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2.0: FORCES THAT AFFECT THE EARTH

2.0: FORCES THAT AFFECT THE EARTH

  

2.0: FORCES THAT AFFECT THE STRUCTURE OF THE EARTH

 What are forces ?

 Forces are the processes that operate (work) within or on the earth’s crust
There are different forces that affects the earth’s but can be grouped into two major types :-

         1.Internal forces (endogenetic/endogenic processes)
         2.External forces(exogenetic /exogenic forces )

 

 INTERNAL FORCES (ENDOGENETIC/ENDOGENIC)

 These are forces that operate within (inside) the earth’s crust
                               OR
These are forces which operates beneath (under)the earth’s surface. These forces are generally referred to as TECTONIC FORCES.

 A word tectonic is derived from a Greek word tecton which means builder

  Tectonic means building

  - internal forces(tectonic forces) whic
h are divided into

        (1)Earth movement (Diastrophism)

        (2)Vulcanism/ Vulcanicity/Volcanic erruptions

 

EARTH MOVEMENTS  
These are also known as Diastrophism
Definition: is the movement of the solid parts of the earth towards each other or away from one another or side way.

Types of Earth movement.
        Earth movements are classified into two (2) main groups:-
                                              (i) Vertical or radial movements

                                              (ii) Lateral or horizontal movements or tangential.

 

I ) VERTICAL OR RADIAL MOVEMENTS.
   
These are the upward and downwards movements or forces .These forces cause the uplift (epeirogenic) and the downward movement (cymatogenic).

   
These forces which causes the vertical earth movements operate from the interior upward toward the surface or downward from the surface to the interior.

These forces cause

 (a) The crustal rock to fault. When faults develop produce feature like plateaus, basin, Block Mountain (host) and escapements.
 (b) Sea level changes due to the upward lift of the land or sinking of the land.

  NB: This changes in the sea level is not eustatic change (not eustatic movement)but is due to vertical forces.

      The eustatic change is the changes of the sea level due to ice melt during ice ages

 

II LATERAL/HORIZONTAL MOVEMENTS
These are also referred as organic forces (movement) because they are responsible for the build of the mountains (Orogenesis means the process of mountain building)
lateral forces are of two(2) types:-

                               1) Compressional forces
                               2) Tensional forces.

 Compressional forces: - Are forces which move towards each other ie move against each other.
                                  - They tend to shorten the crust (the land) i.e. they squeeze the land.

They normally cause :-
                     (i)  Folding of land hence fold mountains
                     (ii) Break the land to form faulting which may produce features like block mountains, rift valley and faults.
                              

Tensional forces: - Are forces that tend to stretch the land i.e. the force move away from each other, they pull the land away.

                          –The forces cause faulting of the crust and produce features like faults, Block Mountains, rift valley.

 

 FEATURES PRODUCED BY COMPRESSIONAL FORCES
1.Fold Mountains.
         Fold Mountains are formed when a piece of the crust is subjected to compressional forces. These forces tend to shorten the crust so the land end up in folding(wrinkling or crumpling) hence develop fold.

-The earth’s crustal rocks produced upward fold known as anticlines (up fold) and downward folds known as synclines (down fold)

                    

          




             The anticline the crest part while the syncline is the trough part.





                                                                  TYPES OF FOLDS

 

i)simple folds or symmetric fold:- the fold has equal limbs.

 


      

ii)Asymmetric fold :-is the fold with one limb steeper than the other.

       iii) Over fold:-one limb is pushed over the other.

       

 


 

 

iv) Recumbent fold:-the limb is pushed over the other completely.

       

 



v)Overthrust fold(nappe):- when temperature is very great a fracture occurs in the fold (such that one limb breaks off)and the one limb is pushed over the other limb along the line of thrust plane

       



                                                                                      

 

Example of fold mountains are Atlas (N. Africa), Aplas(in cape ranges in s. Africa),Himalayas(in Asia),Urals(in Rocky of N.America), Andes(s. America), Appalachian(U.S.A) Great divide range in Australia.

 

2) Rift valley (grabens).

     This can be formed by both tensional and compressional forces.

 Rift valley formed by compressional forces

 

How  it formed:-

 (a)layers of rocks are subjected to compressional force

 



  b)Fault develop                      



c)The side blocks(outer blocks) move (thrust)upward over the center block

                                 



(d)later the over-hanging sides of the rift valley are removed (worn & back)by erosion
                         



               3) Block mountains (Horst) by compressional

 Are formed when compressional force cause the formation of series of fault, such that the central block forced up (to form a block).Examples usambara, Ruwenzori, Vosges and Black forest.

 
How is block mountains formed by compressional force:-

 a)A piece of crustal rock is subjected to compressional force

                       



b) A series of fault develops.

                



c) The side blocks (outer blocks)and central block move (thrust) upward.
                    



 NB: Where series of faults occur both block mountain and rift valleys develop.

 

FEATURES PRODUCED BY TENSIONAL FORCES

 Tensional forces can frm due to fault there are several features which may be produced they include Rift valley, block mountains(horsts) and fault.

 1)Rift valley (Grabens)

 A rift valley is an elongated trough with steep sides.

A rift valley can either be formed due to tensional or compressional forces .

