Structure of Earth

Structure of the earth
When studying plate-tectonics the best starting point is examining the structure of the earth. The earth is very similar to a peach in its structure. In the centre is a solid core. Surrounding the core is the inner core, then the mantle, which is covered in the earths 'skin' or crust.
figure 1. Cross section of the earth (source: Wikipedia) The inner core is the centre of the earth and is the hottest part of the earth. It is a solid mass of iron and nickel. The temperature of the core is around 5500°C
The outer core is the layer around the inner core. It is also made up of iron and nickel though it is in liquid form.
The next layer is the matle.This layer is made up of semi molten rock, known as magma.
The final layer is the earth's crust. This layer is between 0-60km thick.

Continental Drift and Plate Tectonics
In 1912 Alfred Wegener published a theory to explain why the Earth looked like a huge jigsaw. He believed the continents were once joined forming a supercontinent he called Pangaea. Over 180 million years ago this supercontinent began to "break up" due to continental drift.

During the 20th Century, scientists developed the theory of Plate Tectonics. The theory suggested that the crust of the Earth is split up into seven large plates (see map below) and a few smaller ones, all of which are able to slowly move around on the Earth's surface. They float on the semi-molten mantle rocks, and are moved around by convection currents within the very hot rock. See why do plates move? for more details.

The are two types of tectonic plates - continental plates and oceanic plates. Continental plates are lighter (less dense) than oceanic plates. Oceanic crust is much younger in geologic age than continental crust. Continental crust is on average thicker than oceanic crust.
figure 2. The Earth's main plates

Why do plates move?  The earth's tectonic plates are in constantly moving like giant 'rafts' on top of the semi-molten mantle below. However this movement is slow and rates vary from less than 2.5cm /yr to over 15cm/yr. The movement of the earth's crustal plates is believed to be due to convection currents which occur in the semi-molten mantle. These convection currents are created by heat from within the earth - much of which is generated by radioactive decay in the core. So how do convection currents cause plate movements? As semi-molten rock in the mantle is heated it becomes less dense than its surroundings and rises. As it reaches the crust above, it spreads out carrying the plates above with it. As the semi-molten rock then cools, it gradually sinks back down to be re-heated. (see diagram above) Plate Boundaries   The point where two or more plates meet is known as a plate boundary. It is at these locations where earthquakes, volcanoes and fold mountain form. There are four main types of plate boundary. These are constructive, destructive, conservative and collision margins. Plate Boundary Diagram Description Landforms Example Tensional / Constructive (divergent ) plate boundaries Constructive plate boundaries occur when two plates move away from each other. Ocean ridge and volcanic islands North American and Eurasian Plate Compressional / Destructive (subduction zones) plate boundaries Destructive plate boundaries occur when an oceanic plate is forced under (or subducts) a continental plate. Fold Mountains and Oceanic trenches Pacific Plate and the Eurasian Plate Conservative (transform faults) plate boundaries Conservative plate boundaries occur when two plates slide past each other.   North American Plate and the Pacific Plate Collision plate boundaries Collision plate boundaries occur when two continental plates move towards each other. Fold Mountains Indo-Australian and the Eurasian Plate Hotspots You should be aware that whilst most volcanoes / earthquakes occur along plate boundaries, there are exceptions. For example the volcanic Hawaiian islands which can be found in the middle of the Pacific Plate are formed due to a Hotspot. Hotspots are plumes of molten rock which rise underneath a plate causing localised melting and the creation of magma resulting in volcanic activity. See this animation for further explanation of hotspot activity. Key Terms Constructive Boundary (Divergent) - where two plates move away from each other resulting in new crust being formed. Destructive Boundary (Convergent) - where two plates move towards each other - in the case of a plate consisting of continental crust meeting a plate consisting of oceanic crust, the oceanic crust will be subducted and destroyed as it is less dense. Conservative Boundary - where two plates move alongside each other - although crust is neither created or destroyed here, earthquakes usually occur here. Collision Boundary - where two plates of continental crust move towards each other creating fold mountains. Volcano - a vent through which lava, ash etc. is erupted (often, but not always cone-shaped) Earthquake - a sudden movement of the earth's surface
	reference:http://www.geography.learnontheinternet.co.uk/topics/structureofearth.html retrieved on 25th april 2014

EARTH MOVEMENT

Earth movements

Brief description

This unit deals with the formation of features on the earth's surface. The forces, which produce physical features, are of two types: internal forces and external forces.

