Home ADVANCED LEVEL EXTERNAL FORCES ~ PHYSICAL GEOGRAPHY FORM 5 & 6

EXTERNAL FORCES ~ PHYSICAL GEOGRAPHY FORM 5 & 6

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EXOGENETIC FORCES WHICH SCULPTURE THE SURFACE OF THE EARTH

These are external forces.  They operate on the surface of the earth.

a) Denudation:  

These are destructive forces which lower the earth’s surface

i. Weathering / mass wasting

ii. Erosion

iii. Transportation

b) Deposition – Constructive process which raise the eland

DENUDATION

Denudation refers to all processes that involve the breaking, wearing away and lowering of the surface of the earth. Denudation involves several process such as weathering, Mass wasting, Erosion and Transport of materials.

Deposition:This is the laying down or release of the rock particles on the surface,It can be by:

1.Water producing features like flood plains.Natural Levees , alluvials fans,beaches etc.

2.By ice producing features like out wash plains ,clay plains,moraines ,eskers drumlins and kames.

3. By living organisms producing features like coral reefs.

4.By wind producing feature like loess plains and sand dunes(barchans and seifs)

5. By evaporation and precipitation producing features like salt deposits.

I. WEATHERING

– Is the physical disintegration and chemical decomposition/decay of rocks in situ / when exposed to weather.

Weathering does not involve transport, in weathering there is no large scale movement thus weathering doesn’t involve transport.

In situ in the original or appropriate position.

Types of weathering

There are two types of weathering

i. Mechanical / Physical weathering

Is the breaking down of rocks into small particles / fragments without involving (undergoing) any chemical means / changes

Mechanism (How does it take place)

i) Temperature change:  This is best noticed / occurs mostly in hot deserts / tropical deserts.  (Dominant) where there is large diurnal range of temperature during the day, temperature is very high (450c).  The rock surfaces are intensively heated which causes rapid expansion which causes cracks on the surface of the rock (Parallel cracks).

During the night, temperature falls rapidly (20C).  Range is 430C. Rapid contraction on the rock surface which will increase cracks which are vertical / perpendicular to the surface.

–   This process takes a long time (100years) and results to the peeling of rocks on the surface “Onion Peeling”

The process is called EXFOLIATION.

–   A smooth mould which is called exfoliation dome remains after exfoliation has taken place

–   Exfoliation particles are called Screes.  These are particles which collect on the foot of a hill / rock mass  This happens on daily bases.

Why do we have diurnal range of temperature?

Because of absence of cloud cover.  The incoming solar radiations reach at maximum and causes high temperature.  And there is rapid cooling because the heat is not retained hence it removes.

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ii. Frost Action : – This is a process which is dominant in temperate regions and in high mountains (mountainous regions)

–  The process occurs during summer where there melting and during winter there is freezing.  It is a process which occurs seasonally.

–  When image002  freezes it expands by 9% of its original volume this develops a crack.  When the image002  melts it will fill the crack and when the next winter reaches the water will freeze and will expand the crack. This is continuous action which happens seasonally and eventually will lead to the disintegration of the rock.

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iii. Action of living organisms: (biotic / biological).

a)  Plants action – Root Penetration. As the plant grows the roots grow also and penetrate in the surface which causes cracks on the ground.

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b. Animal action.

a) Micro-organism (barrowing animals i.e. roddents,rats):

These animals make some holes on the soil whereby their activities in the soil allow weathering to reach the bedrock. This makes the soil loose hence the rocks break down.

b)Macro-organism (trampling animals and man i.e. cows, elephants) :

These animals encourage soil erosion also poor agricultural methods used by man and deforestation expose bed rock to weather.

iv. Alternating / alternate wetting and drying: –

 This can be dominant in coastal areas.  During high tides the rock is exposed to water which causes contraction to the rock and during low tides the rock expands because it is not exposed to water.  This happens daily as a result it weakens the rock and hence it breaks (it is exposed to weathering).

v. Salt Crystallization:

When salt water is in the rock, during dry time the water will evaporate and salt crystals will remain, these crystals are solid they will occupy space

by expanding the rock and when the salt water returns the rock will contract.  This continues and will hence develop a crack.

vi. Pressure release or unloading

It always occurs in large scale than other processes discussed it resulted from the unloading rocks during exhumation.  As the overlying materials are removed the consolidating pressure are released and the rock tends to rebound.