 

Formation of a rift valley by tensional forces

 a).The line of weakness develop to form normal fault (Normal faults are formed )

 b). The central block subsides(sinks)to form a rift valley

                        



3. Block mountains (horsts) by tensional

 These are tables like mountains formed due to fault

 They are note extensive like fold mountains ,they have almost flat surface

 Example Vosges and Black forest mountains(Europe) -Usambara, Uluguru in -Ruwenzori and Uganda boarder-Mount Sinai in asia.

 
Formation of blocks(horsts)by tensional forces.

 a)A piece of crust is subjected to tensional force.

                   



b)A series of fault develo
                           



  c)some parts subside leaving some standing high as block

              



where series of fault develop both block mountains and rift valleys are formed.

 Fault scarp.

 This is an escapement which is a steep slope when the land falls from a higher to a lower level

-it is formed soon after displacement

                         



    

 

Note

 First -Tensional forces cause fault only

        -Tensional forces cause the normal fault

Second-compressional forces cause faulting and folding
         -compressional force causes the reverse fault

 Faulting: is the process which involves the fracturing of the crust rock due to tectonic forces to form faults.

            This process is associated with the movement. The rock block (i.e. rock displacement

  A fault –is a fracture in the crust due to stress or strain associated with rock displacement.

 Fault is cause by both tensional and compressional forces

 Joint: is a crack in the rock due to stress but is not associated with rock displacement

              



                 

 -The paths of a fault on the surface of the earth s called the fault line (line of fault)

-some of the fault line may cover hundreds of kilometers.

 

FOLDING   :is the wrinkling ,bending or crumpling of the earth crut producing upward folds(anticlines) and downward folds (synclines)

 -folding is caused by compressional forces.

 

 Significance of features produced by earth movements(effect of feature produced by earths movement).

 -Both vertical and lateral force cause either folding and or faulting which in turn from fold and block mountains ,plateaus , rift valley basin and other feature s

These different features have the following positive (good)effects and negative( bad)effects.

 

 Positive (good) effects

 1.climatic influence mountain both fold and block receive orographic rainfall due to relief influence the windward side receive heavy rain at ll temperature are modified into cool(cold)

 2.source of river : d to heavy rains and snow on mountains many rivers originate from them. Water used for irrigation.HEP generation, industrial and domestic uses.

 3.source of minerals:-different minerals are obtained from fold mountain such as coal in Appalachian mountain gold in Colombia, silver in Peru.

 -other mineral from block mountains example dolomite

 4.attract tourism

 Mountain skimming attract a lot of tourists for sporting activities (skidding) e.g. the cross Alps

 -also rift valley forms a very attractive area due to presence of different lakes like Tanganyika , Natron, Manyra, Naivasha, etc ,presence of different volcanic cones and crater within it

 5.slope of mountains have fertile soils for agriculture

 Negative (bad) effects

 1.The leeward side receive little or no rainfall hence hinder agricultural and a lot

 2.sometimes folding causes some minerals to be buried deeper hence not easy to exploits

 3.fault may cause some rivers to disappear I the ground

 4.both mountains and rift valley hinder communication network

 some mount sides are too steep or ruge, or have thick fog hidering both land and air transport

 -earth movement also cause earthquakes and volcanic

2.4: vulcanicity

Concepts from this subtopic

a)       Differentiate vulcanicity from volcanicity

b)      Explain causes of vulcanicity, volcanicity and resulting features

c)       Classify features resulting from the process of vulcanicity

d)      Locate distribution of major volcanic zones in the world

e)      Assess economic importance of volcanoes

Vulcanicity is the process wich molten materials and gases are forced onto the earh’s  surface or within the earth’s crust.

 volcanicity refers to the process whereby molten materials are forced onto the earth’s surface.

When molten materials are still inside the earth’s crust is called magma

When molten materials are forced and cool on the earth surface is called lava

Vulcanicity lead to formation of intrusive and extrusive features

Causes of vulcanicity

  1. it is caused by excessive weight of overlying rocks on the mantle.
  2. Friction at boundaries of tectonic plate
  3. Surface water infiltrate in ground and forced out with high pressure

Types of vulcanicity

  1. Intrusive vulcanicity
  2. Extrusive vulcanicity

Intrusive vulcanicity refers to ejection of molten materials within the earth crust

Extrusive vulcanicity refers to the ejection of molten materials on the earth surface

This two types result to intrusive volcanic features and extrusive volcanic features

(a) Dyke: It 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



(b) Sill: This is an intrusive feature which lies horizontally along the bedding planes. It is formed when magma cools and solidifies horizontally a long a bedding plane. It is a small scale intrusive feature.



 

(c) Lacolith: This is an intrusive feature which looks like a mushroom. 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.



(d)Lapolith: This 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.



(e) Phacolith: This 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.



(f) Batholith: This 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 Gabon (the chailu massif).



  (b) Extrusive volcanic features

These features are formed when molten materials reach the earth’s surface through vents or features.

volcano is formed from accumulation of molten rock which flows out through a vent on the earth’s surface

Types of volcano

  1. Active volcano
  2. Dormant volcano
  3. Extinct volcano

Active volcano is a one which have regular eruptions. For example mount Oldonyo Lengai in Tanzania and Mount longonoti in Kenya.