Earth movements e.g. folding, faulting, down warping and up warping, volcanicity and earthquakes have formed features on the earth's surface.

Earth movements; These are movements which are lateral and vertical, they exert great forces of tension and compression and although they usually take place very slowly they eventually produce very impressive features like the rift valley escarpment in the photograph...

FOLDING AND FAULTING

Introduction:

The forces which produce physical features are of two types i.e. internal forces and external forces.

Internal forces are lateral and vertical forces within the earth surface leading to earthquakes, vulcanicity, faulting and folding.

External forces operate on the earth’s surface leading to denudation , glaciation river action and wave action.

Earth movements have led to the formation of features on the earth's surface of East Africa. Major features such as mountains plateaus, plains, rift valley and others.

The movements are lateral and vertical and exert great forces of tension and compression, though take place very slowly they eventually produce impressive features.

Brief description:

This subtopic describes the two processes of folding and faulting ,the features produced and their importance to human activities.

Lateral forces

        

Earth movements cause sedimentary rocks to be displaced i.e. to be pushed out of the horizontal plane so that the rocks are tilted or inclined.

Earth movements can also cause folding and faulting of the sedimentary rocks. Folding results from lateral forces (compressional forces).
Lateral forces of compression cause folding while either lateral or vertical forces of tension or compression cause faulting.

The process of folding

The nature of folds

The layers of rock which bend up form an up fold or anticline. Those which bend down form a down fold or syncline. The sides of a fold are called the limbs .if compression continues the simple folds are changed first to a symmetrical folds, then into over folds and finally into over thrust folds.

 A simple fold

The centre line of the up fold or down fold is called the axis. In simple folding, the folding process is never intense.

The process gives rise to mountains and valleys. The anticlines became the mountains and synclines the valleys. The sides of a fold are called the limbs.

A symmetrical fold

One limb steeper than the other.

An over fold or recumbent fold

An over fold is formed when one limb is pushed over the other limb. This process occurs when the compressional forces from one side are greater than from the other side.

An over thrust fold

When pressure is very great a fracture occurs in the fold and one limb is pushed forward over the other limb forming an overthrust fold.

In east Africa folding did not cause high mountains as the crust consists of hard basement rocks which fractures when folded.

However minor folds are found at the boarder between Uganda and Tanzania/Karagwe where layers of mudstone were compressed into a U shape.

There is also some folding in the layers of limestone on the East Africa coast. Folding is evident in Bukaba region in the North of Tanzania.

The results of folding(features formed)

Note: Fold Mountains can be found in the North West Africa i.e. Atlas Mountains and in South Africa i.e. Cape Ranges.

Influence of Fold Mountains on human activities

• Fold Mountains often receive heavy rain or heavy snow falls which may give rise to important rivers. For example the Alps in Europe.

NB:The process of folding was not significant in East Africa and therefore didn't create any significant land forms.

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.

 RESULTING OF FAULTING

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.

VULCANICITY

This sub topic describes the processes of Vulcanicity, the features produced and their importance to the people.
              
What causes vulcanicity

Deep inside the earth heat and pressure exerted by the crust cause rocks to melt forming molten rocks known as magma, which is forced to rise along the lines of weakness (fault lines).

Vulcanicity: – involves a process through which molten rock (magma) and gases from interior moves on to the earth surface along lines of weakness (fault lines).

Features formed: – on reaching the surface the magma which becomes lava, cools and solidifies either on the earth surface forming extrusive volcanic features or magma may solidify below the earth ‘s surface forming intrusive volcanic features.

