also cause sheet erosion. It is common in dry areas with strong winds. Such areas also have limited or no vegetation cover. This results in sheet erosion and the formation of sand dunes and drifts. Uncontrolled sheet erosion caused by water creates small channels in the soil. These channels are called rills. Advanced erosion, called rill erosion, then takes place as the small channels widens over time.

With continued erosion, the rills get enlarged to become gullies. These are large gaping channels that carry large volumes of soil during heavy downpours. Such a type of erosion is called gulley erosion.

5.1.3 Causes of erosion

Soil erosion can be caused by natural factors as well as human factors.

i) Natural causes of soil erosion

               • Heavy rainfall

               • Steep slopes

               • Winds

               • Drought

               • Increase in wild game population

Each of these causes is explained below.

a)  Heavy rainfall: Excessive rainfall causes surface run-off. When this occurs on bare land, it causes soil erosion.

b)  Steep slopes: They increase the speed of flow of water on the surface. This increases the rate at which erosion can occur.

c)   Winds: Strong winds are capable of causing sheet erosion. This occurs in areas with little or no vegetation. Vegetation acts as wind breakers, lack of which promotes erosion.

d)  Drought: Vegetation dries up during long periods of sunshine and unreliable or no rainfall at all. Reduced vegetation cover increases chances of erosion.

For example, animal hooves easily loosens the soil, making it easy to be carried away. Winds and water also carries such loose soil away easily.

e)  Increase in wild game population: When wild animals are many, they eat and trample on vegetation. This may be to a point when the land is left almost bare. In the long run, this can cause erosion.

ii) Human causes of soil erosion

Some human activities cause the loosening of the soil making it easy to be carried away. Clearance of vegetation exposes the land to agents of erosion.

Activity 5.2

In groups of five:

 a)  Study the following photograph carefully.

      Explain the factors that may easily cause soil erosion 

      How can soil erosion be avoided in the area shown by the photograph?

b)   Explain how the following poor methods of farming can cause soil erosion:

        • Bush fallowing

        • Monoculture

        • Ploughing near riverbanks

c)   Carry out a field study from the local environment to find out how the following human factors can cause soil erosion. Research widely from various geographical documents as well as the Internet.

  Complete the following table to be used in your class presentation.

5.1.4 Effects of erosion on landscape

When soil erosion is not controlled in its initial stages, it may lead to serious problems. Some of these are:

  a)  Sheet erosion and rill erosion causes the fertile top soil to be carried down slope.

  b)  Gulley erosion creates deep trenches into the soil making it unsuitable for settlement, agriculture and construction.

   c)  Gullies also make the land to lose its original beauty.

The following photograph shows land that has been destroyed by gully erosion.

Soil erosion can also lead to:

•  Soil degeneration: This is the loss of soil fertility. It occurs when the fertile top soil is carried away by agents of erosion.

•  Siltation in rivers and lakes: When soil is washed away by running water, it may end up in nearby rivers or lakes. This causes the water to be brownish in colour.

•  Desertification: If the soil is eroded to a point where it cannot support plant growth, this can easily cause desertification.

•  Landscape destruction: Soil erosion makes the land to look ugly.

5.2 Weathering

Soil erosion is the removal of the top soil by agents of erosion. On the other hand, weathering is the soil forming process.

Activity 5.3

In groups of three, use the Internet, dictionary and other geographical documents to:

•   Identify other differences between soil erosion and weathering.

•   Explain which of the two processes occur first.

5.2.1 Forms of weathering

Weathering occurs when rocks break down into smaller particles in situ (that is, without movement). Further breakdown of these rock particles results into even smaller particles that form soil.

There are some factors that affect the rate at which weathering takes place. These include the removal of the weathered materials by processes such as erosion and mass wasting. This is because erosion and mass wasting exposes the rock to further breakdown by agents of weathering.

Agents of weathering are the elements in the environment that directly influence the breakdown of rocks into smaller particles.

There are three forms of weathering. Each is explained in the following table.

We will discuss more about these forms of weathering in Unit Seven.

5.2.2 Causes of weathering

Weathering is caused by the following factors:

    a)   Climate (temperature and rainfall)

    b) The slope

    c) Plants, animals and human activities

    d)  Time

Let us look at each of these factors:

a)   Climate

The main elements of climate that cause weathering are sunshine and rainfall.

 • The amount of sunshine determines the heat responsible for expansion of the rocks. At night, contraction occurs within the rocks. This alternate expansion and contraction causes the rocks to break down.