Dormant volcano is a one that erupt rarely but still shows sign of eruption. Such sign includes rumbling, gaseous emission and lava flows. For example is mountain Kilimanjaro and Meru in Tanzania

Extinct volcano is one which shows no sign of eruption, although it was formed through volcanic activities. For example is Sabinyo in Uganda and Mpoli in Tanzania

Volcanic activities may form various features on the earth surface as follows

(a) 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.



(b) Composite volcano (strata –volcano or complex cone)

It is a volcanic cone which is made of alternating layers of lava and pyroclasts (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 Oldoinyo Lengai in Tanzania; mount Kenya, Longonot, Suzwa and Elgon in Kenya.



(c) Volcanic plug (plug dome / spine)

It is formed when a mass of viscous acid magma is forced out through a  vent from the ground. Examples are mount palace in France; Hoggar mountains in Algeria



(d) Crater

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



(e) Caldera

This 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 and Embagai in northern Tanzania, Vesuvius in Italy etc



(f) Acidic lava cone

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


(g) Shield volcano

Is an extensive cone with gentle slope sides formed when basic lava is poured onto the earth surface and spread in a large area



Other associated volcanic features

  1. Solfatara is volcanic hole emitting sulphurous gases as a dominant and water vapour
  2. Fumarole is a volcano which emit steam, mud and gases like Sulphur.
  3. Moffete is a volcano which emits carbon dioxide gas
  4. Hot spring is outflow of superheated water from the ground. Example of hot spring is Amboni Tanga
  5. Geyser is forceful emission of superheated water into the air.

 



Condition necessary for hot spring and geyser formation

  1. Presence of fault and joints
  2. Presence of hot rock materials in the ground
  3. Presence of underground water (aquifer)
  4. Presence of high pressure. This lead to geyser formation.

The world distribution of major volcanic zones

  1. zone of recent mountain building; for example mountain building such as south east Asia.
  2. Zone of divergent boundaries. For example mid- Atlantic ridge in Atlantic ocean.
  3. Zone of convergent boundaries. For example Island of japan.
  4. Faulting regions. Such as rift valley in Tanzania.
  5. Continental coastlines

The economic importance of volcanic activities

  1. It produce fertile soil for agriculture activities.
  2. It provides minerals. For example gold and diamonds
  3. It facilities electric power production.
  4. It provides building materials. Like sands
  5. it attracts tourism. For example mountain Kilimanjaro.
  6. It creates employment opportunity.
  7. It helps for water storage. For example underground water

Negative effects of volcanic eruption

  1. Death of people
  2. Air pollution, it produce harmful to atmosphere
  3. Destruction of properties. For example houses
  4. Destruction of agricultural crops.

2.5: Earthquake

Concepts from this subtopic

  1. Define earthquake, epicenter and focus
  2. Describe how earthquake can be detected
  3. Explain causes and effect of earthquake
  4. locate the possible areas where earthquake is likely to occur on world map

Earthquake -These are sudden movement or vibration of the earth's crust along the line of weakness

q  -Earthquake is caused by shocks that are transmitted in form of waves from their point of origin inside the earth.

q  -The point of origin of earthquakes inside the earth is called Focus

q  -Epicenter is the point where shocks is experienced from the focus point

Focus is point in the earth’s crust where an earthquake originates

Epicenter is the point on the earth’s surface that is vertically and immediately above the point of origin

Earthquake occurrence leads to waves production from focus, the waves produced are known as seismic waves

There are two types of seismic waves

  1. Body waves
  2. Surface waves

(i) Body waves These are waves that travel through the earth's crust

They are two types

(a) Primary waves (P-waves) these are waves that travel forward in a horizontal direction

q  They resulted in the forth and back movement

q  These wave have a short wavelength and travel fast in the interior of the earth

(b) Secondary waves (S waves)

q  These are waves travel forward that cause the earth to move side to side  at right angle to the direction of the wave movements.

q  These waves are medium size wavelength, they cannot travel in liquids

(i) Surface waves These are waves that travel on the earth surface

There are two types of surface waves

(a)Love (L) waves

q  These are waves that cause the part of the earth to move side to side at right angle to direction of wave movement.

(b) Rayleigh ( R) waves

q  These are waves cause the earth to move vertical circular movement for example. Waves of the ocean.

How earthquake can be detected

q  Seismograph or seismometer is an instrument used to detect and measure the seismic wave by recording seismic waves

q  Seismograph plots the intensity in form of seismogram.

q  Seismogram is a graph that shows the intensity of the earth’s vibration.

q  Richter scale is measurement of severity of earthquake by its magnitude. The scale reading ranges from 0 to 9.

q  Magnitude is energy released by the vibration

q  The mercall scale is used to measure intensity of earthquake which varies place to place

q  Intensity is damage caused by vibration

Causes of the earthquake

Natural causes of earthquakes

(i)Volcanic eruption

q  it occurs when magma moves under the influence of intense heat and pressure from within the earth crust

q  it resulted to shaking the part of the earth

(ii) Plate tectonic movement

q  This is caused by faulting of the crust leads one plate to slide from another or move against each other  hence waves are produced due to shaking of the earth.