The stresses created open up joints and bedding planes.  Rarely the process result to direct disintegration but is seems to be a wide spread means by which rocks are weakened and make more susceptible to other weathering processes.

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ii) Chemical weathering

This is a process of decaying or decomposition of the rock when exposed to weather.  In such process there is chemical change which weakens the rock, and makes it easily to break down by other forces / weathering agents.

Chemical weathering occurs under the following processes:

i.      Oxidation 

ii.       Carbonation        

iii.      Hydration     

iv.      Hydrolysis   

v.       Solution

i. OXIDATION:

Is the addition of Oxygen to the rock mineral. A process whereby certain mineral compounds takes additional oxygen.  In that process it becomes weak and easily broken down. This is common in clay which contains iron compound when iron rusts it is oxidation.

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ii. CARBONATION – This process takes place where there is plenty of carbon dioxide and moisture.

Is a chemical process where weak carbonic acid reacts with calcium carbonate to form calcium bi carbonate which is soluble compound which is readily removed in solution by ground water.

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Carbon dioxide will dissolve in rain image011O which will form weak carbonic acid, when reaches on the ground will react with lime stone in the rock to form calcium bicarbonate.  The solution will come out of the rock and make the rock weak.

iii.  HYDRATION

Some rocks take in additional water molecules which causes stress to the rock / expansion and that expansion causes internal stress and fracture.  In this case the rock becomes weak and easily broken down by other processes.

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iv. HYDROLYSIS:

This process involves hydrogen (in the water) combining with certain metal ions (in a mineral) that is the water and the mineral react chemically which gives rise to the formation of different chemical compounds.

H2O reacts with minerals rocks and carbon dioxide to form clay minerals, silica and potassium carbonate which is soluble and leaches out leaving the rock weak.

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v. SOLUTION – Nacl + H2O = salt solution.

Where rock comes into contact with water and salt it disintegrates.

vi. BIOTIC WEATHERING    –Associated by plant roots, where by root tips are secretes acid which react with rocks and they disintegrate

Decaying organs produce organic acids which when come into contact with rocks it disintegrates.

Organic acid of decaying vegetation – weathering of rock

FACTORS WHICH INFLUENCE THE RATE OF WEATHERING

1.THE NATURE OF THE ROCK – soft or hard

a) Mineral composition.  –  This varies from one rock to another and hence affects its resistance some rocks tend to be stable, while others are unstable depending on their mineral composition.  For example, rocks like granite with quartz are stable and very resistant to weathering hence weathering process will be slow which rocks which are unstable like basalt which have lower silica undergo weathering very fast.  Iron minerals undergo oxidation very fast; rocky salt (sodium chloride) rocks dissolve quickly in water.

b) Plane of weakness or rock structure – Through joints, weathering takes place very fast, since acids can penetrate easily into the rock and cause the rock to decay during chemical weathering.  And frost action tends to be fast in the areas where the rocks have some cracks.

c) Color- rocks with dark minerals (olivine) heat faster than rocks with light minerals.  Thus, the rate of weathering by temperature change is higher to rocks with dark minerals.

2. CLIMATE

Variation of climate cause differences in the rate and type of weathering.  The main climatic controls are of temperature and humidity.  However the role of climate in weathering is extremely varied region to regions depending in temperature and rainfall patterns.

Hence type of climate determines the rate and type of weathering.

a) Equatorial regions – These are characterized by high temperature and rainfall throughout the year.  Chemical composition of rocks is very active in these latitudes due to high temperature and high rainfalls totals.

b)Tropical (Savannah) regions – These are characterized by the seasonal variation of rainfall and temperature i.e. there is dry and wet seasons.  Due to the presence of both seasons, both chemical weathering and mechanical weathering takes place. Chemical weathering during wet seasons and mechanical weathering during dry seasons.

c) Hot Deserts – These are characterized by large diurnal temperature range i.e. day temperatures are extremely high while night temperature fall more rapidly.  There is low amount of rainfall and excessive evaporation.  Both mechanical and chemical weathering take place mechanical by the process of exfoliation and (frost action) and chemical by salt crystallization due to excessive evaporation.

d) Mountain regions – These are characterized by high humidity and low temperature.  Hence, frost action is active in tropical regions where there is an existence of mountains with height above 430m like Elgon and Ruwenzori, frost action is also common.