Extrusive and intrusive features. (Bunnet)

Extrusive features

1. Lava - that moves to the surface differs in chemical composition, this party explain the different types or shapes of volcanoes and nature of eruption whether explosions or quite.

Acidic lava: very thick, traps a lot of gas , water, rich in silica content violent eruption and lava solidifies very quickly builds up steep cones.

Basic lava: this is a form of fluid poor in silica content flows for a very distance before it solidifies, it builds up gently sloping cones, lava plateau and plains.

Pyroclasts (Heated lava): when lava is heated, it breaks down into small fragments or pieces (pyroclasts) ,which usually falls to the ground and form layers of Ash/cinder. Layers of ash mix with layers of lava to form composite cones.

2. Volcanoes: These are hills or mountains formed when magma erupt and piles up around the vent until a cone is formed. These are basically four types of volcanic cones.

Types of volcanic cones

Ash/cinder cone:These are small but steep sided hills formed when explosive eruptions throw lava high into the air, breaking up lava into small fragments or pieces known as pyroclastics (fire rocks). The erupted materials build up or accumulate around the opening known as vent; layer after layer forming steep sided cones of about 150 metres high. examples in Kenya are Suswa and Menegai hills and Teleki hills south of Lake Turkana and Longonat. The hills are characterised by a large crater /caldera at the top because of violet eruptions.

Crater- circular depression caldera - an enlargement of a crater on top of
at top of volcano. Mountain as a result of violent eruption.

FORMATION OF A CALDERA

Examples are found on Menegai, Suswa, Longnot in Kenya, Napaka, and Elgon in Uganda. Ngorogoro in Tanzania.

There are also low land explosion craters. explosion craters are flat floor depressions formed when explosions eruptions of gases blow off the rocks at the surface leaving a shallow circular depression, depressions are filled with water to form explosion crater lakes. Examples are; L. Katwe, L. Nyamuruka, L. Munyanyang, L. Saka, and Kyegere and others. Found in western Uganda, in Kasese , Kabarole and Bushenyi districts.

Basic lava cone/shield volcano: This is a hill with gently slopping sides. Its formed when hot fluid lava, with low silica content, flows from one or two faults in a quiet eruption. It spreads out in flat layers. The layers build up a broad volcano with gently sloping sides, shaped like a shield and hence the name shield volcano. Its common in Uganda- Rwanda boarder along the Nyamulagira ranges.

Acid lava dome: These are doom shaped hills, thick lava rich in silica content, solidifies quickly on reaching the surface, forming a volcanic dome or cone of viscous lava, examples – sernal volcano domes in Tsavo national park in Kenya.

Volcanic plug: The thick lava (viscous) lava, at times solidifies in the vent, after prolonged erosion, the plug is exposed to the surface examples are the Tororo rocks in Eastern Uganda.

A composite cone: A composite cones are large mountains formed when volcanic eruptions alternate between quite and violent eruptions. An exposure period releases gas and ash and cinder layers. Then the eruption changes to a quite period erupting lava over the top of ash layer, when the cycle of ash and lave is repeated over and over in alternating layers, a composite volcano is formed.
At times lava is diverted side ways from the vent forming dykes, Corrects or parasite cone .Examples are  Mt. Kenya, Kilimanjaro in Tanzania, Mahambura in  South west Uganda.



Lava plateau: formed as a result of quite eruptions, when lava moves out slowly along cracks /faults .the lava fluid lava flows for a very long times on the plateau before it solidifies, forming lava plateau.

Examples are; Kisoro lava plain in Uganda, Yatta, Laikipia and Turkana plateau in Kenya.

Hot springs /geyser: These are features produced when heated water in volcanic areas flow out queitely in form of hot springs or erupt periodically shooting out water /steam in the air.

Examples – Kitagata in Bushenyi.

                    Sempaya in fortportal

                    Majimoto in Tanzania

                    Majiyamot in Kenya

kitagata hotspring in Bushenyi

Lava dammed lakes: These are formed when lava blocks channels. Water pools behind leading to formation of lakes. Examples L.Bunyonyi and L. Mutande in southwest Uganda.