(iii) Falling of heavy objects from atmosphere. For example, meteorite

Human causes of earthquakes

      (i)            Mass wasting

q  refers to the movement weathered material down the slope due to gravitational force

q  Mass wasting can be of rock fall or land slide that cause the shaking of the earth

(ii) Movement of a big number of animals resulted to earth tremor on local scale

(iii) Human activities, for example mining activities, railway transport and road construction

Effect of earthquake

  1. It leads to the death of people

q  700 people died in Francisco 1906

  1. It causes destruction of properties.

q  For example losses of houses

q  It happened in kegera region 2016, September, 10th.

  1. It leads to fire outbreak

q  This is caused by falling of electric poles

  1. It causes sea floor to rise or subside

q  The depth of the sea in some areas of the coast of Morocco decreased from 40m to 15m after the 1960 earthquake

World distribution of earthquakes

(a)    Mid-oceanic ridge like the entire of the mid Atlantic Ocean

(b)    The ocean deep trenches and volcanic islands of the Pacific Ocean

(c)     The region of crustal compression (Young fold mountains) stretching across southern Europe and Asia linking the Atlantic and Pacific Oceans;

(d)    Rift valley areas such as the Great East African Rift Valley and areas which are vulnerable to earthquakes in Tanzania (Examples are Kagera Region in North-Western Tanzania and Rungwe in Southern Tanzania highlands)

e)      The circum-pacific belt, which includes the West Coast of North and South America, Japan, the Philippines and East Indies

f)        The belt from Atlas in North West Africa, the Alpines in Southern Europe to the Himalayas; and

g)       Along all the boundaries of the tectonic plates

Precautions to be taken prior to earthquake incident

  1. People should not construct building in fault areas
  2. Provision of education. It based on training people what to do during earthquake. For example should no run when earthquake occurs and stay away with large buildings
  3. preparedness of earthquake, for example ambulance and helicopter services
  4. Emergency communication plan should be made.

What to do during earthquake occurrence

  1. If you are inside move outside and stay in open space or slide under the table or bed and cover your to protect yourself from sharp object.
  2. It you are outside avoid staying near buildings, electric poles and electric cables
  3. If you are in moving car stop quickly safe way. Stop area with no building and trees
  4. Avoid using roads, bridges or humps that might have been damaged by the earthquake.

External forces of the earth

External forces of the earth are those forces that operate and modify the land surface.

Those forces operate on the earth surface and influenced by either wind, water, ice temperature, gravitation force and human activities 

External forces involve mass wasting, weathering and (Erosion, transportation and deposition) by water, ice, wind and wave action 

(a) WEATHERING 

Weathering refers to the disintegration of the rock exposed on the surface. 

Types of weathering 

  Mechanical weathering 

  Chemical weathering 

Mechanical weathering 

Refers to the disintegration of the rock without chemical composition. This process is caused by temperature, frost and rainfall.

process of mechanical weathering occurrence

1. Insulation (thermal expansion) weathering. This weathering is brought about changes temperature. This process results to the following effects;

(a)    Exfoliation; it refers to the peeling off the outer layer of the rock like an onion.

(b)     Granular disintegration; it is breaking up of the rock which consist of different minerals. These minerals expand and contract separately through temperature changes. 

(c)      Block disintegration; this takes place when the rock with homogenous minerals breaks into regular blocks due to changes in temperature. This is common when the rock is well joined. This process can be aided by chemical weathering.  

(d)    2.Frost shattering (frost action); It occurs when water freeze in rock caves.

(e)    this is common temperature areas as well as highlands and deserts.

(f)      3. Crystal growth (Salt crystallization); this happens when salt crystals are deposited in the rock cracks or pore during evaporation

(g)    4. Pressure release; this occur where denudation has taken place to a great extent. As the materials are removed in a certain area the pressure is released due to the reduction in weight(unloading).

(h)    5. Alternate wetting and drying; some rocks expand after absorbing water and contrast when they become dry.

(i)      Biological (biotic) weathering 

(j)      Refers to disintegration of the rock due to action of living organism.

(k)    -By plants, the roots of plants especially of trees can force apart the joints and cracks in the rock leading to breaking of the rock 

(l)      -By animals, the burrowing animal tend to break up the surface rock example rodents, rats

(m)  -By human being, when cultivating and quarrying, human being can end up breaking the rock into small particles. 

  Chemical weathering 

This is the process which involves the decomposition or decay of the rock.

chemical weathering includes the following process.

(a)    Oxidation; is the process which rocks minerals are oxidized when they combine with oxygen in the presence of water.



(b) Carbonation this occurs in areas with limestone rocks and chalk which react with directly with carbon dioxide contained in rainwater to form weak carbonic acid. 



(c) Hydration; this is the process in which certain minerals absorb water and swell causing internal stress and fracturing of the rock. Since hydration is associated with other processes the rock can change into new compounds.  

(d)Hydrolysis; this involves the reaction between H2O and minerals in rock producing new chemical compounds. In this process hydrogen of water combined with the iron in minerals hydrolysis is common the reaction between feldspar and water where by feldspar breaks down to form clay and silica.



(e) Solution; this is the process in which some soluble minerals simplify dissolve in things like salt rock, which on dissolving leads to the disappearance of the rock. 

(f) Biotic chemical weathering 

Some plants produce h umic acid which lead to the decay of the rock such plants are like lictuns and moss. 