3. RELIEF (Slopes).

At steep slopes the rate of physical weathering will be fast but chemical weathering is retarded because H2O moves (no penetration of water).

On lowland, physical weathering will be slow because weathering is protected by weathered materials and chemical weathering will be faster because water is there and will influence weathering.
Under relief there is aspect -is the position of a place in relation to sun rays. North facing slopes do not face sunlight hence it is less developed and weathering takes place slowly but in the south facing slope, weathering takes place fast and well developed soil and good vegetation.

  •   In the tropical regions there is no aspects
  •   In mountainous regions frost action takes place actively.

4. ROCK AGE.

Old rocks are more susceptible to weathering as they had enough time been much subjected to different weather forces compared to young rocks.  It is thus, the rate of weathering is higher to old rocks compared to young rocks.

5.  BIOTIC ACTIVITIES

Biotic activities contribute a lot to weathering in varied ways and include;

– Penetration of plant roots causes physical destruction of rocks.

– Some plants and animals secrets acid from their bodies leading to decomposition of rocks.

– Burrowing of animals also cause mechanical weathering

– When the living organisms in the soil respire, give out carbon dioxide gas.  The gas dissolves in water to form carbonic acid, which cause decomposition of rocks.

– In other way round, thick vegetation cover, such as tropical forests acts as a protection against physical weathering and also helps to slow down the removal of weathered materials.

– Human activities, poor methods of agriculture expose the bed rock to weathering.

Study Questions

1.  Discuss exhaustively the role of water in the weathering process.

2.   Where and for what reasons mechanical weathering is a dominant process.

3.  “Mechanical weathering and chemical weathering processes are interdependent and complementary” Discuss.

4. Weathering is not influenced by force of gravity but mass movement is influenced by force of gravity. justify

II. MASS WASTING (Mass Movement)

Is the down slope movement of weathered materials under the influence of gravity.  In this movement, there is no transporting agent.H2O is involved as a lubricant and not a transporting agent. Water helps to reduce friction of particles within weathered materials.  And also water adds to bulkiness, this will facilitate the process of mass wasting.

Types of mass wasting

Two types according to the speed of movement

a)      Slow mass movement

1.      Soil creep

2.      Solifluction

3.      Talus creep

4.      Mudflow

b)     Rapid movement

1.      Slump

2.      Rock slide   or sometimes both slump and rock slide are called “land slide”.

3.      Rock fall

1. SOIL CREEP

Is a steady downward movement of soil on all sloping land.  Rain water lubricates soil particles and enables them to slide over each other.

Ø  Is the slowest and imperceptible movement of weathered material, mainly fine soil down a gentle slope.

Ø  Soil creep can be manifested through mounds of soil behind the walls, tilting and cracking of walls, bending of trees, fences and telegraph poles as well as cracking of the road.

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In equatorial regions creep is often disguised by dense vegetation cover.

2. SOLIFLUCTION

Movement of weathered materials under frost areas. It is limited to mountain and cold climate areas where thawing causes a saturated surface layer to creep as a mass over underlying frozen ground.  (Saturated soil, gravels and weathered rock).

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3.TALUS CREEP

This is also a very slow movement of angular waste rock of all sizes (talus or scree) down a slope.  It is common on the sides of the mountains, hills and scraps.  It takes place where free- thaw action is common especially in the highlands and high latitude regions.

Large talus sheets move mass especially in mountains where freeze – thaw is frequent.  Talus moving down a valley in a long stream is a rock glacier.

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4. MUD FLOW

Movement of large volumes of unconsolidated materials which are super-saturated with water, the materials flow as semi-liquid mud (as slurry) with boulders and gravel embedded in mud.

Large volumes of unconsolidated material, super-saturated after heavy rain, become plastic and flow common in acid and semi arid region.

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5.SLUMP.

Massive rocks overlying weak rocks saturated by heavy rain common on over steepened slopes.  Large masses of rock and debris.

 

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6. ROCK SLIDE.

Is sliding movement of the slab of rock down the steep slope, no rotation is involved. It can be triggered off by earth quake or human activities like mining or cultivation.

Surface rocks sliding over a slip surface formed by bedding or fault planes dipping sharply down slope.

rockslide.jpg 7. ROCK FALL

Is a falling movement of individual rock blocks with boulders along a precipitous (steep) slope of a mountain or along road cuttings or cliffs.

