GEOGRAPHY FORM 3: TOPIC 2 - FORCES THAT AFFECT THE EARTH

FORCES THAT AFFECT THE EART  (Geomorphic Processes)

The earth can be affected by two forces which may result into various land forms. Forces that act on the earth can be grouped into internal and external forces.

 

Internal forces (endogenic forces)

These are forces which operate within the earth’s crust. Internally forces include vulcanicity, earthquakes ,faulting and earth movement which is accompanied by horizontal (lateral) and vertical (radial) movements. These forces may result into formation of several land forms such as mountains and valleys.

 

External forces (exogenic forces)

These are natural forces that operate on the earth’s surface. The forces mainly act on the earth’s crust or close the surface of the earth, such as Weathering, Mass Wasting and erosion, transport and deposition due to action of wind in the desert, river, ice and wave action along the coast. Often the features produced by these forces can be seen on the surface of the earth.

 

INTERNAL FORCES

 

Can categorized into two

 

(i)                 Earth movement

(ii)              Vulcanicity and earthquakes

 

Earth movements(diastrophism)

These are movements that occur within the rocks of the earth crust (lithosphere). They are caused by internal forces, which are also called as tectonics forces. These forces causes the earth to move either vertically or horizontally and form various landform such as faults, folds, plateaus, valleys and mountains.

 

Earth movement can be classified into two

(i)                 Vertical or Radial movement(orogenic)

(ii)              Horizontal movement(epeirogenic)

 

Two types of earth movements

 

Vertical or radial movements(orogenic)

These are upward(uplift) and down ward(warping) movements which cause the crustal rocks to be uplifted or move downward.

 

Resulting features from vertical movement

 

Uplift of the land may cause

 

1.      Plateaus

2.      Block mountains(horst)

3.      Emerged coast (raised beach, raised cliffs)

 

Subsidence (down warping) cause

1.      Basins e.g. lake Victoria, lake Chad

2.      A rift valley

3.      Submerged coasts (rias, fiords, longitudinal coast, estuarine)

 

Draw the diagrams to show how vertical and horizontal movement operates

 

 

Horizontal or lateral movements (epeirogenic)

 

These are side way movements caused by tension and compression forces operating within the earth’s crust.

Horizontal movement can be caused by

(i)                 Tension forces

(ii)              Compression forces

 

Types of forces causing horizontal movements

 

Tension forces

These are forces which cause the crustal rocks to move away from each other. They cause tension within the earth’s crust.

 

Compression forces

These are the internal forces which cause the crustal rock to move towards each other. They cause the crustal rocks to be squeezed. The crustal rocks fold or even crack.

 

Shear / tear forces

These are internal forces which causes the crustal rocks to move in opposite directions past each other.

                                 Various processes results from earth  movements

 

Folding

Is the bending or wrinkling of the crustal rocks caused by compressional forces. These forces cause the rocks to bend upward or down ward. It occurs in sedimentary rocks which are fairly young.(flexible).

 

Meaning of some terms

                       i.             Anticline is an up fold which bend upwards

                     ii.            Syncline is a down fold which bend downwards

                  iii.             Crest is the upper most part (summit) of the anticline.

                   iv.             Trough is the lowest –part of the syncline

                     v.            Axis is a line drawn vertically through the Centre of the anticline.

                   vi.             Limbs are the slopes of a fold

                      

                                 

Types of folds

 

Simple (symmetrical) folds

Are types of fold whereby both limbs are equal in steepness. They are formed by weak forces. They cause the rock to bend evenly.

Asymmetrical folds

 

Are types of folds where one limb is steeper than the other. They are formed by compressional forces of unequal strength. One force is stronger than the other resulting to a fold with one limb steeper than the other.

Overfold

Is a type of fold where one limb is pushed over the other.The anticline is pushed over the limb of the next fold.

 

Ø  Draw a diagram  to show overfold.

 

 

Features produced by folding

 

Rolling planes

These are very slight anticlines and synclines. They are formed when very weak compressional forces act on a land turning it into a gently  sloping anticlines and very wide synclines.

 

Ø  Draw a diagram to indicate Rolling planes.

 

Fold mountains

 

These are the most outstanding features resulting from folding. They form the world’s highest mountains

Examples

v  Atlas mountains in North Africa

v  The cape ranges of South Africa

v  The Himalayas of Asia

v  The  Alps in Europe

v  The Appalachians in North America

v  The Rockies in North America

v  The Andes in South America

 

Intermontane plateaus

 

Within some parts of fold mountains, there are areas that resisted folding while the rocks at their edges were folded. These unaffected areas are called intermontane plateaus.

 

Draw a diagram to show intermontane plateaus

 

 

Intermontane basins

These are the basins found between the fold mountains

 

Draw a diagram to illustrate intermontane basins

 

 

Effects of folding

1.      Fold mountains modify the climate of an area. Wind ward side receives heavy orographic rainfall while the Lee ward (rain shadow) side receives very little or no rainfall at all.

2.      Due to heavy rainfall on the wind ward side, heavy and thick forests are formed which support lumbering.

3.      Catchment areas for most rivers. Heavy rains and snow in these area, give rise to rivers that provides water for H.E.P generation, irrigation and other uses

4.      Expose valuable minerals near the earth’s surface. This makes them easily available for mining.

5.      Tourists attraction. The fold mountain landscape sometimes covered by snow provides a beautiful scenery which is an attraction to tourists who bring in foreign exchange. For example the Swiss Alps.

6.      Discourage settlement. This is due to rugged topography of fold mountains.

7.      Mountains are barrier to transport and communication. This makes difficult to construct transport routes.

 

 

 

Faulting

Is the breaking or fracturing of rock due to stress caused by tectonic forces. Faulting can be due to compressional or tensional forces. This process results in formation of faults.

GUIDING QUESTION

 

What is the difference between faults and joints

 

Terms associated with a fault

1. Down thrown is a land that is displaced downwards.
2. Up thrown: Is a land that is displaced upward
3. Fault line: Is a position a fault that can be traced on the surface of the earth when the displacement is horizontally.
4. Fault plane: Is a surface of separation of the land created by a fault.
5. Heave: Lateral / horizontal displacement
6. Hade: Inclination of the fault to the vertical plane displacement.

Draw a diagram to indicate the terms associated with faulting

 

 

Types of faults

Normal faults

These are faults caused by tensional forces

Reverse faults

These are faults cause by compressional forces.

 

Shear / tear faults

These are faults caused by horizontal forces that move in opposite directions within the earth’s crust.

 

Illustrate the diagram above

 

Features resulting from faulting

A fault scarp or escarpment

This is the fault which has steep slope on one side and gentle slope on another side. It occurs when vertical displacement has taken place. For example: Lake Manyara scarp, West of lake Manyara in Tanzania.

Block mountains (horst)

Refers to a table like mountains formed due to the influence of faulting that leads to rising of crustal rocks. A block mountain can be formed by either tensional or compressional forces. This is when the earth’s movements cause parallel faults which results into uplifting of some parts.

 

                     Block mountains (a horst)

 

 

Draw the diagrams to show the formation of block mountains by tensional forces (a horst)

Formation of block mountains by compression forces (a horst)

 

 

Rift valley

A valley is a depression or an elongated depression which is formed on the earth’s crust. Rift valley is a trough or hollow which may result from both vertical and lateral movements of the earth’s crust. It is formed when two faults develop parallel to each other. It is formed either by tensional forces or compressional forces.

 

Illustrate the diagram of Rift valley

Formation or rift valley by tensional force

 

This is formed when tensional forces moves away from each other. These forces of tension produce faults and the central block between the two parallel faults subsides to form a rift valley.

 

Formation of rift valley by tension forces

 

Examples of rift valleys

1.  The great rift valley (East African Rift valley) which starts in Syria in the middle East and passes through Ethiopia; East Africa and into Mozambique.

2.      The Benne rift valley of West Africa

3.      The Rhine rift valley Central Europe

4.      The Baikal rift valley in Southern Siberia

 

Draw a sketch map to show the East African Rift Valley.

 

Plateau

Are the slight raised highlands with extensive flat or undulating top surface.

Basin

Are plate like depression which are shallow and broad. They are formed as a results of down warping.eg Victoria basin.

 

Effects of faulting

        i.            Disruption of transport lines such as roads, railways, pipelines, power transmission lines, telephone line. This is due to the fact that faulting causes parts of the land to be disconnected.

      ii.            Subsidence of the land can lead to destruction of properties and loss of lives.

    iii.            Causes the change of a river course. Rivers may start flowing along the line of fault.

    iv.            Formation of lakes. Depression may occur at the floor of the valley. When these depressions are filled with water they form lakes e.g. lake Tanganyika (the deepest lake), lake Nyasa, Eyasi, Turkana,Edward, Albert, Baikal of Russia the world’s deepest lake.

      v.            Appearance of hot springs and geysers and other volcanic features.

    vi.            Stimulate the development of tourism industry. The rift valley and other features resulting from faulting are attractive to look at. It is a source of foreign exchange.

  vii.            Expose valuable minerals on the scarp slopes for easy extraction.

viii.            Block mountains modify the climate of an area by bringing orographic rainfall on the windward side. This makes these slopes favourable for agriculture and settlement.

    ix.            Block mountains are catchment areas where most rivers have their sources. The rivers provides water for irrigation and generation of hydro electric power.

 

 

Vulcanicity

 

Vulcanicity is the process by which solid, liquid or gaseous materials are forced out from the interior of the earth into the earth’s crust or onto the surface of the earth.

 

This process operates in the interior of the earth where these materials are under very high temperature and great pressure. These materials are known as molten materials.

 

The difference between vulcanicity and volcanicity

 

Volcanicity refers to all forms of igneous activities involving materials that reach the earth’s surface. It is also known as extrusive vulcanicity.

 

Intrusive vulcanicity refers to igneous activity in which materials do not reach the earth’s surface but remain within the rocks of the earth’s crust. It is also termed as “PLUTONIC”.

 

 

 

Origin of vulcanicity

 

Ø  Inside the earth at depth of 16km or more, rocks are under very high temperature and pressure. Some rocks melts to form molten rock materials called magma. When these molten rock materials come out of the ground and reaches the earth’s surface, is called lava.

 

Ø  Magma and lava may be viscous (very thick) or less viscous (very fluid) depending on its chemical composition (amount of silica and basic oxides). Magma escapes from the interior through:

 

1.      Vents (vent eruption) are holes through which magma comes out of the ground onto the surface of the earth.

2.      Fissures (fissure eruption) are the cracks or faults through which magma comes out of the ground onto the earth’s surface.

 

Types of vulcanicity

 

These are two types

(i)                 Intrusive Vulcanicity

(ii)               Extrusive Vulcanicity

 

Intrusive (internal) vulcanicity

 

Ø  This occurs when the magma cools, solidifies and forms features within the earth’s crust before it reaches the earth’s surface. The features (land forms) formed by this process are sometimes termed as intrusive (internal) features.