The following are the differences between mechanical and chemical processes

 


 Factors affecting weathering

q  Nature of the rock

q  Climate

q  Vegetation

q  Chemical composition

The significance of weathering 

  The weathering process produces new substances such as clay that used in pottery and brick making. 

  When the rocks are weathering they become weak and this makes easy for people to exploit them by quarrying 

  Weathering helps in modifying the land forms

  Weathering can produce feature which may result to tourism activities such as Bismark rock at Mwanza. 

  Weathering resulted to formation of soil. The materials for soil formation is rock particles 

2.7: Mass wasting

Concept from this subopic

a.      Define mass wasting

b.      Identify type of mass wasting

c.       Describes factors which cause mass wasting

d.      Assess the effect of mass wasting to environment

Meaning of mass wasting

Mass wasting is movement of weathered material down the slope under the influence of gravitational forces.

Types of mass wasting

        I.            Slow mass wasting

      II.            Rapid mass wasting

Slow mass wasting refers to the slow movement of weathered material down the slope that cannot be noticeable.

 Types of slow mass wasting

i.                    Soil creep

ii.                   Talus creep

iii.                 Solifluction or slugging

 

i.                    Soil creep

is the slowest and unobservable movement of weathered materials mainly fine soil, down a gentle slope.

Soil creep can have manifested by bending of electric poles and trees, also accumulation mound of soil behind the wall of house.



   Ii.        Talus creep or scree

is a slow movement of angular waste rock of all sizes down a slope.

It is common on the sides of mountains, hills and scarps.

It occurs where freeze and thaw is common.



Solifluction (slugging)

This is movement of gravel mixed with soil saturated with water, down a slope.

It is common in mountain and cold climatic area. This cold region cause thawing which lead to flow of layer underling frozen ground.

This movement lead to deposition of materials called solifluction lobes.



 Rapid mass wasting

This is a fast and sudden movement of rock materials.

Types of rapid mass wasting

        i.            Mudflow

       ii.            Earthflow

     iii.            Landslide

     iv.            Rockslide

       v.            rock fall

     vi.            avalanche

i. mudflow

This is rapid movement of saturated soil containing gravels and boulders, down a slope.

It is common in arid and semi-arid region



ii.  earthflow

It takes place after heavy rainfall where soil and other weathered materials mix with water break away from the slope in a short time.


iii. landslide

Refers to slumping of the land or rock.

It is a rapid movement of a large mass of debris, rocks or earth down a hill or mountain side.

This is caused by lubricant action under influence of water and deep undercutting of base slope.

iv. Rockslide

This is a sliding movement of a rock down a steep slope.

This movement does not involve rotation of rock.



Rockfall This is a free fall of rock masses from the top to the foot of a cliff.



Avalanche 

is a sudden sliding or falling of a large mass of snow, ice and loose rock materials down a mountain side.



Factors affecting the nature and speed of mass wasting

(a) The nature of the materials and the extent of saturation

 (b) The angle of slope. The steeper the slope the faster the rate of movement.

(c) Climate: This includes the amount and nature of rainfall, annual and daily temperature ranges. Heavy rain or alternate freezing and thawing encourages movement.

(d) The influence of vegetation: 

 (e) Human activities: Mining, building and animals keeping influence instability of the earth which might cause movement of the earth.

Effects of mass wasting on the environment

(a) Formation of fertile soil: The materials which are transported down slope tend to accumulate at the foot of the hills or mountains. 

 (b) Land degradation: Mass wasting erodes the land on steep slopes thus making it unsuitable for agriculture.

(c) Tourist attraction: Features resulting from mass wasting such as rockfall and avalanches attract tourists. For example, the avalanches of Switzerland attract a large number of tourists which in turn boost the economy of the country.

 (d) Formation of lakes: weathered materials down the slope might become barrier where river flow which lead to formation of lake.

(e) Damage of property and loss of life: Mass wasting can destroy buildings, means of transport, and lines of communication and can even cause loss of life

EROSION AND DEPOSITION BY RUNNING WATER, ICE, WIND AND WAVE ACTION

Concepts from this sub topic

a.            To define the concept of erosion and deposition

b.            Examine how agents of erosion and deposition operate on the landscape

c.             Examine erosion and deposition features for each agents

d.            Assess the importance of erosional and depositional features to human beings.

THE WORK OF RIVER

A river is a large amount of water flowing in a defined channel from a source

A river performs triple functions namely

  1. erosion
  2. transportation
  3. deposition of eroded materials

River erosion

This occurs in four process

  1. hydraulic action,
  2.  corrasion or abrasion,
  3.  attrition and
  4. corrosion or solution
  5. Hydraulic action is an erosive process caused by the force of moving water against river banks and river floor.
  6. Corrasion also known as abrasion is an erosive process responsible for wearing away of the bed and banks of a river, using the load carried by the river.
  7. Attrition involves disintegration of the load itself due to constant collision of fragments with each other and colliding with river banks or the river bed.
  8. Corrosion also known as solution is a process by which rock is weathered through chemical reaction and solution.