Precipitous slopes in mountains where well jointed rocks may be loosened by freeze – thaw.

Rocks accumulate as a talus slope along valley sides.

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FACTORS WHICH AFFECT THE NATURE AND SPEED OF MASS WASTING

a). The degree of saturation and nature of weathered material

The more saturated the weathered material is the faster the rate of movement. Because there is more friction between particles.

Depth of weathered materials, weakly bedded and steep dipping the faster the rate of movement.

b). Gradient.

Steeper the slope, the faster the rate of movement and vice versa.

c). Climate.

Amount of rainfall, nature of rainfall , the annual and day temperature ranges. Heavy rain or alternative freezing and thawing encourage movement.

Heating and cooling.

d). Vegetation cover.

Absence of vegetation cover to hold the materials will increase the speed of mass wasting.

e). Human activities.

Mining, overgrazing and keeping of animals are among the ways in which man has affected the stability of the surface and facilitates the rate of move.

f). Earth movement

Especially earth quake which can disturb the rocks and encourage mass wasting.

  Effect of mass wasting

i. Loss of life.

Example, for those who build their settlement down the hills, rock fall many occur for mountain climbers the avalanche may fall.

ii. Destruction of property.

Example, those farms which are on the foot of the hill or on a slope may be destructed by mass wasting (soil creep, land slide, rock fall) buildings, roads, railway lines, river all these become blocked by mass wasting and land slide.

iii. Attract tourism.

The resulting feature after mass wasting has occurred can arrange the rocks in such a way that is attractive to the human eye. Also human beings like to see the effects of a disaster first hand.

iv. Land degradation.

Removal of fertile soil from the land. It leaves scars which is less value than the before land.

v. Formation of fertile soil on the foot of the hill where weathered materials have accumulated.

vi. Can dam a river to form temporary lakes. But the weathered materials are loose hence water will remove the weathered materials and the river will continue to flow.

PRECAUTIONS

–  Afforestation and reforestation on slope lands. This will help to the stability of weathered material, will reduce rate of move.

– Control human activities. Especially poor methods of agriculture (contour cultivation will stabilize the farm).

–  Avoid establishing settlement in areas which are prone to mass wasting.

–  Making of terraces across of slopes.

Study questions.

Carefully distinguish mass wasting from weathering.

Mass wasting weathering
1. Influenced by gravity 1. Disintegration and decomposition of rock.
2.  Movement of weathered materials 2. Types include physical and chemical
3. Types include slow mass movement and rapid. 3.Generally restricted to material breakdown in place
4. Processes range considerably in rate 4. Is a surface phenomenon

EROSION

Is the detachment and removal of weathered materials from the surface of the earth surface by agents of erosion.

OR Is the process of breaking up and wearing away of exposed rocks by moving water,wind and moving ice.

Agent of erosion;-

There are four agents of erosion

i. Running water

ii. Wind

iii. Glaciers (moving ice)

iv.  Waves and tidal currents.

EROSION BY RUNNING WATER

What is running water?

Is any water which falls on the ground and flows down slope under the influence of gravity.

When water in running on the surface performs three functions ,that is Erosion, Transportation and Deposition.

What happens when water fall on the surface of the earth?

  • Percolation – When H20 is absorbed in the ground.
  • Evaporation-Rain
  • Surface run off- Is most effective agent of erosion over the earth surface

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Surface run off cause’s soil erosion

Types of erosion

1. Sheet erosion.

Uniformly removal of upper soil without well defined channels.

Removal over a large area of a top layer of soil and other fine materials by a thin sheet of H20 flowing over a fairly smooth surface.

2. Rills erosion

Removal of upper soil from surface with well defined channels called rills. The impact of rills is more effective than sheet erosion.

3. Gully erosion.

If rills are not checked they will collide and come larger and to form big channels called gullies. Removal of soil by these gullies is what is called gully erosion. Gullies are also grooves or depression of v-shaped.

4.Splash erosion.

Is the rain drop impact on the surface. Particles are displaced by rain drops (loose dry materials).

Impact (result).

i. Formation of small channels which will leads to formation of badlands. ( smooth land – bad land which is less useful)

a)      Bad land

b)      Gullies

c)      Rills.

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