 

Ø  Those intrusive features which are formed very deep underground, are called “plutonic” features. Those intrusive features which are formed underground but near the surface (shallow depth) are called “Hypabbysal features”.

                                                                                                                      

 

 

Features (land forms) formed due to intrusive vulcanicity

 

Dyke:

 Is a wall like feature cutting across the bedding planes. It is formed when magma cools and solidifies vertically across the bedding planes. Examples are Mwadui dyke one of the sources of diamond in Tanzania

 

 

Sill

Is an intrusive feature which lies horizontally along the bedding planes. It is formed when magma cools and solidifies horizontally along a bedding plane. It is a small scale intrusive feature.

Lacolith

Is an intrusive feature which looks like a dome. It is formed when the magma cools and solidifies in anticline bedding place. Sometimes, it can be exposed to the earth’s surface following denudation processes. It is also called as laccolite.

 

 

 

Lapolith

 Is an intrusive feature which looks like a saucer in shape. It is formed when magma (molten rocks) cools and solidifies in a syncline bedding plane.

Batholith

Is a very large mass of magma which cools and solidifies in the earth’s crust. Sometimes it forms the root or core of a mountain. Batholiths are made of granite and they form surface features only after they have been exposed to the earth’s surface by denudation. Sometimes batholiths resist erosion and form uplands.Examples of batholiths are found in Zimbabwe, Tanzania, Zambia and the Chailu Massif in Gabon.

 

Phacolith

Is a lens – shaped mass of igneous rock. It is formed when magma cools and solidifies at anticline and syncline in folded rocks. Example is the Gordon hill in UK.(United Kingdom)

Extrusive vulcanicity (external)

 Is the cooling and solidification of molten materials called on the earth’s surface which form various extrusive land forms like volcano.

When the molten rock materials (igneous materials) reaches the earth’s surface, are called lava.

 

Volcano

Is a cone shaped hill or mountain formed when lava and pyroclastic accumulates on the ground.

 

Types of lava cones

There are two types of lava cones

(i)                 Acidic lava cone

(ii)              Basic lava cone

 

 Acidic lava

 Usually forms volcanoes with very steep sides and narrow base.

 

 Basic lava

 This forms volcanoes with gentle slopes and a broad base

 

Acidic lava cone

This is a cone made of viscous lava. Normally lava cones have high height and break into small fragments. Acidic lava cools faster than basic lava because it is viscous.

Basic lava cone (shield volcano)

This is a volcano with a broad base and relatively gentle slope. They are lower in height. They are made up of basic lava. Eruptions are less violent because the magma is very fluid. On reaching the surface, lava flows in all directions and spread far before it cools and solidifies. Examples are Mauna Loa of Hawaii and basaltic dome of Nyamlangira near lake Kivu in DRC.

 

 

 

Ash and cinder cone

This is a cone made up of ashes and stones that erupted from beneath (interior) the earth to form a concave cone. The slopes of a cone are usually concave due to the spreading tendency.

 

Composite volcano (strata –volcano or complex cone)

It is a volcanic cone which is made of alternating layers of lava and ash. It has small cones on the slopes of the main volcano. They are called as parasitic cones or side cones or cone lets.

Examples of composite volcanoes includes: Mount Kilimanjaro, Meru and Oldonyo Lengai in Tanzania; mount Kenya, Longonot, Suzwa and Elgon in Kenya.

 

Volcanic plug (plug dome / spine)

This is a big rock which blocks the top of the pipe. It is formed when lava solidifies quickly to block the pipe or vent. Examples are mount palace in France; Hoggar mountains in Algeria.

 

Lava plateaus

Is an extensive and flat land form which is formed when molten magma flows onto the earth’s crust through fissure. Examples are found in Ethiopia highlands, Bihe plateau in Nigeria and Deccan plateau in India.

 

Draw a diagram of lava plateau

 

 

Crater

Is a small depression on the volcanic cone or mountain. It is sometimes filled with water to form a crater lake(Ribbon lake). It is formed when volcanic eruption ceases and leaves a hole on the basic lava cone. Examples are: Kibo peak of mount Kilimanjaro.

      

Caldera

Is a large depression on top of a volcanic cone. It is formed when a strong and violent eruption blow off the top part of a volcano resulting in a much wider crater whose edge is at a lower altitude than the original crater (explosion caldera). Some caldera are formed as a result of block subsidence (collapse caldera). Examples of caldera are Ngorongoro in northern Tanzania and Vesuvius in Italy etc

 

Hot springs (thermal springs)

Is a natural outflow of super heated water onto the earth’s surface from underground. If there is a depression where water collect, they form pool of boiling water.

 

Geysers / steam jets (natural fountains)

Is a jet of hot water or steam ejected explosively from the holes or fissures in the ground.

 

Fumaroles

Are holes in the ground through which steam or gases are emitted. If the gases are mainly carbon dioxide, the hole is called a mofette. If the gas is composed of sulphurous compounds, the hole is called a solfatara.

 

 

Types of volcanoes

There are three types of volcanoes

 

Types of volcanoes

        i.            Active volcano

      ii.            Dormant volcano

    iii.            Extinct volcano

 

 

Active volcano

Is the volcanic mountain which is still experiencing periodic eruption from time to time.Example Vesuvius(Italy),Krakatoa(Indonesia),Mufumbiro(Uganda),OldonyoLengai,Nyiragongo,Mauna Loa, and Mt Cameroon

 

Dormant volcano

Is a volcano which has erupted once in the past but have remained inactive for fairly long time. Example Kilimanjaro and Mount Meru.

 

 

Extinct / dead volcano

Is a volcano which has not erupted for very longtime and has not shown any sign of eruption

Kenya ,Elgon,Ngorongoro and mount  Rungwe.

 

Active	Dormant	Extinct
Nyamlagira  (2010) Congo DRC Nyiragongo  (2009) DRC Oldoinyo Lengai (2007) Tanzania Meru (1910) Tanzania  Cameroon – Cameroon Krakatoa– Indonesia Mauna Loa Hawaii	Kilimanjaro – Tanzania Mount Kenya	Ngorongoro Tanzania Rungwe Tanzania Mt. Kenya – Kenya Elgon – Kenya Muhavura – Uganda Mauna Loa Hawaii

 

 

Distribution of major volcanic zones in the world

Most volcanic areas are found in continents bordering the Pacific Ocean, an area referred to as “Ring of Fire”. The ring of fire is the name of the area around the Pacific ocean where many of the world's volcanoes are found besides volcanoes, there are also more earth quakes in ring of fire than the rest of the world. Many islands like the Hawaiian islands are formed from volcanoes.

 

 

 

A world map showing distribution of volcanoes

 

Economic importance of volcanoes

1.      Formation of fertile volcanic soil. Weathering of the lava poured upon the earth’s surface helps the formation of soil which helps in agriculture and forestry. Examples are the soils on the slopes of mount Kilimanjaro, which support the growth of coffee, banana, tea and other crops.

2.      Materials for construction activities. When magma solidifies, it forms hard rocks that can be quarried and used to construct roads, bridges, houses.

3.      Development of tourism industry. This earns foreign currency to the country. This is because of the spectacular features formed due to volcanicity such as mountains, crater,caldera, lakes, geysers and hot springs.

4.      Vulcanicity brings minerals from deep the earth’s crust to close or onto the earth’s surface. Various minerals and gemstones are mainly found in the volcanic regions  diamond in Mwadui is mined from volcanic plugs and dykes. Gold and silver are associated with batholiths.

5.      Geysers can be harnessed to generate geothermal electricity.

6.      Hot water from hot springs is pumped into homes during winter to heat up homes. This is done in cold countries like Iceland and New Zealand.

7.      People use hot springs and pools of hot water as medicine. They bathe in the water for the purpose of curing certain diseases.

8.      Some crater lakes are source of salts and other minerals while others support fishing activities, for example lake Chala. Some lakes are used as a source of fresh water for domestic and industrial activities.

9.      the climate of an area. The windward side receives heavy orographic rainfall which encourages settlements and farming.

 

EARTHQUAKE

Refer to the sudden shaking or vibrations of the earth’s crust. This is due to sudden and rapid displacement of tectonic plates.

 

Occurrence of earthquake

The majority of earthquakes occur in narrow belt which mark the boundaries of tectonic plates.The main types of regions where they occur are

 

1.      The mid ocean ridges

2.      The ocean deeps and volcanic islands

3.      Regions of crustal compression

 

Nature of the earthquakes

The point at which an earthquakes originates is called focus, and sometimes it is several kilometres below the surface. The point on the earth’s surface immediately above the focus is called the the epicenter.

The epicenter is the point where the shock waves first hit the surface. It is the shock waves which give rise to an earthquakes.

 

                                                 

 

 

Types of earthquake waves

 

Earthquake waves are also known as seismic waves.

There are two main types of earthquake waves

1.      Body waves

2.      Surface waves

 

 

BODY WAVES

Are seismic waves which are travelling within the rocks before they reach the earth’s surface.

 

 

Two types of body waves

 

A.    Primary waves (p –waves) these are very strong body waves which travel very fast and through any state of rock.

            

B.     Secondary (s – waves) these are the weak body waves that travels slowly and only through rocks which are solid in nature               

 

 

 

 

SURFACE WAVES

 

These are the earthquake waves that operate on the earth’s surface. These are the earthquake waves that cause destruction on the earth’s surface

 

 

 

Types of surface waves

1.      Love (L) waves

2.      Rayleigh

How can earthquake be detected

 

Earthquakes is detected and measured by using a seismograph. Seismograph has a sensitive instrument which record seismic waves called seismometer. Seismometer records the impulses on a graph like record sheet called a seismogram. It is from the seismogram that reading and interpretation of seismic waves are made. The strength of an earthquake is usually measured by its magnitude.

 

Magnitude refers to the total amount of energy realized by an earthquake. It is measured by a richter scale. This scale ranges from 0 to 8.9 . The higher the magnitude value the stronger the earthquake.

 

Intensity refers to the effects produced by the earthquake. Intensity vary from one place to another. While intensity of an earthquake varies, its magnitude does not vary. Intensity is measured by an instrument known as mercalli scale. It ranges from undetectable, moderate, strong to major catastrophe.

 

Causes of earthquakes

1.      Faulting of the lithosphere caused by tectonic movement where one plate slides over another plate.

2.      Vulcanism can cause occurrence of the earthquake. This is due to the fact that the magma moves under the influence of intense pressure from within the earth’s interior.

3.      Mass wasting like land slide and rock fall can cause occurrence of earthquakes, but this is for local scale.

4.      Falling objects from the atmosphere such as meteorites may lead to the shaking of the earth’s crust.

5.      Man’s influence through his activities such as mining using explosive like dynamites, transport vessels like trains and heavy trucks.

6.      Construction of large water reservoirs. The eight of water in the reservoirs and increased fluid pressure in the rocks can reactivate inactive older faults by increasing strains and causing earthquakes.