Those four functions lead to erosion of the river into three ways

  1. Vertical erosion. This involves the down-cutting of the river bed which consequently deepens the river channel.
  2. Lateral erosion. This involves the side cutting of the river banks and as a result, the river channel is widened.
  3.  Headward erosion. This involves upward erosion that lengthens the river, stream at its head

River transportation

Four ways of river transportation

  1. Solution
  2. saltation
  3. Traction
  4. suspension.
  5. Traction is a way in which a river transports its load where large boulders and rocks are rolled along the river bed.
  6. Saltation is the type of river transportation process which involves small pebbles and stones being dragged, slid or bounced along the river bed. This kind of transportation is common near the source of a river.
  7.  Suspension is a river transportation method where a suspended lighter load, is carried along in the water. This is common near the mouth of a river
  8.  Solution is a type of river transportation where the dissolved load is transported downstream through solution. Limestone can be transported in this form.

River deposition This is a geographical process in which sediments, soil and rocks are accumulated on the river bed, often at the lower stage of the river

  Factors which influence River deposition are as follows

  Decrease of the volume of the river

  Decrease in the velocity (speed) of river water

  Presence of an obstacle

  Stagnation of water when the river enters the sea or lake or the flat land.

The long profile of a river is stretch of a river course from its source to its mouth is called the long profile of a river. The river profile is divided into three main sections, namely the

  1.  upper,
  2.  the middle
  3.  the lower courses





The upper course of a river

This course is also known as the young stage of a river. At this stage, a river is characterized with a very steep slope. Water flows with high speed resulting into vertical erosion. Vertical erosion leads to the development of a steep- sided V-shaped valley.

Features produced in the upper course of a river

  1. V-shaped valley

In the upper stage the river flows with high speed due to the steep slope of the river bed. As the river flows, it performs vertical erosion which results into a steep sided V-shaped valley



(ii) Interlocking spurs

As the river flows, it erodes the landscape in the upper course, winds and bends to avoid the areas of hard rock. This creates interlocking spurs which resemble interlocking parts of a zip



(iii) Rapids are formed by a sudden steepening of the slope because of unequal resistance in the successive rocks in the river profile. It is where the river flows faster than its normal speed



(iv) Waterfalls occur when there is a sharp break in a riverbed. This is a result of erosion of soft rock which lies horizontally under hard rock.




Plunge-pool These are big pot holes formed at the base of waterfalls. These are formed due high velocity of falling water into the river bed

Pot holes is a hole remained after moving pebbles


(v) River gorges and canyons

Gorges are deep and narrow elongated steep sided valleys or troughs formed as a result of vertical erosion of a river.

 A canyon is a widened deep gorge.

Conditions for formation of gorges include the following

(a)It is formed where river erosion cuts down more rapidly than the forces of weathering.

(b) It is formed when a river cuts through limestone rocks in arid regions. 

(c) When a waterfall is formed on a resistant rock layer that is horizontal or is gently dipping upstream. 

(d) When the river flows in a faulted river bed which makes it erode more vertically than laterally


The middle course of a river (mature stage of the river)

q  It is characterised by a gentle slope which decreases water velocity.

q  The major function of a river at this stage is transportation of the load and lateral erosion that opens up the valley to attain a U-shaped valley

q  Boulders and pebbles are deposited at this stage.

Features formed at this stage

      (i)            Meanders are formed when a river performs lateral erosion, which leads to the development of curves as a result of deposition of the river load on the concave side, for examples of river meanders are found along river Ruaha in Iringa, Tanzania.

    (ii)             Bluff is a steep cliff overlooking a plain or body of water, It may be composed of beach sand, or soil or a rock formation


The lower course of a river (Old stage)

q   The main function of this stage is deposition

q  The gradient of the river is very low and so deposition occurs due to insufficient energy of the river to carry its load.

q  The load carried becomes heavier and deposition takes place both on the river floor and on the river banks

Features associated with lower course of the river

      (i)            A flood plain refers to an area of gentle or flat land bordering a river with deposited sediments

q  The flood plain is characterised by deposition of a load of sand and silt carried by the river

q  It turns the area suitable for agriculture activities due to high accumulation alluvial soil


(ii)An ox-bow lake is a crescent-shaped lake that is formed from river meanders cut off from main stream in the flood plain.

Development of  ox-bow lake

   A the river has just developed meanders

    At B the meanders has become more pronounced forming a loop

  At C a river has at through a neck isolating at a bend, which later former an Ox bow lake.


(iii)Natural leeves These are ridge like or embankments produced as a result of deposition of sediments along the banks of the river in the flood plain.

 


iv. Deferred Tributary (Yazoo stream)

Is a lower course tributary forced to flow alongside the main valley or (consequent) valley. For long distance before joining the main stream.

 


(v)A delta is a low – lying swampy plain formed due to accumulation of alluvium.