Intrusive features:

These are formed when magma solidifies underground. Magma forms in many different shapes and sizes, the most common are: -

1. Batholiths: these are mostly massive rocks. They are dome shaped, formed at great depth and at times they are exposed at the surface by denudational forces as inselbergs. Examples are mumbende hills, parabong and labwa hills in Acholi. Mumias and Kisii batholith in Kenya.

2. Dykes: formed when magma solidifies into vertical cracks, cutting across rock layers. When affected by erosion dykes may stand as a ridge. There are ridges in most of Turkana and Sukulu hills South of Tororo town.

3.  Sills: formed when lava solidifies in between rock layers, after prolonged erosion, Sills may be exposed as escarpments and while they occur across a river valley they cause water falls and rapids. Examples are Thika and Thompson falls in Kenya, Karuma falls and Bujagali in Uganda.

4. Laccolith: this is magma which solidifies in a shape similar to a mushroom. After prolonged erosion, it may form upland.

5. Lappolith: This is a sauce shaped magma, after erosion. It can be exposed as a shallow basin. Examples are Arenas in Ankole.

Advantages of Vulcanicity

1. Volcanic mountains and lava plateau provide fertile volcanic soils which support agriculture, e.g. Arabic coffee is grown on mountain Elgon, Mufumbiro, Kenya and Kilimanjaro, coffee earns the country foreign currency and provides employment, other crops grown include wheat, tea, pyrethrum, maize, bananas, vegetables, irish potatoes.

2.  The highland areas are densely settled. This is due to the fertile soils and cool climate, e.g. Bugishu, Kigezi, Kenya and Kilimanjaro highlands. In addition some of these areas have large towns like Kabale, Mbale, Moshi, and Nairobi. This has led to development of commercial activities.

3. The volcanic features especially mountains are tourist attractions. They provide sporting activities like mountain climbing. They generate income in form of foreign exchange and provide employment to local people.

4. Volcanic mountains influence climate, lead to formation of geographic or relief rainfall which is important for agriculture. Some mountains are ice capped. They are a source of many rivers, which provide water for domestic use and generate hydroelectric power.

5. There are forest reserves on the slopes of mountains like Elgon, Mufumbiro Kenya, and Kilimanjaro which are valuable source of timber and firewood. The forests also act as wildlife conservation areas e.g. Bwindi impenetrable forests, has the largest population of gorillas, which promote tourism.

6. Lava or magma is rich in minerals e.g. Tororo rock volcanic plug is a source of limestone for the cement industry. Iron,tin uranium are found in the Mufumbiro ranges.


Lake Katwe which is an explosion crater is a centre for salt mining. The Kimberlite rock in Tanzania is centre for gold mining. Minerals provide revenue and employment.

7. Hot springs or Geysers are potential source of Geothermal power (electricity).in Kenya ,the Olkaria Geothermal Power Station near lake Naivasha in Kenya, generates electricity.

8. There is fishing in lava-damned lakes, which provides food and employment.

The intrusive features Batholiths, dyke, sills, laccoliths and lappoliths once exposed to the surface as inselbergs have the following advantages;

• Good sites for quarrying. They are sources of stones used for construction.
  
  
• Sills and Dykes once crossed by rivers create waterfalls which are good for hydro power generation.
  
  
  
• They are tourist attractions.

Disadvantages of Vulcanicity

• Volcanic features especially mountains are communication barriers due to steepness.

• It is very expensive and risky to construct roads and railways in the hilly areas.

• Volcanic eruption leads to loss of lives and property.
  
  
• Heavy rainfall and steepness lead to soil erosion, mass wasting and landslides which are common in Kigezi and Elgon areas.

• Mountains act as barriers to rainfall especially on the leeward side (rain shadow areas) this causes aridity.
  
  
• Intrusive features like sills and dykes form waterfalls and rapids which hinder navigation of rivers.

• Where there are Inselbergs and Batholiths make agriculture practising difficult
• reference: http://www.elateafrica.org/elate/geography/earthmovements/earthmovementsintro.html retrieved on 25th april 2014