 

Effects of earthquakes

1.      Displace parts of the earth’s crust either vertically or horizontally.

2.      Raise or lower parts of the ocean floor. The Agadir earthquake in Morocco in 1960 raised the sea floor off the coast from 400metres to 15metres after the earthquake.

3.      Raise or lower coastal rocks for instance the Alaskan earthquake of 1899 raised some of coastal rocks by 16metres.

4.      Landslides and open up deep cracks in the surface rocks. Good example is the El Asnam earthquakes in Algeria of 1954,destroyed an area of radius 40km and opened surface cracks up to 3metres deep.

 

5.      Loss of lives and properties like buildings, roads, railways and bridges.

 

External forces that affects the earth

 

External forces are the forces that operates on the earth’s surface. They are responsible for destruction, modification and formation of land forms.

 

They all lead to external land forming processes. Denudation is derived from a Latin word “Denudare” which means “to lay bare”. Denudation is the destruction, wastage and removal (wear away) of parts of the earth’s surface

 

Processes of denudation

Ø  Weathering

Ø  Mass wasting

Ø  Erosion

Ø  Transportation

Ø  Deposition

 

Weathering

Refers to the process of breaking and disintegration of rocks that exposed to the agents of weather like temperature and rain.The process of weathering prepares the rock materials for transportation by the agents of erosion.

 

Types of weathering

1.      Mechanical or physical weathering

2.      Chemical weathering

3.      Biological weathering

 

 Mechanical weathering

Is the disintegration of rock into small particles by mechanical processes without involving chemical changes.

 

Causes of mechanical weathering

1.      Temperature change

2.      Frost action

3.      Pressure release

4.      Rain water

5.      Crystal growth

 

Temperature changes

The changes temperature especially in arid areas has led to rock disintegration and formation of various landforms.

Usually arid areas experience high temperature during the day and low temperature during the nights which lead the rock surfaces to alternate heating during the day and cooling during the night.

This causes the outer parts of the rocks to expand during the day and contract during the nights, this sets up powerful internal stresses in the upper parts of the rock hence crack appears and cause the outer layer of rock to pull away.(peel off).

 

 

1. Exfoliation or spalling or onion weathering

Is the peeling off of the outer layer of the rock like an onion. It is common in arid or hot climates. When an arid area experiences high temperature during the day and low temperature during the night rock surfaces alternately heat and cool.

 

This causes the outer parts of the rocks to expand during the day and the contract during the night. When this happens repeatedly over time, shells of rock peel off from the outer part of the rock. Exfoliation leads to the formation of exfoliation domes (rounded rock masses and talus or screes (small rock particles).

 

There are numerous exfoliation domes in the Egyptian, Sinai and Kalahari deserts.

 

    

 

2. Granular disintegration

Is the breaking up of rock which consists of different minerals. These minerals expand and contract separately through temperature changes.

 

Put a diagram relevant to the point above

 

3. Block disintegration (block separation)

It takes place when the rock with homogeneous rock breaks into rectangular blocks due to changes in temperature. This is common when the rock is well jointed. This process can be aided by chemical weathering.

 

Frost action (frost shattering)

Is the action of ice on rocks. It is common in very cold areas(temperate),highlands and deserts regions where water freezes. When temperature falls the water in the crevices(cracks) of rocks freezes. When water freezes its volume increases and expands causing widening of the cracks and joints.

When temperature rises, the ice melts and water enters deeper into the cracks. With repeated freezing and thawing(melting) the rocks breaks into fragments. Eventually the whole rock is broken into blocks.

 

 

How does freeze-thaw take place

        

Pressure release

It is also known as unloading. This is weathering of rocks that were at one time buried deep below the surface but denudation has exposed the rocks on the surface. The upper part of the exposed rocks expands due to reduction in pressure. Eventually shells of rocks break off from the parent rock beneath. It leads to the formation of exfoliation domes and granite tors.

 

Put a diagram relevant to the point above

 

Alternate Wetting and Drying (Rainwater) or (slacking)

Some rocks expand after absorbing water and contract when they become dry. Repeated wetting and drying leads to the formation of cracks in the rock and hence it becomes weak such that it can easily be broken into smaller fragments.

 

 

Put a diagram relevant to the point above

 

Crystal growth (crystallization)

It happens when salt crystals are deposited in the cracks or pores during evaporation. As the deposition goes on the crystals become larger and exert stresses upon the rock causing it to disintegrate. The process is common in hot desert, and particularly along the coast.

 

Chemical weathering

Is the process by which rocks are decomposed, dissolved or loosened by chemical processes hence rock become weak and easily broken. Chemical weathering involves the following:

       

Processes of chemical weathering

 

1. Solution

Is the process in which some soluble minerals simply dissolve in water like rock salt which leads to its disappearance.

 The resulting features are:

        i.            Clint – sharp ridges

      ii.            Grikes – the grooves

 

Put a diagram relevant to the point above

 

2. Carbonation

Is the process whereby carbon dioxide in the atmosphere dissolves in rain water, when it rains to form weak carbonic acid. The weak carbonic acid can change insoluble minerals in rocks such as calcium carbonate to soluble minerals in rock such as calcium bicarbonate to be dissolved in water and washed away down through the soil.

   

             

 

3. Hydrolysis

Is the process involves hydrogen(in water) combining with certain metal ions(in a mineral) which is, the water and the mineral react chemically which give rise to the formation of different chemical compound which is easily broken eg feldspar is broken in this way.

 

4. Oxidation

Is the reaction between rocks containing iron and oxygen in the presence of water. The ferrous state of rocks changes into ferric state forming a yellow or brown crust which is easily broken (crumbled).

5. Hydration

Is the process whereby some minerals rocks absorb water and swell, hence become loose and break easily. Example haematite change to form limonite.

 

 

Biological weathering

Is the type of weathering which is caused by either plants or animals, can be physical or chemical weathering.

 

Ø  Plant roots can grow into rock joints hence forcing the rock to break apart.

Ø  People contribute much to the weathering of rocks through farming; quarrying; mining; construction.

Ø  Burrowing animals and birds give out acidic materials which break the rock surfaces.

Ø  Micro organisms such as bacteria produce chemical materials which lead to rock break – up.

Importance of weathering

1.      Formation of soil. Rock particles are raw materials for soil formation.

2.      Produces new substances such as clay that is used in pottery and brick making.

3.      Weaken the rocks and make it easy for people to exploit them e.g. by quarrying

4.      Accelerates soil erosion

5.      Formation of fascinating features which attracts tourists who bring foreign currency.

Example, exfoliation domes.

 

Mass wasting / movement

Is the movement of weathered materials down the slope under the influence gravity. Is the movement of rock materials produced by the agents of denudation (weathering and erosion) under the influence of gravitational force.

 

Factors influencing type and speed of mass wasting

1.      The nature of the material.

A weak rock having loose materials tend to move fast, except where the materials is stable, also saturated materials move fast.

2.      The extent of saturation.

 Saturated materials move easily than dry materials. Water increases the weight of the materials also water acts as a lubricant along the bedding planes which facilitates movement.

3.      The angle of slope.

 The steeper the slope, the faster the movement of materials. Movement is slow on gentle slope. On flat region there is no movement completely.

4.      Climate.

 Areas receiving heavy rainfall experience massive mass wasting such as landslides especially rock fall on steep slopes. In cold regions, alternate freezing and thawing encourages mass wasting

5.      Vegetation

Plant cover helps to hold materials such as soil together and therefore reducing the movement on the surface. But dense forests facilitates much water to enter the ground. This makes the rock materials saturated which ease mass movement.

6.      Human activities

             Various human activities such as cultivation, construction, mining, clearing of vegetation,  

             grazing animals facilitates mass wasting.

7.      Tectonic movements

            Volcanic eruptions and earthquakes accelerate mass wasting.

 

Types of mass wasting

1.      Slow mass wasting

2.      Rapid mass wasting

 

Slow mass wasting

Is a slow but steady movement of rock debris and soil down a slope. Sometimes the movement is so slow that it cannot be detected easily.

 

 

Four processes of slow mass wasting

1.      Soil creep

2.      Talus creep

3.      Rock creep

4.      Solifluction

 

 

Soil creep

Is a very slow movement involving soil and other fine materials along a very gentle uniform slope. It covers a wide area.

 

 

 

 

 

Factors influencing soil creep

 

1.      Alternate heating and cooling of soil particles

2.      Movement of animals.

3.      Mining activities

4.      Wetting and drying of soil.

Evidence of soil creep

1.      Fence posts and telegraph poles appear inclined down a slope and pushed into new position

2.      Stone walls built across a slope appear to bulge down slope or may even be broken.

3.      Upper slopes are left bare when soil particles move down the hill.

4.      At the base of a slope, there appears an accumulation of soil. This can be seen on roadsides.

 

Talus creep

Is the mass of broken rock particles and rock pieces that accumulate at the base of a rock mass such as a cliff. This is a very slow movement of talus (scree) down a slope.

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Rock creep

Is a slow movement of individual rock blocks down a slope.

 

 

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Solifluction

Is the movement of a mixture of soil, gravel and weathered rock which is frozen. During winter, the soil is frozen. During spring the top soil begin to thaw (melt) causing it to move down a slope. The subsoil remains permanent frozen (permafrost).

 

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Rapid mass wasting

Is a sudden and fast movement of loose weathered materials down a slope.

 

Processes of rapid mass wasting

1.      Earth flow

2.      Mud flow

3.      Land slides

4.      Slump

5.      Debris slide

6.      Rock slide

7.      Rock fall

8.      Debris fall

9.      Avalanche

 

1. Earth flow

Is a rapid movement of saturated earth. It occurs in humid regions on hillsides.

 

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2. Mud flow

Is the flow of large quantities of mud down moderate and steep slopes. Mud flow are more fluid  than earth flow. An earth flow can turn into a mud flow when there is an increase in rainfall.

 

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3. Land slides

It takes place when large quantities of loosened surface rocks and soil suddenly slide down a steep slope such as a cliff face, a valley side. Land slides are caused by the lubricating action of water and the pull of gravity.

 

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Forms of land slides

 

A. Slump

Is a movement of large mass of soil, rock and other loose materials down a steep slope. It occurs when sedimentary rock strata overlie weaker rocks such as shale or clay. The weaker rocks beneath then becomes saturated with water. Erosion at the base of a cliff can also results in a slump.

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B. Debris slide

Is a rapid movement of a whole mass of accumulated rock debris produced by weathering down a hill slope.

                       

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D. Rock slide

This is a movement of individual rock masses.

 

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D. Rock fall

Is the most rapid form of mass wasting involving free falling of individual rock blocks or boulders from a steep slope or vertical slope. Very common in areas of heavy rainfall like Lushoto.

 

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Land Slide

                    

 

E. Debris fall

Is a free falling of loose materials or debris of various sizes over a very steep (vertical)slope.