 • Rainfall provides water that dissolves minerals in rocks causing them to crumble. Some rocks absorb water, which enlarges and softens them. This causes them to break down easily.

b)   Slope

After rocks have broken down, the slope determines the rate of removal of the pieces. This exposes the parent rock to further weathering. Some materials may flow down slope freely. Sometimes, these materials may need to be swept by water. On a flat surface, the rate of removal is slow, so is the rate of weathering.

c)  Plants, animals and human activities

  • Organic acids released from the decomposition of plants and animals assist in chemical weathering.

   • Plant roots grow deep into the ground, penetrating into rock joints. This opens the joints up, leading to physical weathering.

   • Human beings accelerate the rate of weathering through activities such as digging, mining and blasting.

d) Time

This refers to the duration the rocks are exposed to agents of weathering. The longer a rock is exposed to agents of weathering, the greater the rate of weathering.

5.2.3 Impact of weathering on landscape

Weathering shapes the land surface. This is seen in the varied landforms that result after rocks breakdown. The following are examples of how land is shaped as a result of weathering:

a)  When the rocks seen on the earth’s surface undergo weathering, they reduce in size. The harder parts of the rocks may remain protruding on the earth’s surface as the rocks wear down. This has created rock outcrops and inselbergs in some places.

b)  Some rocky highland areas may also undergo weathering leading to the creation of flat topped hills.

c)  Soluble parts of some rocks lead to the formation of holes on the surface. Water may disappear into such holes, which may further dissolve minerals underground. This creates underground caves.

d)  Some rocks get weathered by peeling off. The rock layers appear like layers of an onion. Such a process is called exfoliation. The resultant feature is called an exfoliation dome.

e)  The summit of some mountains get weathered, leaving behind the hard solid rock in the middle of the main vent.

5.3 Mass wasting

Mass wasting is the movement of weathered material down slope under the influence of gravity. Mass wasting occurs when weathered materials upslope are lubricated by rain water.

5.3.1 Forms of mass wasting

The movement of materials during mass wasting may be slow or rapid.

5.3.2 Causes of mass wasting

The movement of weathered materials down slope depends on:

      • The weight of the weathered materials

      • Earth or tectonic movements

      • Nature of the material

      • The angle of slope• Human activities

      • Vegetation

Activity 5.4

In groups of five, discuss each cause of mass wasting mentioned above. Use various geographical materials and the Internet in your search for explanation.

Support your explanation with illustrations and photographs obtained from the Internet or newspapers.

Present your report to the class.

5.3.3 Effects of mass wasting

The effects of mass wasting are both positive and negative.

Activity 5.5

In pairs:

1. Read the following effects of mass wasting and classify each as either positive or negative.

•   Mass wasting causes fertile soils to move down slope. This promotes crop farming down slope.

•   Areas upslope that loose the fertile soils due to mass wasting loose fertility making them unsuitable for crop farming.

•   Rapid mass wasting may causes destruction of property, infrastructure and even death of animals and people.

•   Mineral deposits that may be underground get exposed upslope in areas where mass wasting originates from.

•   The materials that flow down slope may block roads. This prevents transport.

•   Materials carried down slope may block river channels in river valleys. This can change the flow of rivers or form temporary dams.

•   Areas upslope that experience mass wasting may be prone to erosion. This is because the top soil will have moved down slope, exposing the softer underlying soils.

2. Discuss other effects of mass wasting.

Present your findings to class.

5.3.4 Measures of controlling erosion and mass wasting

The various measures used to control erosion and mass wasting are called soil conservation methods.S

oil conservation is a responsibility of each one of us. However, the government plays an important role to support this initiative. For example, by:

 •   Choosing alternative sites for infrastructural development.

 •   Creating public awareness on the need to conserve the environment to avoid cases of erosion and mass wasting.

Activity 5.6

1. Carry out a field visit and observe the method used to prevent or control soil erosion. This may be one or a combination of the methods mentioned below.

   a) Afforestation

   b) Reafforestation

   c) Agro-forestry

   d) P lanting cover crops

   e) Ploughing across the contours

    f) Using soil embankments on slopes

2.Identify the ways through which the government is participating in soil conservation measures.

Prepare a report for class presentation.

END OF UNIT REVISION QUESTIONS

In your notebooks:

     1.  Define soil erosion.

    2.  Identify the agents of erosion.

    3  .State different forms of weathering.

    4.   Discuss different forms of mass wasting.

     5.  Describe the causes of soil erosion and mass wasting.

     6.  Explain the measures of controlling erosion and mass wasting.