GEOGRAPHY

EARTH MOVEMENT

ACTIVITIES

Introduction
Faulting
Exercise 1
Vulcanicity
Exercise 2
 

FAULTING

A fault is a break, crack or a fracture in the Earth crust produced by vertical and lateral movements within the earth’s crust.

Faulting is one of the internal land form bulding process which shapes the earth’s surface. It is caused by earth movements, which create forces of tension and compression that are either lateral or vertical.

If forces of tension and compression are created by earth movements the rocks of the earth crust may fracture or crack. thus the faults are formed . The line along which rocks have fractured is called a fault.

Tension causes a normal fault, compression causes a reverse fault and lateral movement produces a tear fault.

Escarpments is a steep sided feature bordering a rift valley which develops when faulting is accompained by upward or downward movement of adjoining parts of the crust.

Types of faults

Normal faults

These are formed by the forces of tension. If the rocks are under tension ,faults will be formed and the centre block may sink down relative to its neighbours. Normal faults are common in East Africa.

Reverse faults

These are also known as thrust faults and are caused by the forces of compression when the strata or layers are compressed or pressed together the rocks will crack and faults will be formed. One block of rocks may override another to form a black mountain or a horst. Such faults are common in western Uganda. The Rwenzori mountain is an example of a block mountain formed by thrust faults.

Tear faults

When lateral movement is taking place particularly during an earthquake, tear faults may occur. Tear faults are also known as wrench or transform faults. If a tear fault occurs across the course of a river, then the river’s course may be slightly offset.

 

FEATURES FORMED (PRODUCED) AS A RESULT OF FAULTING

Faulting has produced a variety of physical features over the surface of the earth. Among the most remarkable features are;

  • Block mountains

  • Tilt blocks

  • Rift valleys or grabens

  • Escarpments or fault scarps

  • Rift lakes.

Block Mountains

A block mountain is also known as a Horst. It is formed when the middle block which is bounded by more or less parallel faults is made to rise or is uplifted by the compressional forces. The best example of a block mountain in East Africa is mount. Rwenzori ranges, also known as the mountains of the moon. Others include Mathews ranges and Nyiru Ndoto in Northern Kenya.

Tilt Blocks

Tilt blocks are formed when one side of the middle block is uplifted higher than the other side. The top of the middle block will not be flat but will be tilted. E.g. west Kenya tilt block, which rises to about 1900 metres towards lake Victoria.

Escarpments

Escarpments are steep cliff-like slopes. Escarpments are said to have been formed during the formation of the rift valleys.

Some escarpments are steep and may extend several hundreds of kilometres.When escarpments are eroded, they become fault scarps.

Examples of such escarpments in East Africa include:

  • Mandi(Kenya)

  • Butiaba (uganda)

  • Eldeyo marakwet (Kenya)

  • Kikuyu (Kenya)

  • Mau (kenya)

  • Lake manyara (Tanzania)

  • Nyando (Kenya)

  • Keiyo (Kenya)

  • Nyandarua (or Aberdare) (Kanya)

  • Chuya (Tanzania.)

RIFT VALLEYS

A rift valley i san elongated traugh bound by two in-facing escarpments.

Rift valleys are long, narrow depressions on the earth surface bounded by more or less parallel faults. A rift valley is also known as a graben.

Rift valleys are thought to have been developed either from the action of tensional forces in the crust or from the action of compressional forces.

The East Africa rift valley system extends southwards from the Red sea. Through Ethiopia and East Africa to Malawi.

The east Africa rift valley covers a distance of approximately 5,600km .it is divided into two branches that is, the western rift valley and the eastern rift valley.

The Western branch stretches from lake Albert in Uganda to lake Malawi. The eastern branch stretches from lake Turkana in northern Kenya to lake Malawi.

The width of the rift valley varies from place to place. The average width is between 50km and 60km.

On the floor of the rift valley there are a number of rift lakes and volcanic craters such as longonat crater and menengai craters.

Origin of the rift valleys

A number of theories have been put forward to explain the origns of rift valleys. However, two have remained popular thus, there the two theories which attempt to explain the origin of rift valleys. One relies on the forces of tension and the other on the forces of compression. Both theories depend on upward swells, along the sides of which faults develop.