 

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F. Avalanche

Is a sudden sliding and falling of a large mass of snow, ice and loose rock materials down a mountain side. It is very common in temperate (cold) region.

 

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Effects of mass wasting

1.      Formation of scars and barren land.

Scar is a barren piece of land that vegetation and the top soil have been removed.

2.      Soil erosion.

Mass wasting cause the soil to be removed from highland to lowland.

3.      Formation of new land forms.

 At the bottom or base of a slope, materials accumulate to form hill rocks. At the top of a slope, there is a formation of scars and depressions.

4.      Formation of lakes.

 Materials moving may end up in valleys where they accumulate and block the flowing river. At the upstream, water collects to form a lake.

5.      Changing of river courses.

Landslide may block a river course. This may force a river to change its course.         

6.      Formation of fertile soil.

 At the base of a slope, the collected materials may form a very fertile soil.

7.      Damage to property.

 Properties like telephone, power transmission lines ,roads, railway lines, buildings, farm may be destroyed.

8.      Loss of life.

Rapid mass wasting such as mud flow, slumps, rock fall and avalanches are known to have caused great loss of people’s lives.

 

 

Erosion, transportation and deposition by running water

 

The hydrological (water) cycle

Is the movement of water occurring between the atmosphere and the land surface powered by solar energy. Water moves down to the earth’s surface from the atmosphere by the process known as precipitation.

On the surface water forms over land flow (surface runoff). Some water collects on the surface as ponds and lakes

 

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Surface water returns to the atmosphere as water vapour through evaporation and transpiration. These two process are together called evapo-transpiration.

 At high altitude, water vapour and condense (condensation) occur to form saturated (dew point) clouds are formed. Further cooling result in the water droplets joining together to give larger droplets which fall to the ground as rain (precipitation).

 

Action of running water

The process associated with running water are known as fluvial processes.

 

Surface runoff

It is common when the level of rain is higher than the rate at which water can infiltrate into the ground, surface runoff occurs. If the runoff has sufficient flow of energy, it transports loosened soil particles down the slope.

 

Four Types of erosion caused by rainfall and surface runoff

1.      Splash erosion.

Is the rain drops falling onto the ground creates displacement of soil particles.

2.      Sheet erosion.

Is the erosion which results in the removal of a uniform cover of the soil. It is common in gentle slope, which are bare of vegetation.

3.      Rill erosion.

 Is the uneven removal of soil through small channels on the surface called rills. It leads to the production of small grooves on the surface called rills.

4.      Gully erosion.

 Is the type of erosion which takes place when rill erosion becomes more concentrated leading to the production of deep troughs into the land.

 

River action and the features it produces

Definition of some terms

 

Ø  River. Is a large natural stream or mass of water flowing in a valley from its source to its mouth.

Ø  River source. Is the starting point of a river. It can be mountains, forest, melting point etc.

Ø  River mouth. Is the terminal or end of a river. It can be an ocean or a lake

Ø  Tributaries. Are small streams that flow into big river, at the river source.

Ø  Distributaries. Are small streams that divide from the main stream, at the river mouth.

Ø  Confluence. Is the point at which a tributary joins the main river.

Ø  Watershed / divide. Is a boundary separating a catchment area and the next.

Ø  Catchment area. Is an area where the river source is found.

Ø  Drainage basin. Is a unit area of land that is drained by a river system.

Ø  River system. The tributaries and the main river together make up the river.

Ø  Upstream. Is the side of a river towards the source.

Ø  Downstream. Is the side of a river towards the mouth.

Ø  Interfluves. Are areas of high ground between tributaries.

Drainage Basin 

 

 

 

Types of rivers

1.      Permanent or perennial rivers.

 Are rivers which flow throughout of the year. e.g. river Nile, Congo, or Niger.

2.      Intermittent or seasonal rivers.

Are rivers which flows only during wet season in the regions, which receive seasonal rainfall.

3.      Ephemeral rivers.

       Are rivers which appear during rainy season in areas which receive very little amount

       of rain especially in desert region. flow only for hours or days following rainfall. They

       tend to disappear immediately after the rain season has stopped.

 

River regime

 

Refers to seasonal variation in the volume of water in the river channel. Water volume tends to fluctuate in most of the rivers.

Classes of river regime

1.      Simple regime

 Is the regime which experience one period of high water and one period of low water each year. It is common in tropical areas where there is one period of wet and one period of dry season. Examples are river Ruvuma, Ruvu or  Wami etc.

 

2.      Double regime

 Is the regime in which there are two periods of high water volume. It is very common in areas which experience two peaks of high rainfall (double maxima) like the equatorial region.eg Congo and Niger.

 

3.      Complex or composite regime. Is a feature of many of the largest rivers with extensive basin covering various climatic regions and receiving numerous tributaries, each perhaps with different regimes.eg Nile river.

 

 

Two ways of river flow

1.      Laminar flow.

 Water flow is layers parallel to the bed.

 

 

 

2.      Turbulent flow.

 Water flows in a circular like manner

 

 

 

 

 River erosion

River erosion involves four processes. These are

1.      Hydraulic action

2.      Abrasion (corrasion)

3.      Attrition

4.      Solution (corrosion)

 

Hydraulic action

Refers to the force of moving water which is able to remove loose materials such as gravel, sand and silt.

 

Abrasion ( corrasion)

Refers to the wearing away of the sides and bed of a river channel by the load. The more the load the river is carrying, the greater the erosive effect on the river channel.

 

Attrition

Is the process by which the load itself is broken down. The load are in constant collision with each other as they travel down the river.

 

Solution (corrosion)

River water directly dissolves soluble minerals which are found within the rocks over which is flowing. The dissolved minerals are then carried in solution.

 

Three ways or types of river erosion

 

Head ward erosion

This is the erosion by which a river increases its length by a river itself cutting back at its source. Rain wash and soil creep also help a river to extend its channel up the slope.

 

 Vertical erosion

Is the erosion by which a river deepens its channel. It takes place on the river bed. It increases the river depth.

 

Lateral erosion

Is the wearing away of the sides of a river channel. This is particularly effective along the river banks. It results in increase of the river depth.

Factors influencing the rate of erosion

 

River energy.

 A strong river erodes its valley faster than a weak river. The energy of a river depends on: -

a)      River volume.

The large the amount of water in a river, the more effective erosion by the river will be.

b)      Speed or velocity.

 The speed of a river is determined by the gradient or slope. The steeper the gradient the higher the velocity of water. Fast flowing river cause serious erosion.

 

Nature of the bedrock.

If the rocks are soft or less resistant to erosion, they will be eroded faster resulting in a deep channel.

 

Nature and amount of load.

 When the load comprises large and hard materials like boulders, it causes more erosion by corrasion than when the load comprises small and softer materials.

 

River transportation

The materials transported by a river are called river load

Four processes of river transportation

1.      Suspension.

 Is the transportation of very light and insoluble materials such as mud, clay, silt. These materials floats on the surface water.

2.      Saltation. (Hydraulic)

 is the bouncing  of large and heavier materials that cannot be held in water. They are uplifted for a short distance and they land back on the river bed.

3.      Traction.

 Is the pushing and rolling of much heavier materials such as boulders which cannot be uplifted by the force of water.

4.      Solution.

Is when soluble load dissolves in water and carried in solution. These materials are carried for a very long distance.

 

Factors influencing the ability of a river to transport its load.

 

1.      Energy of a river.

The energy of a river depends on.

a)      volume. The larger the amount of water in a river, the greater the amount of load it carries.

 

b)      Speeds. A fast flowing river can carry a greater amount and variety of the load.

 

2.      Nature and amount of load.

Small and light particles can be transported over long distance while large and heavy materials are carried for short distances. The dissolved load is carried all the way to the river destination due to its light weight.

 

River deposition          

Deposition by a river occurs when the river’s ability to transport its load decreases,at that point a river begins to drop some of its load. The materials deposited by rivers are called alluvial (alluvium) deposits.

 

Factors influencing river deposition

1.      Decrease in river energy.

River energy decreases when one or both of the following decreases.

             (a) Decreases in the river volume cause the river to drop some of its load.

      (b) Speed / velocity. A decrease in river speed causes the river to start to drop   

           some of its load starting with the heaviest one.

 

 

2.      Nature and amount of load.

The heavier load are deposited first while the load in suspension are carried farther and the load in solution is carried farthest.

3.      Width of the river.

When the width of the river channel increases, water spread out over the wider surface area. This cause reduction in volume per unit area which in turn cause deposition to take place.

4.      Obstacles in the channel.

 Presence of obstacles such as rock boulders, trees stumps cause deposition to take place.

5.      Freezing of river water.

 During winter, in cold lands rivers freeze together with the load. When thawing begins to take place in summer, materials will be deposited  as melting takes place.

6.      When a river enters a stagnant water.

Large amount of materials can be deposited when a river enters a sea or a lake where river speed is slowed.

 

Features of river erosion

                                    

 

 

 

 

  

1.      V – shaped valley (stream cut valley.)

 Is a river valley that has a V – shape, it is narrow and steep sided.

 

 

 

                                  

 

 

2.      Gorge. Is a long narrow deep and steep sided river valley. It is formed when vertical erosion erodes the bed of a river valley increasing its depth.

 

                

 

 

 

               When gorges becomes so big they are called canyon. Examples: The Grand canyon of   

              the Colorado river in USA.

 

 

            3.  Pot holes

              Are shallow pot like circular depressions on the river bed. They are caused by abrasion

               process when water swirl

 

 

 

 

4. Rapids

Are sections in the river course where the river bed is suddenly steepened causing the river to flow swiftly at that point. Water does not fall or drop but flows swiftly and remains in contact with the ground. They are formed by a hard rock overlying a soft rock. A series of rapids is called a cataract.

      

5. Water falls

Is a sharp break in the river bed making it vertical or nearly vertical causing water to fall. This break in gradient causes the water to drop or fall from the upper level to the lower one. A series of water falls is known as cascade.

 

 

 

 

6. Interlocking spurs

A spur is a portion of highland projecting into the valley. The river flows around them.

 

 

                                 

          

 

7. Boulders

These are large rock blocks broken up by river erosion. They remain untransported because of their heavy weight.

 

 

 

 

 

8. Plunge pool

This is a depression formed when water hits the ground at the bottom of water fall. It is filled with water.

                         

                          

River deposition features( landforms )

 

1. Flood plain

Is a wide and gentle sloping area formed as a result of deposition of alluvial materials. It forms very fertile soil for agricultural activities.

 

                   

 

 

2. Meanders

Are the windings or bends of a river along the valley.

 

 

 

                      

3. Ox bow lake

Is a body of water that occupies a crescent shaped depression formed on the flood plain of a river.             

                                 

 

GUIDING QUESTION

 

 

With the aid of diagrams, indicate various stages on how Ox bow lake is formed

 

 

4. Braided River

Is a form of a river which split into several channels then rejoin and split again.