Condition necessary for delta formation

  1. Presence of erosive power to the upper course of the mounth
  2. A river must have a large load
  3. The velocity of a river must be sufficiently low to allow most of its load to be deposited in the river’s mouth.
  4. The river’s load must be deposited faster they it can be removed by the action of tides and waves.
  5. There must be gentle slope to the river mouth

Stages for delta formation

  1. Accumulation of alluvium
  2. Distribution of tributaries


(iii) Development of lagoons


(iv) Extension of lagoons


Types of delta

      (i)            An arcuate delta Refers to a type of delta which is triangular in shape.

q  It is composed mainly of coarse sediments such as gravels and sand. Arcuate deltas have a large number of distributaries.

q  Examples of arcuate deltas are the Nile delta in Egypt and the Rufiji delta in Tanzania


(ii) A digitate delta It is a type of delta which looks like the foot of a bird, with claws. That is why it is referred to as a bird’s foot delta.

q   It is composed of very fine material called silt.

q  This type of delta has long distributaries which are far apart and dominated by levees.

q  Weak currents and sea waves contribute much in the formation of a digitate delta

q  The delta is characterized by few distributaries

q  Examples of such deltas are Mississippi River delta in USA


(iii) An estuarine delta This refers to a type of delta that develops in the submerged mouth of a river, called an estuary.

q  Its shape is that of an estuary.

q  Due to the presence of tides, this type of delta does not extend out to the sea.

q  Waves and currents remove the sediments as they are deposited


A cuspate delta This is a tooth -like delta, formed where a river reaches a straight coastline with a strong wave action.

q  There is uniform spreading of materials on either side of the river mouth


Sample of questions

1. Draw a long profile of a river and outline the characteristic features of each stage.

2. With the aid of diagrams, describe why erosion is dominant at the upper stage and deposition at the old stage of the river.

 3. What are the conditions necessary for the formation of an ox-bow lake?

River rejuvenation

River rejuvenation is a renewal of the erosive power of a river.

q  The river is characterized by vertical erosion at the upper course which deepens its valley.

Causes of river rejuvenation

  1. uplift or fall of the land.
  2. It is also formed where a new and an old river meet
  3. when there is increase in the river volume which can be caused by heavy rainfall or melting of ice.

Features formed due to river rejuvenation

(i)Knick points These are formed when there is a break of slope of the river bed due to occurrence of faults along the river.

q  They are marked with the presence of waterfalls

q  (ii)Paired terraces These are formed when the vertical erosive power of the river undercuts the river banks to form steps or benches on the sides of the river

q  (iii)Incised meanders These are formed when vertical erosion of the river cuts the meanders deeply into the river floor. The slopes of the meanders may differ due to undercutting and lateral erosion of a river. One side may be steeper than the other side of the slope


 

A. DRAINAGE

Drainage is the plan or layout of the river with its tributaries until it reaches its destination i.e. lake, main river, swamps or an ocean .therefore the concept drainage includes rivers , swamps, lakes, waterfalls ,flood areas

Note:-the common drainage shown on the map is rivers, swamps, lakes and ocean. But expect to see even waterfalls especially on coloured topographical map.

 

DRAINAGE PATTERNS

 -Simply means the network displayed by a river and its tributaries

-Drainage of the river usually posses different network/system depending on the way how tributaries convey to the main river and the general appearance, hence drainage pattern

 


THE FOLLOWING PATTERNS ARE COMMONLY DISPLAYED IN TOPOGRAPHICAL MAPS

 I. DENDRITIC PATTERN
-Is a pattern in which its tributaries convey (join) to the main river at an acute angle resembling to the shape of tree trunk and its branches



Note:- Dendritic pattern are common in areas of gentle slope and of uniform(homogeneous)rock hardness. Therefore it can be made from granitic or metamorphic rock

 
II.TRELLISED PATTERN
Is the pattern in which its tributaries convey or join to the main river at almost right angle.


 Note:- This type is commonly found in areas with severe cracks or fractures mostly to the rocks with an alternate hard and soft rock. Therefore this is associated with sedimentary rocks.

 
III. RADIAL PATTERN

-Is the pattern or layout in which its tributaries flow outward from the center.(summit) or at the peaks of mountains

                                                     OR

 -is a pattern that resemble a spoken ring of bicycles where stream flow out in every direction from the center.

 

-Therefore radial drainage pattern is commonly associated with volcanic mountains /region composed of granitic rock or igneous rock


 

IV. CENTRIPETAL DRAINAGE PATTERN

            -this is the pattern in which almost all streams are following from all direction converging to the center can be to the swamp, lake or depression.

                                                        OR

          -Is opposite to radial as in this type the streams flow toward a common depression center

 


Note:-The determinant factor of stream flow is a slope .Therefore the drainage pattern can be associated with sedimentary rock.

 

V. RECTANGULAR PATTERN /DRAINAGE.
    -Is a pattern which resemble trellised, but it has tributaries joining the main river at a right angle. The pattern is common in areas which are faulted.
    -Therefore can be found along sedimentary rocks/granitic rock or any faulted rock.


 

VI. BRAIDED PATTERN

       -Is the pattern in which its distributaries tend to split into several channels which rejoin and split again .The Congo River has braided channels between Lisala and river Ubangi.



VII. ANNULAR PATTERN.

       -Is the pattern with series of streams flowing on flanks or around the dissected dome, depression or crater. Where there are an alternate band of soft and hard rocks.



 Note:-This type is not so common, but is found around Lake Bosumtwi in Ghana .Is commonly found in areas affected by back tilting.