Theory 1: Tensional forces

 

 

a) Tension forces act on the layers of rock.

 

 

 

 

 

b) Gradually two parallel faults appear and the central block begins to subside (sink).

 

 

 

c)Land in between sinks in forming a rift valley. The land on either sides stays in place.

 

 

After subsidence a depression with steep fault scarp sides i.e. a rift valley is formed. It is trapped in position by later pressure.

Theory 2. Compressional forces

 

a) Layers of rocks are subjected to compressional forces.

 

 

 

 

 

b) Faults develop and the outer blocks move upwards (Reverse faults are formed).

 

 

 

 

 

c) Central block stays in place and a rift valley is formed.

 

RIFT  VALLEY LAKES

These have been formed on the floor of the rift valley and they vary in size, depth and salinity. Examples of the salty lakes are Natron, and Magadi. The rift valley has several in land water basins which contain lakes.

Rift valley lakes of East Africa

Kenya

Uganda

Tanzania

L. Turkana

L. Albert

L. Tanganyika

L.Baringo

L. Edward

L.Rukwa

L.Nakuru

L. George.

.L.Natron

L.Elmentata

 

L.Eyasi

L.Naivasha

 

L.Manyara.

L. Natron   

L.Magadi

  

FAULTED AREAS IN EAST AFRICA

There are faulted areas in east Africa outside the rift valley such as:

Kavirondo rift at Kisumu.

Northern face of which is the Nandi scarp

Usambara mountains have fault scarps

Benefits of the Rift valley to the people of East Africa

  • The beautiful scenery attracts tourists

  • Lakes in the Rift valley provide fish. Forexample L.Tanganyika, L.Albert e.t.c.

  • Rift valley lakes provide water for domestic and agricultural use

  • Lakes help in navigation (water transport).

  • Forests on the slopes are sources of timber

  • Gentle slopes are used for crop farming and settlement due to fertile soils

  • Rift valley lakes are used for mining forexample soda ash from L. Magadi

  • Areas of little rainfall (rain shadow) provides pasture for grazing|

  • Study purposes or research

  • Wildlife conservation. Forexample game parks in rift valley areas.

Problems faced by the people living in the Rift valley areas of East Africa.

  • High temperatures lead to shortage of water.’’

  • Earthquakes (tremors) which destroy property

  • Little rainfall or drought in the Rain shadow areas

  • Poor means of transport and communication because of the steep escarpments

  • Salty lakes because of high temperatures and high evaporation rates

  • Soil erosion and land slides especially on the steep slopes

The East African Rift Valley

 Importance of faulting

Faulting has resulted into the formation of high mountains in East Africa. For example the Rwenzori in western Uganda, the southern highlands and the Usambara mountains in Tanzania and the mathew ranges in Kenya. These are the most productive areas, where both cash crops and subsistence crops are grown.

These highlands receive abundant and reliable rainfall.

Rift valley lakes for example L. Tanganyika, lake Turkana, lake Naivasha and lake Baringo are fishing grounds.

Some of these lakes have fresh water which can be used for irrigation and also for domestic purposes and industrial use.

L.Magadi contains vast deposits of soda ash, which is one of the most important minerals in Kenya.

Faulting presents an impressive scenery which can be used for tourism. For example L.Nakuru has millions of colourful flamingos and other birds.

Some highlands have been made into National parks and game reserves e.g. the slopes of the Nyandarua and Rwenzori mountains . These parks attract many tourists.

Faulting can also cause the formatiom of waterfalls such as the Karuma falls, Murchison falls.

Problems caused by faulting

Escarpments and mountains hinder transport development.

Rift valleys are very hot and only suitable for grazing because they are in the rain shadow unless irrigation is practised as with the case of mubuku.

There is severe soil erosion and mass wasting on the step slopes which result in the destruction of soil surface, crops and at times people's property. For example Rwenzori mountains.

It is difficult to settle on the steep areas on the rift valley escarpments.

Teachers' guide
Scheme of work
Lesson plan
Learners'activities