 

                  

 

 

5.      Natural levees

Are ridge like features raised above the flood plain on the banks of a river. They form natural barrier to floods. They also form a good site for building bridges.

 

                        

 

6. Differed tributary (Yazoo stream)

This is a tributary which flows parallel to the main stream for a long distance before joining the main stream.

 

 

 

                    

7. Delta

Is a low lying swampy plain formed due to deposition of alluvial materials at the river mouth.It is called marine delta while in the lake, it is called a lacustrine delta.

 

                     

 

Conditions for delta formation

 

1.      A river must have a large load

2.      The velocity of a river must be low as it enters a water body (mouth). This will allow most of its load to be deposited in the river’s mouth.

3.      The river’s load must be deposited faster than it can be removed by the action of tides and currents.

4.      The river course must be free from obstacles such as lakes and swamps.

 

GUIDING QUESTIONS

 

With the aid of diagrams, explain three stages for the formation of delta(Reference-PHYSICAL GEOGRAPHY IN  DIAGRAMS FOR AFRICA by R.B Bunnett)

 

 

Types of delta

 

1.Arcute delta

Is a delta consists of both coarse and five sediments and it has the shape of an inverted cone. It is crossed by numerous distributaries. Examples: Niger, Nile, Ganges, Indus, Hwang – Ho etc.

 

 

                        

 

Birds foot (digitate) delta)

Is a delta consists of very fine materials called silt, and it has a few long distributaries bordered by levees. Examples the Mississippi delta, the Omo river in Ethiopia.

 

 

                        

Estuarine delta

Is a delta formed from materials deposited in the submerged lowland river mouth. It takes a shape of an estuary. Examples: River Seine in France, Vistula in Poland.

 

 

 

 

4.Cuspate delta

   Is a delta with tooth like shape. It is formed where a river reaches a straight coastline along which wave action is vigorous. No lagoons here and good example include Ebro river in Spain and Tiber river in Italy.

 

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5.Lacustrine delta

Is a type of delta which is formed in a lake, example the lakes around lake Geneva in Switzerland.

 

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Drainage patterns

Is a layout of the main stream with its tributaries.In most cases it depends on geomorphological and geological structure of the place.

                                                   

Common drainage patterns

 

1. Dendritic pattern

This pattern has a shape like the trunk and branches of a tree with the tributaries joining the main stream fairly acutely from many directions. This pattern develops on rocks of uniform structure and hardness.

 

 

 

  2. Trellis pattern

In this pattern the tributaries joins the main stream at right angle. They develop in a region which is made up of alternate belts of hard and soft rocks which all dip in the same direction

 

 

            

 

 

3. Radial pattern

This pattern develops on a dome or cone shaped upland; such as a volcano. The rivers flow outwardly forming a pattern like the spokes of a wheel.

 

 

 

 

                           

4. Centripetal (concentric pattern)(Annular pattern)

This is a pattern in which rivers flow from all directions into a common inland basin such as a lake, swamp or a sea.

 

                   

 

 

 

Drainage systems

 

1. Accordant  drainage system

Is a normal drainage system of a river. River flow is influenced by the slope or rock structure following areas of weak rocks. It is the most common drainage system where by river flows according to slope.

Good example of accordant drainage include dendritic, trellis, radial, centripetal, rectangular and others.

2. Discordant drainage system

Discordant drainage is opposite of accordant drainage whereby a river flows without being guided by either the slope of the land or dominant rock structure in a given area.

 

Types of discordant drainage system

 

A. Antecedent drainage system

Is a system in which a river maintains its original direction of flow across land that is under going uplift.

 

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B. Superimposed drainage system

Is a system where a river flow over a new set of rock of different structure after the original rock structure is removed by erosion.

 

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The long profile of a river

This is the entire length of a river course from its source to its mouth.

 

Three (3) stages of a long river profile

1.      The upper / youth / torrent stage

2.      The middle / mature / valley stage

3.      The lower / old / plain stage.

                 

 

 

 

Upper stage	Middle stage	Lower stage
§  It is found at the river source §  Due to steep slope, water flows with very high speed.   §  Vertical erosion is dominant         §  Very low volume   §  The valley is deep, narrow and steep sided with v – shaped §  Common features are erosional such as water falls, rapids, pot holes, interlocking spurs and gorges.	§  It is found at the middle part of a river §  Medium steepness of slope cause water to flow in a medium speed §  Lateral erosion is dominant and deposition starts. §  Medium volume   §  The valley becomes wider with an open V – shape §  Common features are such as meanders, bluffs and slip off slopes.	§  It is found at the river mouth §  The slope is very gentle so water flows slowly.     §  Deposition is dominant   §  Maximum volume       §  The valley is very wide with a U – shape     §  Common features are depositional such as flood plain, oxbow lakes, levees, braids differed tributaries

 

 

 

River capture (river piracy,abstraction, beheading)

 

Is the process whereby a powerful river diverts the course of a weak adjacent river into its course.

 

                      

                                  

Conditions for river capture

1.      There must be two adjacent rivers flowing parallel in the same direction.

2.      One river must be powerful than the other.

3.      The powerful river must flow at a lower level and it must erode its valley both head ward and vertically more faster than the weak river.

 

Features (landforms) associated with river capture

 

1. Elbow of capture

     Is the bend where water from the weak river turns into a pirate river.

 

2. Wind gap(Dry valley)

     Is a section of a valley just below the elbow of capture which become dry.

 

3. Misfit stream (beheaded river or underfit stream)

     Is the remainder of the river below the elbow of capture contains less water and becomes

     too small for its valley.

4.Rejuvenation and incision of the strong stream

Rejuvenation occurs due to increase in volume of water which in turn can lead to the formation of all    features under river rejuvenation like incised meanders, gorges, knick points and water falls.

 

River rejuvenation

Is the renewal of the erosive activity of a river when a river starts eroding its channel more powerful than before.

Causes of river rejuvenation

 

1.      Change in base level.

Base level is the lowest level to which a river can erode its bed. It is normally the sea level. (Dynamic rejuvenation)

 

             Causes of change in base level

       (a)   Fall in sea level (eustatic change)

                  (b)   Land uplift (isostatic change)

                   (c)   Unequal regional subsidence of the land

 

2.      Increase in river volume (discharge)

River volume increases due to the following reasons.

(a)   An increase in precipitation

(b)   Increase in temperature (ice melts)

(c)   River capture  (static rejuvenation)

 

 

 

3.      A change in rock resistance

Less resistant rock can allow  a river to renew its erosive power which is rejuvenation.

 

 

Effects (features) of river rejuvenation

a)      Knick point

Is a point where there is a sudden break in slope in the long river profile.

b)     Parred terraces

Are steps or benches on either side of a river valley formed as a results of undercutting of the river due to renewed erosion.

c)      Incised meanders

These are new meanders that form within the old meander. They are of two types.

       (i) Ingrown meander. The valley has one side steeper than the other (asymmetrical)

       (ii) Entrenched meanders. The valley sides are both steep (symmetrical)

d)     Gorges and Canyons

Are steep sided troughs formed when the undercutting becomes concentrated into river bed. Canyon is bigger than gorge ,also Canyon is more impressive.

 

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e)      Valley within a valley

Is a new valley formed within the former valley.

 

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Importance of rivers and their features

1.      Provide water for domestic and industrial use

2.      Irrigation water

3.      Transport(navigation)

4.      Fishing grounds

5.      Production of hydro electric power (HEP)

6.      Building materials (sand, gravel and pebbles)

7.      Valuable minerals

8.      Tourist attractions

9.      Alluvial fertile soil

10.  Natural boundaries

 

 

Negative importance

1.      River floods

2.      Barrier for transport and communication

3.      Waterborne diseases

4.      Dangerous animals

 

 

Underground water

Is the water that exists below the surface of the earth.

 

Other names

1.      Ground water

2.      Subterranean water

 

 

 

Types of underground water

1.      Rain water(Meteoric water)

Is the underground water formed when rain falls on the ground then infiltrate in the ground.

 

2.      Melted water.

 In regions that experience the winter season, snow accumulate on the ground during summer season as ice melts water penetrates into the ground to form underground water.

3.      Oceanic water

 Is the underground water originated from lake and sea water that has seeped in the ground through available spaces.

 

4.      Magmatic water(Juvenile water)

Is the type of water that gets trapped in the rocks beneath the surface during igneous activity.  It is usually hot and contains a lot of minerals.

 

 5.Connate water.

     Is the type of water retained within the sedimentary rocks since the time of their   

    formation. It is also called as fossil water.

 

 Factors influencing ground water

The amount of water that gets into the ground depends on its rate of infiltration. Infiltration is the process whereby the surface water enters the ground. The rate of infiltration depends on: -

 

1.      Precipitation.

 The higher the precipitation the higher chance of much water to infiltrate in the ground.

 

2.      Slope.

 The flatter the surface the greater the amount of water that would infiltrate the ground. The steeper the slope, the less the amount that would infiltrate the ground.

 

3.      Nature of the rocks.

 Infiltration and percolation depends of the rock permeability. Rock permeability is the ability of a rock to allow water to enter and pass through it.

 

4.    Vegetation cover.

       A surface covered by vegetation increases the rate of infiltration.

        (i)     Reduces the speed of falling rain drops

                (ii)     Slows down the rate of surface run off

          (iii)   Open the ground for surface water to infiltrate the ground.

 

5.      Level of saturation of the ground.

 If the ground is dry, the rate of infiltration is higher. If the ground is saturated with water; the rate of infiltration will be low and even cease.

6.      Evapo – transpiration.

The higher the rate of evapo – transpiration, the higher the rate of infiltration.

 

 

Two types of rocks according to permeability

 

Permeable rocks.

These are rocks that allow water to enter and pass through.

 

Two types of permeable rocks

                       i.             Porous rocks. Are the permeable rocks where water enters and pass through the pore spaces between the rock grains.

 

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                     ii.            Pervious rocks. Are the permeable rocks where water enters and pass through cracks, fractures, joints or fissure

 

Put a diagram relevant to the point above

 

 

 

Impermeable rocks. Are the rocks that do not allow water to enter and pass through. Hence infiltration will be high if the rocks are permeable in nature.

 

Put a diagram relevant to the point above

 

 

 

Water table and levels of saturation

 

Water table

   Is the upper limit of the zone of saturation.

 

Three zones of saturation

 

1.      Zone of non – saturation.

      Is the first zone in which water just passes as it goes downwards.

      The zone does not saturated.

 

 

2.      Zone of intermittent saturation.

 Is the second zone which is saturated during wet season only. They become dry during dry season.

3.       Zone of permanent saturation.

 Is the zone which is always get saturated with water, usually does not dry

 

 

Features of underground water

1.      Springs.

Is a place where underground water flows out naturally onto the earth’s surface.

 

Ways of the formation of springs

a)      When a permeable rocks overlies an impermeable layer.

 A spring will occur where the impermeable rock is exposed to the surface.