 

 

Action of Ice

q  Glaciation is the process of covering the earth with glacier or masses of ice.

q  Snow is tiny droplets of water (water vapour) in the atmosphere may also freeze to form white powdery flakes

q  Ice sheets is extensive area covered by permanent thick layer of ice and snow

q  Glacier is a mass of moving ice. For example glaciers on top of mount Kilimanjaro in Tanzania

q  A snowline is a line on a hill slope or mountain side that represents the lower limit of permanent snow.

q  The snowline is as high as 5 500 metres above sea level

Glacier erosion

  1. Plucking
  2. Abrasion
  3. Sapping

      (i)            Plucking involves the removal of blocks of bedrock which have been loosened by ice

    (ii)            Abrasion

Occurs when rock debris frozen into the base of the glacier is dragged over the surface bed rock thus eroding the surface.

(iii) Sapping

Involves wearing of rocks by alternate freezing and thawing of water at the base of cracks along the mountain sides.

Highlands glacial erosional features

      (i)            cirque This is also known as a corrie. It is a steep sided, semi-circular basin curved on the side of a mountain by a glacier

    (ii)            An arêtes This is a steep-sided-knife edged ridge separating two cirques or corries in a glaciated highland area. It is formed by plucking back of the cirque. Examples of arêtes are found on the sides of mount Matterhorn in the Swiss Alps, in Switzerland

  (iii)            A pyramidal peak It is a sharp peak formed when the sides of the cirques are cut and deepened by frost action


(iv) u-shaped valley It is a steep-sided valley with a flat floor produced by vertical and lateral erosion of moving ice. Hanging valleys are produced by unequal down cutting on the tributary valleys and the main valley

Lowlands glacial erosional features

(i)Roche moutonnee It is formed when a resistant residual mass of rock rises above the surrounding land surface. The upstream side is smoothed by ice abrasion but the leeside of the rock is plucked to form a steep slope


(ii) Crag and tail This is a mass of resistant rock with a steep ice smoothed rock face at one end and a gentle slope of a rock glacier drift at the other side of the rock


Glacial depositional features

      (i)            Moraine These are rock particles and fragments mainly transported and deposited by glacier.

q  There are four types of moraines

  1. Lateral moraines
  2. Medial moraines
  3. Ground moraines
  4. Terminal moraines
  5. A lateral moraine is the load carried along the sides of a glacier
  6. A medial moraine is formed when adjacent lateral moraines join
  7. terminal moraine is a bulk of the debris transported at the front of the glacier
  8. A ground moraine is carried as load at the bottom of the glacier


(ii)Drumlins

These are elongated hills or ridges of boulder clay usually oval and half egg-like shape. They occur about 1 kilometer long and between 25 to 100 meters high

(iii) Eskers

These are long, narrow, meandering ridges of sand and gravel. They are about 40 meters high and their materials occur in layers. They are formed when ice retreats leaving bonded materials following the pattern of the stream

(iii) Outwash plains

These are wide, gently sloping plains of fluvial glacial deposits of sand, gravel and silt which result from massive volumes of melt water, spreading materials

(iv) Erratics These are boulders of varying sizes which are deposited by ice sheets and glaciers when the ice melts. Erratics help in tracing the direction of the quaternary ice flow but when they occur in large numbers, they hinder farming activities.

(v) Boulder and clay plain

It is a uniform boulder plain resulting from deposition by ice sheets and glaciers. It is an extensive flat low-lying land consisting of boulders and clay that were randomly deposited by ice sheets

(vi) Kettle lake is depression or hole in an outwash plain formed by retreating glaciers or draining flood water.


Importance of glaciation

  1. Formation of lake
  2. Attraction of tourists
  3. Formation of fertile soil. Especially moraine formed to the bottom of mountain
  4. Sports during winter season
  5. Generation of hydro electric power. For example in area of hanging valley that form water fall

The action of wind and water in desert areas

q  Wind is air in motion. The frequently blowing wind in desert and semi-desert regions normally act as a key agent of erosion, transportation and deposition.

q  A desert is a landscape characterised by sparse or no vegetation and receives very little or no rainfall for prolonged period of time.

q  Determinants of wind action The action of wind could be slow or fast depending on the speed and strength of the wind, nature of the load being transported, obstacles in the path of the wind such as vegetation cover as well as weather conditions

q  the speed and strength of wind determine the distance and size of the load transported. Winds with high velocity are usually stronger, and therefore transport more heavier materials than winds of lower velocity.

Wind erosion

The mechanism of erosion by wind involves three processes, namely

  1. Deflation
  2. abrasion
  3. attrition.

  Deflation refers to the blowing away of rock waste that lowers the land surface and produces depressions that are sometimes very extensive.

Abrasion This refers to the constant blasting, grinding and scratching of rock surfaces by sand and rock fragments carried by wind, which hurls these against the rock.

q  Abrasion contributes to the formation of features like rock pedestals, zeugens, yardangs and ventifacts.

Attrition

refers to the breaking up of rock particles through collision against each other as they are transported by wind from one place to another.

q  This process results into the formation of sand dunes and barchans

Features formed by wind erosion

      (i)            Rock pedestals are tower-like structures or shapes composed of alternate bands of soft and hard rocks. They are also called mushroom rocks. Example of the rock pedestals is in the Gara Mountains in Saudi Arabia


(ii) Yardangs are elongated rock ridges of about 100-500 metres long with gentle slopes separated from one another by furrows.

q