                     

Put a diagram relevant to the point above

 

b)      When water enters well jointed rocks on a hill.

 Water comes out of the ground at a point where the water table reaches at the ground surface.

Put a diagram relevant to the point above

 

c)        When an aquifer lies on the upper side of a dyke

   Occur where the aquifer and dyke intersect.

Put a diagram relevant to the point above

 

d)      Where an impermeable layer underlies a limestone escarpment.

 A spring forms at the foot of the scarp slope.

                          

Put a diagram relevant to the point above

 

 

e)      When faulting occurs

 Springs can form at the foot of the scarp slope.

 

2. Wells and boreholes

       Are deep holes dug in the ground commonly for getting underground water.

 

Three types of wells

1.      Permanent wells. Are the holes sunk deep into the layer of permanent saturation, water is permanent.

 

2.      Intermittent wells. Are holes sunk only into the zone of intermittent saturation. It will contain water during the rainy or wet season but remain dry during the dry season.

3.      Dry wells. Are the holes sunk just into the zone of non saturation, it is dry except at the time of flood.

 

 

 

3. Artesian basins and artesian wells

      Artesian basin is a basin which consists of a layer of permeable rock lying between the two layers of impermeable rock.

    When the permeable rock between the two impermeable rocks is saturated with water; it is called aquifer. If a well is sunk into the aquifer of an artesian basin, it will form an artesian well. Water will come normally to the surface by hydraulic pressure.

        

Put a diagram relevant to the point above

 

 

Conditions for the location of Artesian Well

1.      There should be a permeable rock layer lying between two impermeable rock layers.

2.      The permeable rock layer must be exposed in an area of sufficient precipitation.

3.      The basin must dip towards the region where the land surface is lower than it is at the exposed end of the previous formation.

 

Problems associated with artesian wells

a)      Water in some wells is hot or salty or both.

b)      If the rate of drawing water from artesian well is greater than the rate at which it is being replaced. The water table may be lowered and ceases to be artesian well and it becomes an ordinary well.

Significance of underground water

1.      Source of rivers

2.      Provide water for domestic and industrial uses.

3.      Improves agriculture through irrigation

4.      Encourage the development of settlement

5.      Hot water is used to warm up houses

6.      Tourist attraction (hot springs)

7.      Some hot springs contain minerals which are believed to be a cure of some diseases like skin diseases.

8.      Source of minerals. Especially mineral salts as the water evaporates.

 

Water action in limestone areas

Limestone are sedimentary rocks which are well jointed made up of calcium carbonate (caco3). The landscape of limestone is commonly known as karst scenery

Water action in limestone areas is the carbonation process and solution.

 

 

Surface features in limestone areas

 

1. Grikes and clints

   Grikes are the irregular gullies found on the surface.

   Clints are the separating limestone blocks on the surface.

 

Put a diagram relevant to the point above

 

2. Swallow holes / sink holes

    Are vertical holes in the ground through which rain water or river may disappear into the

     ground beneath.

 

Put a diagram relevant to the point above

 

 

3.Dry valleys

   Is a part of a river valley through which water no longer flows. It forms on the down stream

   side of a swallow hole.

Put a diagram relevant to the point above

 

 

4. Doline (dolina)

    Is a round hollow on the limestone surface formed after several small hollows merge.

 

Put a diagram relevant to the point above

 

5. Uvala

     Is a very wide depression with a fairly flat floor formed when several dolines merge.

 

Put a diagram relevant to the point above

 

 

6. Polje

      Is the largest of the surface depressions formed in limestone regions. It is formed when

      uvalas merge or when an underground cave collapses, or sinking caused by tectonic forces.

Put a diagram relevant to the point above

 

7. Gorge

       Is a long, deep, narrow steep sided valley formed when the roof of an underground cave

      collapses.

 

Put a diagram relevant to the point above

 

 

 

 

Underground limestone features

 

1. Caves

    Is a large hole inside the earth with an opening at some place on the surface of the

    earth. When a cave becomes extremely big, it is called a cavern.

 

2. Stalactites

     Are long slender needle – shaped features which hang down from the cave roofs.

 

3. Stalagmites

    Are long slender needle shaped features which grow upward from the floor of a cave.

 

4. Natural pillars

    Are columns of limestone formed when stalactite and stalagmite joins.

 

5. Gorge

    Is a long deep narrow steep sided valley that develop when the underground cave collapses.

 

6. Underground river

     Is a stream that flows below the surface after the surface stream has disappeared through

     the swallow holes.

 

Draw one diagram to show the following features cave,stalactites,stalagmates,natural pillar and underground river.

 

 

Characteristics of a limestone (karst region)

1.      Its rocks are sedimentary consist of calcium carbonate.

2.      The surface is both stony and broken (bare rocks)

3.      Absence of surface drainage due to its permeability

4.      Very  thin soil layer

5.      Poor vegetation due to poor soil and drought

6.      Rugged escarpments, gorges with cliff like sides and depressions are common features.

Values of limestone region

1.      Tourism due to the presence of attractive features.

2.      Limestone rocks are raw materials for cement manufacturing

3.      Pastoralism activities

4.      Discourage settlement

 

 

 

 

Features produced by the action of ice

 

Meaning of some terms

 

v  Ice

            Is frozen water. It is common in temperate regions and high altitude areas.  Precipitation

            is mostly in form of snow.

 

v  Ice age

            Is the period in the earth’s history when large areas were buried beneath thick sheets of

             ice and glaciers. The last ice age occurred in the Pleistocene.

 

v  Pleistocene period.

 The Pleistocene ice age began about two million year ago. The period in which we live now is called an inter – glacial period.

v  Ice age is a period when the temperature of the air falls below 0⁰C, some of its water vapour condenses and freezes into ice crystals that fall to the earth as snow.

v  Permanent snow cover The area where snow does not melt. For example Greenland, Antarctica.

v  The snow line is a level above which there is a permanent snow cover.

v   Firm or neve is the granular mass of snow formed when snow is compressed down due to its weight.               

v  Glacier ice is formed when the air spaces disappear the neve is converted into a compact mass of ice.

v  Ice sheet is a continuous mass of ice covers a large land surface. Examples are ice sheets of Greenland and Antarctica.

v  Ice berg is large block of ice that floats on the sea.

v  A glacier (mountain or alpine) valley glacier is a moving mass of ice.

v   A piedmont glacier is a valley glacier converge at the foot of a mountain.

v   Ice cap is a permanent ice sheet of limited size covering a plateau or mountain top.

v  Cirque glacier is the ice that occupies a depression.

 

How ice moves

Ice moves through sliding or flowing in the following ways

 

1.      Plastic flowage

 Is the movement caused by great weight of ice that cause great pressure. Some ice particles melt slight and moves down hill before freezing again.

 

2.      Basal slip.

Is when the great weight of the ice causes the layer in contact with the rock beneath to melt. This lubricate the passage that facilitates movement.

 

 

3.      Extrusion flow

 Is when the great weight of ice in the centre of the mass exerts pressure on the layers below. This pressure forces the ice underneath to spread outwards in all directions.

 

Factors that influence ice movement

1.      Gradient.

The steeper the land the faster the ice moves. Effects of gravity is greater on steeper slopes.

 

2.      Season.

 The warmer the season the faster the movement. During summer, ice melts causing faster movement.

 

3.      Friction.

 Friction slow down ice movement

 

4.      Thickness and weight of ice.

The thicker the ice the heavier it would be. Thick ice mass exerts great pressure on the layers beneath casing melting hence facilitating faster movement. The thicker the ice the faster the movement.

 

Glaciations

Ø  Is the sculpturing of the landscape by moving ice. It is the process by which moving ice erodes, transports and deposits materials on the earth’s surface.

Ø  Is the past occupation of an area by a glacier or an ice sheet e.g. Pleistocene glaciations.

 

Glacial erosion

Glacial erosion predominates in the highlands.

Three processes

1.      Sapping.

Is the break up of rocks by alternate freezing and thawing of water at the bottom of cracks.

 

2.      Plucking.

 Is the tearing away of blocks of rock which have become frozen into the sides or bottom of a glacier.

 

3.     Abrasion.

 Is the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.

 

Features of glacial erosion (highland features)

 

 

 

 

 

1.      Cirque (corrie).

Is a semi circular, steep sided basin cut into the sides of a mountain. When these basins are filled with water they are called tarns.

2.      Arête.

 Is a steep sided ridge on the glaciated highland which separate two cirques.

3.      Pyramidal peak (horn).

Is a peak of glaciated highland formed by the steepening of the back walls of several cirques which lie on the sides of a mountain. These peaks become sharpened by frost action.

 

4.      U – shaped valley or glacial trough.

 Is a steep sided flat bottomed, wide valley which contains both glacial erosional and depositional features.

 

Put a diagram relevant to the point above

 

 

5.      Hanging valley

 Is a tributary valley which ends up abruptly high above the floor of the U – shaped valley and separated from it by an almost vertical slope. Hanging valley plunges into the floor of a U – shaped valley as water falls. It sometimes builds alluvial fan on the floor of a U – shaped valley.

 

Put a diagram relevant to the point above

 

6.      Truncated spurs.

 These are the pieces of the former spurs whose ends are planed off by a glacial erosion.

Put a diagram relevant to the point above

 

 

7.      Rock basin.

Is an irregular depression in the floor of a U – shaped valley formed by unequal glacial erosion of the bed rock. When ice disappear in a U – shaped valley, a rock basin may be filled with water to form a ribbon lake; trough or finger lake.

 

Glacial deposition

 

The materials deposited by ice are known as glacial drift.

There are two types of glacial drift

 

1.      Fluvio glacial deposits.

 Are the materials deposited by melt water as it flows away from melting ice.

 

2.      Till deposits.

 Are materials deposited directly as the ice melts.

 

Features of glacial deposition

 

Erratic:

Are large boulders made of rock, different from that of the region where they are deposited.

 

Put a diagram relevant to the point above

Boulder train.

 Is a series of erratic that originates from the same bed rock and are deposited with the apex pointing towards their origin.

 

Boulder clay plain.

 Is a monotonous extensive plain made of clay and boulders, deposited by ice sheet glaciers over a surface.

Drumlins. Are elongated oval – shaped low hummocks made of boulders clay. The upstream sides are usually steeper than the downstream sides. They usually occur in groups.

Moraine

Is the deposition made by ice, usually has the tendency of forming ridges of various shapes.

 

 

GUIDING QUESTIONS

Identify five(5) types of moraines in glaciated lowland.

 

 

Esker.

 Is a steep sided ridge about 40m high that winds  a across country made of gravel and sand.

 

Kame.

 Is an irregular shaped mass of stratified materials formed as a delta on the surface of a stationary glacier or at its margin.

 

Put a diagram relevant to the point above

 

Outwash plain.

Is an extensive and fairly level land whose surface is covered by deposits of clay, sand and gravel. Coarser materials form the plain nearest to the terminal moraine, but the deposits gets finer away from the moraine.

 

Put a diagram relevant to the point above

 

Moraine dammed lakes.

Are lakes formed when the terminal moraine or valley glaciers have blocked stream flowing down slope. The upstream part forms a lake e.g. the great lakes of the USA.

 

Importance of glaciated landscape

 

A.  Positive

1.      Formation of fertile soil. The outwash plain, boulder clay plain and old glacial lake beds. Example is the Canadian prairies forms the largest wheat producing region in the world.

2.      Facilitate navigation. This is in the glacial lakes

3.      Generation of hydroelectric power. In the water falls of the hanging valleys. Examples in Norway and Switzerland.

4.      Tourist attractions. The glaciated landscape has wonderful features and is good for some sports such as skiing and mountain climbing e.g. Switzerland.

5.      Support pastoralism during summer season. Glaciated valleys during summer have good pastures.

6.      Provide building and construction materials; sand, boulders from the outwash plain, boulder clay plain, kame and eskers.

7.      Glacial erosion has exposed valuable minerals, making their exploitation easier.

8.      Fiords provide suitable breeding grounds of fish. They also form good natural harbours.

9.      Source of fresh water for various uses

10.  Forms river sources

 

 

 

B. Negative

1.      Creation of swamps condition. This makes the area not to be fully used for agriculture and settlement.

2.      Some outwash plains are dominated by infertile sands which limits agriculture.

3.      Numerous moraine dammed lakes reduces the size of land available for human use.

4.      Very rugged glaciated landscapes cause a problem for settlement or constructing transport lines.

 

 

 

Wind action in hot and temperate deserts

 

Wind action is also known as Aeolian action. Wind action is very common in hot desert. This is due to:

1.      The ground is very dry, loose and has unconsolidated masses of sand and gravel that can easily be acted upon by wind.

2.      There is absence of vegetation cover.

3.      There are very strong tropical wind storms.

 

Aridity: Is a state of land being deficient in moisture leading to scanty vegetation (less than 250mm of rain)

 

Types of deserts

 

According to level of temperature

1.      Hot deserts. They experience very high temperature (hotness) of about 44⁰C and above. Examples are the Saharan, Arabian, Kalahari, Iranian, Namib, Australian deserts.

2.      Cold desert. They experience extremely low temperature (very cold) of about 0⁰C to -100⁰C Examples are Gobi, Patagonian, Greenland and Antarctica.

 

According to location

1.      Western margin deserts. Are the deserts located on the western coasts of continents. Examples: the Saharan, Kalahari, Namib, Australian

2.      Continental (interior) desert. Are the deserts that are located the interior of continents far away from the coasts. Example Gobi and Arabian deserts.

 

According to surfaces

1.      Sandy desert (ERG)

 Is a desert surface covered mostly in large quantities with sand.

 

                           

 

2.      Stony desert (REG) (SERIR)

 Is a desert surface covered mostly by angular boulders, gravels and pebbles.

 

 

                        

 

3.      Rocky desert (HAMADA)

 Its desert surface mainly made of bare rock.

 

 

4.      Badland.

The land is broken by extensive gullies separated by steep sided ridges.

 

                               

 

Characteristics of deserts

 

1.      Low rainfall. Less than 250mm a year for an arid desert. At least 500mm for semi arid desert such as Kalahari.

2.      High temperature and evaporation.

 

Why desert occurs

The main cause for the deserts formation can generally be stated as the location of an area. Due to the location, we have two points.

a)      The largest deserts (arid and semi arid) occur between 15⁰ and 30⁰ North and south of the equator. Most of them are located in the western sides of continents. These places receives offshore trade winds. These winds are dry. When they receive onshore trade winds, they are also dry due to the presence of the cool ocean currents that parallel the west coasts.

 

Reasons for the formation of continental (interior) deserts

1.      Are very far from the oceans. They don’t get the moderating influence of oceans.

2.      Located on the leeward sides of mountains (rain shadow)

 

Wind erosion

It involves three (3) processes or mechanism

1.      Abrasion.

 Is the erosion caused by the materials carried by the wind. This is done by polishing (hitting, grinding, scarping and blasting)  the rock surface.

 

2.      Deflation.

 Is the process whereby wind blows away rock waste which are dry unconsolidated

like dust and fine sand particles.

 

3.      Attrition.

 Is the process by which wind materials collide against each other and wear away.

 

Features produced by wind erosion

 

1.      Rock pedestals.

Are tower like structures of various shapes. They are made of heterogeneous rocks of differing resistance. The soft rock layers are easily eroded than the hard rock layers.

            

 

2.        Mushroom block.

Is a rock pedestal with a massive rock with a broad, smoothed and rounded top and a very narrow base. It is formed from homogeneous rock of uniform resistance to erosion.

 

Put a diagram relevant to the point above

 

 

3.      Zeugen

These are ridges of  rock mass formed when a layer of resistant rock is underlain by a layer of weak rock.The layers of resistant and less resistant rock lie horizontally to each other. 

 

Yardang.

 

Are elongated rock ridges formed when layers of hard and soft rocks lie vertical to each other. This occurs when the layers (bands) of resistant and weak rock lie parallel to the prevailing winds.

 

  

 

Deflation hollows:

Are depression formed by deflation process. When these depression reach down to the water bearing rocks, they form a swamp or an oasis. Example: Qattara depression south west of Alexandria (Egypt) is 122m below sea level.

 

 

 

Inselberg.

Is a rounded topped mass of resistant rock. It is formed when wind erosion or water erosion has removed all of the original surface except for isolated resistant rocks.                                                                             

                                         

                                           

 

 

Ventifacts.

Are boulders, stones or pebbles that have been broken from massive rocks. Usualy are smoothened and polished on their wind ward side through abrasion.

 

Put a diagram relevant to the point above

 

Wind transportation

Factors influencing wind transportation

1.      Speed and strength of wind.

The higher the velocity of the wind, the greater its strength. A strong wind transports more materials than a weak one.

2.      Obstacles.

 Obstacles tends to block the movement of wind. This reduces wind velocity, hence its strength to carry the load.

 

3.      Vegetation cover or moist surfaces.

 The surface covered by vegetation or the surface of moist materials binds the soil together. This reduces the ability of wind to transport.

4.      Nature of the load.

The lighter the materials the greater the distance they can be transported. Heavier materials are transported for short  distances.

5.      Weather changes.

When the short torrential rains occur, the materials are washed down the slope.

 

Process of mechanism of wind transportation

 

1.      Suspension.

 Is the transportation of very fine and light particles like dust through air currents above the ground.

2.      Saltation.

Is the transportation of medium sized particles by bouncing as they move

3.      Surface creep.

 Is the transportation of the much heavier unconsolidated materials that cannot be transported by wind. They are pushed and rolled along the desert surface.

Wind deposition

As the wind speed is reduced, its energy to transport decreases. The wind starts to drop some of its materials, beginning with the heaviest.

      Factors influencing wind deposition          

1.      Speed and strength of wind.

 The higher the velocity of the wind, the greater its strength. A strong wind transports more materials than a weak one.

2.      Obstacles

 Obstacles tends to block the movement of wind. This reduces wind velocity, hence its strength to carry the load.

3. Vegetation cover or moist surfaces.

 The surface covered by vegetation or the surface of moist materials binds the materials together. This reduces the ability of wind to transport.

4. Nature of the load.

 The lighter the materials the greater the distance they can be transported. Heavier materials are transported for short  distances.

5. Weather changes.

 When the short torrential rains occur, the materials will be washed down and deposited.

Features formed due to wind deposition

1.      Sand dunes.

Are low ridges or hill of sand that accumulates on the surface.

                    

Types of sand dunes

(a)   Barchans.

        Is a crescent shaped dune lying at right angle to the prevailing wind with the   

        horns pointing downwind. It has a gentle slope on the windward side, but steep

        slope on the lee ward side

 

                 

 

 

 

(b)    Seifs or longitudinal dunes.

        Are ridge shaped dunes with steep sides and lying parallel to the prevailing

        winds. They are parallel to each other. The crest of a seif is sharp.

 

 

 

            2.Loess.

                   Is an area covered by dust deposits. The dust is removed by wind from desert

                    surface. It is deposited at the margins of deserts. Loess is easily eroded by

                    wind. In America, loess is called adobe.

 

 

 

 

 

 

Water action in deserts

In few cases, deserts experiences torrential rains of from 100mm to 250mm. Runoff on steep slopes is through rills; they connects to form gullies.

 

Features produced by water erosion in the desert (fluvial erosion)

 

Rills

Are small, shallow grooves formed due to rill erosion caused by surface run off.

 

 Gullies

Are deep steep sided troughs form when rills join together.

 

Put a diagram relevant to the point above

 

 Wadis

  Is a deep, steep sided and flat floored valleys formed due to severe water erosion in the desert.

 

Pediment

Is the gently sloping platform formed when the edge of the desert and semi desert highlands get pushed back by erosion and weathering.

 

Pediplains

Are features that are formed when several pediments exist close to each other, resulting to multi concave gently sloping features.

 

Mesas and buttes

Mesas

 Are large and extensive flat topped residual hills like a table.

Buttes

 Are similar to mesas but much smaller in size.

 

Gorge is a long narrow deep steep sided valley. A big and attractive gorge is known as a canyon.

 

Put a diagram to indicate all features above.

Basin or inland drainage features

 

Desert has no permanent drainage due to

1.      Very little rainfall

2.      High rate of evaporation

3.      High rate of infiltration

It forms temporary lakes and salt flats. In the Sahara, they are called sebkhas, in America playas.

 

Desert water deposition features(fluvial deposition)        

 

Alluvial fans.

Are fan shaped features formed when alluvial materials deposits at the foot of the steep slope. Sometimes the deposition of several alluvial fans form bajada or bahada.

 

Alluvial cones

Are features of alluvial materials coarse in nature deposited boulders and pebbles at the foot of tsteep slope.

 

Bajada (Bahada)

Are gentle undulating slope of alluvial fans and cones which look like hills brought together during deposition of sediment at the foot of steep slope.

 

Peripediment

 Is a feature with a gentle slope formed when the alluvial deposits overlies the edge of the pediment surrounding a Sebkha.

 

Indicate all above features in one diagram

 

Deserts and rivers

Most of the deserts rivers are inland. That means they don’t reach the sea. Very few rivers manage to reach the sea; these are

1.      The Nile in Egypt.

2.      Tigris – Euphrates in Iraq.

3.      The Colorado in the USA

These rivers originate in regions of heavy rain which falls throughout the year and which is sufficient to sustain a permanent flow of water across the desert area.

Action of wave along the coast and the features it produces

Introduction

 

§  Coasts are constantly changing

§  Wave erosion causes some coasts to retreat

§  Wave deposition causes others to advance

§  There are so many variations pertaining to coasts

 

Factors influencing the nature of coasts (evolution of coast)

1.      Wave action and tidal currents

2.      The nature of the rocks forming the coast

3.      The height of the coast

4.      The nature of the climate

5.      Human activities

6.      Relative changes in the levels of the land and sea.

 

Definition of some terms

 

Coast – is a strip of the land where it meets the sea.

Coast line – is the margin of the land. The limit at which wave action takes place.

Shore – Is the strip of land that lies between high water and low water levels.

Shore line – is the line where shore and water meet

Wave – is a moving ridge of water especially on the sea which is caused by the winds and tides.

Tide – is a regular rise and fall in the level of sea; caused by the pull of gravity of moon and sun.

Highest water level or mark. Is the farthest level of the landward side which is reached by the most powerful waves.

Lowest water level or mark. Is the lowest level on the side of the sea which is reached by the lowest tide.

Beach – is the part of the shore covered by sand or shingle.

 

Cliff-is the highest and very steep rock face along the coast line.

 

 

Use Bunnett to draw a diagram of all features above

 

WAVES

Draw a diagram of wave to indicate the terminologies below

 

Crest  is the top part of a wave

Trough is the bottom part of a wave

Wave length is the horizontal distance between two successive crests.

Wave height is the difference in height between the crest and the trough.

 

The height and power of a wave depends on

1.      Strength of the wind

2.      Fetch (distance of open water over which the wind blows)

                       The stronger the wind and the greater the fetch, the more powerful the wave.

 

Two movements of water from a broken wave at the coast

1.      Swash (send)

 Is the amount of water that rushes forward to the beach after a wave has broken.

2.      Back wash

Is  the movement of water back toward the sea by gravity.

 

 

Types of waves

1.      Constructive waves

Are waves whose swash is more powerful than the backwash.

2.      Destructive waves

Are waves whose backwash is more powerful than the swash.

 

Wave erosion

There are for process or mechanism of wave erosion

1.      Hydraulic action

 Is the direct action of moving water within the wave. Its force causes the water to break rocks as well as remove loose materials from the coast line.

2.      Corrosion (abrasion)

 Is the erosion caused by the materials carried by the waves. Boulders, pebbles and sand are hurled against the base of a cliff by breaking waves and resulting in undercutting and rock break up.

3.      Attrition

 Is the break up of boulders and rocks as they collide against each other and against the shore.

4.      Solution

 Is the process where the soluble rocks dissolves in water and carried away in solution form.

 

Factors influencing(affecting) the rate of wave erosion

1.      Type and energy of waves

 Strong and high waves cause a lot of erosion of the coast.

2.      Weather conditions.

Stormy weather produces huge waves which cause serious erosion along the coast.

3.      Depth of water at the shore coasts

With shallow shores cause the waves to break before they reach the coastal land, this cause less effects of wave attack.

4.      Nature of the coastal rocks

 If the coast line consists of hard rocks, the erosion process is very slow.

5.      Gradient of the coast

When the gradient of the coast is steep there will be a lot of erosion of the rock. This is because the waves break directly against it.

6.      Orientation of the coast line

When the coast line lies perpendicular of the breaking waves; wave erosion is likely to be very effective, compared to the parallel coast.

7.      Nature and amount of erosional materials

When the water of waves contains hard boulders, pebbles and shingle, they cause serious erosion through abrasion.

 

 

8.      Human activities

Human being can reduce coastal erosion by building defensive sea walls like groynes  or planting trees but can accelerate erosion by constructing building near the sea or cutting trees especially mangrove trees.

 

 

 

Features resulting from wave erosion

 

 

Cliff:

 Is a steep rock face that is bordering the sea. It may be vertical or nearly vertical.

 

                                                                                                      

 

                

 

 

Wave cut platform:

 Is a fairly flat part of the shore at the base of a sea cliff. It is formed as a result of continual undercutting of cliff due to wave erosion.

 

         

 

 

 

 

GUIDING QUESTIONS

 

QN.    By using diagrams illustrate five (5) stages in which indicate on how wave cut

           platform is formed.(Reference-Physical Geography in Diagrams for Africa by

           R.B.Bunnett)

 

Bays and headlands:

Are the features formed where the rocks making up the coast are of differing resistance to erosion, the sections of the softer rocks are eroded more by waves than those of resistant rock. Those that are eroded more form sea inlets called bays. The resistant rocks stick out to the sea and form headland.

 

 

Caves:

Are tunnel like opening at the base of a cliff face that develops where there less resistant rocks.

 

An arch is a roof like structure formed as a result of continuation of wave erosion inside the cave to the extent of forming a way through from one side of a cave to another side of a cave on the headland.

 

A stack is a isolated piece of rock which is formed when the roof of an arch collapses.

 

A stump is a small piece of rock which is formed when a stack is reduced in size by prolonged wave erosion.

Blow hole (group) : is a vertical hole formed by wave erosion  on the roof of a cave.

 

Put a diagram relevant to the point above

 

 

Geo – is a long narrow inlet of the sea formed when the roof  between  a cave and blow hole collapses.

Put a diagram relevant to the point above

 

 

Wave transportation

Waves transports large quantity of the load such as mud, sand, pebbles, shingle that have been dumped into the sea by

-          Rivers and wind

-          Mass wasting

-          Weathering along the coast

-          Volcanic debris

 

Factors influencing wave transportation

(a)   Wind

(b)   Tides

(c)   Ocean currents

(d)   Gradient of the shore

(e)   Orientation of the coast line

(f)    Nature of the load

 

Wave deposition                                                                                                                               

Waves deposits its materials on the shore through the actions of swash and backwash. Wave deposition takes place when swash is stronger than backwash (by constructive waves).

 

 

Wave deposition (marine deposition) features

Beaches:

Is a gently sloping mass of accumulated materials such as sand, shingle, pebbles and boulders along the coast (shore) when the deposition is taking place in the bay it forms a bay head beach (bay beach).

 

      

 

 

Spit

Is a low, narrow ridge of pebbles or sand joined to the land at one end, with the other end terminating in the sea.

       

 

 

 

Bar – is a ridge of materials usually sand which lies parallel or almost parallel to the coast. A bar

          is not attached to the land.

(a)   Bay bar

Is the bar that forms across the entrance to a bay. A bay bar encloses water in a bay called a lagoon.

 

 

 

(b)  Tombolo:

Is a bar made of sand or shingle that links the mainland to an offshore island.

 

Put a diagram relevant to the point above

 

 

(C)   Offshore bars (Barrier beach)

 Are ridges of materials that are deposited more or less parallel to the coast line.

       

                 

                             

        

Cuspate foreland.

 Is a triangular shaped accumulation of sand or shingle deposits projecting from the mainland into the sea.

Put a diagram relevant to the point above

 

Mudflats and salt marshes

Mudflat is a platform of mud formed by tidal deposits. The mud in the sea is brought by rivers and high tides.

 

Lagoon

Is a part of sea water enclosed by land.

 

Types of coasts

1.      Submerged coasts

2.      Emerged coasts

 

Submerged coasts (drowned coasts)

Are the coasts which were formerly dry lands, now lies under water.

 

Causes

a)      A positive change in base level (Eustatic change)

It happens due to the increase in volume of water in the ocean.

 

b)      Subsidence of the coastal region.

It is caused by the earth movements. It causes the coastal land to sink or subside.

 

Two types of submerged coast

1.      Submerged highland coast

2.      Submerged lowland coast

 

Submerged highland coasts

These do happens when the coasts consist of highland features sink. Submerged highland coasts includes

 

                    (i)            The ria coast.

 A ria is a submerged highland river mouth.

Put a diagram relevant to the point above

 

 

                 (ii)            A fiord coast.

 A fiord is a submerged lower part of a U – shaped valley, common in glaciated highland eg Norway.

 

 

 

               (iii)            Longitudinal (dalmation) coasts.

It happens when the ridge like hills and valleys lying parallel to the coast is submerged, where the hills form islands. The valleys appears drowned to give narrow inlets (sounds).

 

Put a diagram relevant to the point above

 

 

Submerged lowland coasts

Is formed when lowland coast consists of gentle slopes and plains sink.

 It includes:

 

Estuarine coasts

An estuary is a submerged lowland river mouth. When the drowned coast has estuaries it forms estuarine coast.eg River Msimbazi in Dar es Salaam,Tanzania.

 

Emerged coasts

Is formed when part of the land which was formerly under sea water becomes permanently exposed.

 

Causes

1.      Negative change in base level

Is the falling in the volume of ocean water. The sea level drops.

2.      Uplift of the coastal land

 Internal earth movements may cause the coastal land to rise upward.

 

Two types of emerged coasts

                    (i)            Emerged highland coasts

                 (ii)            Emerged lowland coasts

 

Features of emerged highland coasts

                    (i)            Raised beaches

                 (ii)            Raised cliffs

               (iii)            Raised waves cut plat form.

  

Emerged lowland coasts

(i)                 Fall in the sea level along the lowland coasts. Part of continental shelf becomes exposed

forming a wide, gently sloping coastal plan.

 

(ii)              Knick point may be formed due to rejuvenation.

(iii)            Shallow offshore will result into deposition. Various depositional features will be formed.

 

Coral coasts

 Are coasts composed mainly of organic materials, the remains of once living things plants and animals.

 

Coral

 Is a limestone rock made up of the skeletons of very small marine organisms called coral polyps.

 

Coral polyps

Usually lives in very large groups called colony.

 

Conditions favourable for coral polyps to survive and grow

1.      Warm sea temperature of about 21⁰C

2.      Sediment free salt water

3.      Shallow water. Sunlight penetrates to a depth of at least 50m.

4.      Plentiful supply of plant food (plankton).

 

Distribution of Coral Reefs

Extensive coral features develop between 30⁰N and 30⁰S on the eastern sides of continents where warm currents flow near to the coast.

 

Types of coral reefs

 

1.      Fringing reef.

 Is a coral reef that is separated from the coast by a narrow and shallow lagoon.

 

2.      Barrier reef.

Is a coral reef that is separated from the coast by a wide and deep lagoon.

 

3.      A toll

 Is a circular coral reef usually broken in several places that encloses a lagoon.

 

Importance of coastal features

a)      Port development

 Some features like rias, estuaries, fiords favour the development of deep and well sheltered natural harbours.

b)      Fishing

The shallow continental shelves encourage the growth of planktons on which fish feed.

c)      Tourism

Many coastal features such as coral reefs, beaches, caves, cliffs and fiords attract tourists who bring in foreign exchange.

d)       Minerals and building materials

 Coral reefs are used as raw materials  for  manufacturing of cement. Others are stones which are used as building materials.

e)      Marine life

The mudflats, lagoons, salt marshes, fiords, coral reefs and continental shelves provide suitable habitat for marine life.

f)       Land for settlement

Emerged land from the sea forms a coastal plains suitable for

 settlement.

g)      Infertile soil

 Some emerged  coastal lands are sandy, gravel and rocky which cannot

support agriculture.

h)      Transport barrier

 Sand bars and coral reefs are a barrier to water transport.