External Land Forming Processes - Geography Form 3 Notes

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Definition

  • Processes operating on the exterior of the earth resulting in the formation of natural physical features.
  • These are:
    1. Weathering
    2. Mass wasting
    3. Erosion
    4. Transportation
    5. Deposition


Weathering

  • Mechanical breakdown or chemical decay of rocks “in situ” (without movement)

Agents of Weathering

  • Things that work to cause it:
    1. Weather elements:
      • rainfall
      • temperature
      • frost
      • gases e.g. CO2,O2
    2. Plants
    3. Animals
    4. People

Factors That Influence Weathering

Climate

  • Different areas with different climatic elements experience different types of weathering e.g. block disintegration are experienced in arid areas while frost action is experienced in temperate regions and mountainous regions of tropics.

Topography

  • Weathering is faster on steep slopes than on gentle slopes because weathered material is washed away quickly exposing the rock once again to agents while on gentle slopes materials remain in one position shielding the rock from weathering agents.

Nature of rocks

  • Dark coloured rocks absorb more heat than light coloured ones hence break faster due to excessive expansion and contraction.
  • A rock with different minerals may disintegrate faster due to differential expansion and contraction of minerals.
  • A well jointed rock will break faster because physical and chemical agents can penetrate faster e.g. by freezing and thawing.
  • Fine textured rocks have a large surface area on which chemical processes can act e.g. Limestone.

Biological organisms

  • Bacteria facilitate rotting of organic matter producing organic acids which reacts with some minerals causing the rock to break up.
  • Plant roots and burrowing animals penetrate rocks resulting in cracks providing passage for agents such as water to act on rocks.
  • People accelerate the rate of weathering by exposing rocks buried deep below by digging, blasting and drilling.

Types of Weathering

  1. Mechanical Weathering
  • Physical break up of rocks without change in their chemical composition.

    Processes
    1. Block Disintegration/Separation
      block disintegration.PNG
      • Breaking of rocks into blocks along the joints.
      • It’s effective in arid areas because of great diurnal temperature range.
      • Day, well jointed rocks are subjected to intense heating causing minerals in it to expand.
      • In the night the rock is cooled causing it to contract.
      • The rock joints enlarge due to the alternating cooling and contraction.
      • The process is repeated over a long time causing the rock to disintegrate into blocks along the joints e.g. Mundanda rock in Tsavo East.
    2. Exfoliation
      exfoliation.PNG
      • Peeling off of layers of rocks.
      • Also common in arid areas.
      • Day, rock surface is heated more than inner layers because rocks are poor conductors of heat.
      • The surface expands more than inner layers causing strain between the two layers.
      • With time outer layer develops cracks and later peels off and pieces of rocks fall down under gravity e.g. along Mombassa-Nairobi road between Mtito Andei and Voi.
    3. Granular Disintegration
      • Disintegration of rocks into grains.
      • Occurs in rocks with different minerals.
        granular disintegration.PNG
      • When the rock is heated, different minerals expand differently.
      • Internal stress results and with time the rock disintegrates into grains.
    4. Pressure Release/Sheeting/Unloading
      • Disintegration of rocks due to expansion when weight is removed from over it.
      • Soil and other materials lying on top of a rock are removed by erosion and mass wasting (denudation).
      • The exposed rock expands when the weight that was pressing it is removed.
      • The outer layer curves and eventually shells are pulled out from the rock.
      • The result is formation of a high rocky hills called granitic tors e.g. Maragoli and parts of Machakos.
    5. Frost Action
      frost action.PNG
      • Breaking of rocks into angular blocks due to repeated freezing and thawing.
      • Common in temperate regions or mountainous regions of tropics where temperature fall below zero.
      • Water from melting ice collects into small cracks of rocks.
      • It freezes and expands and exerts pressure on cracks widening them.
      • Repeated freezing and thawing causes the rocks to break into angular blocks e.g. on Mt. Kenya, Kilimanjaro and Ruwenzori.
    6. Crystal Growth
      • Break up of rocks due to crystal growth.
      • It occurs in arid areas.
      • High rate of evaporation draws out moisture and dissolved minerals from the rock interior through capillary action.
      • The moisture evaporates when it gets to the surface of the rock leaving behind crystals in the cracks and pores of rocks.
      • The crystals continue to grow exerting pressure on the cracks or pores widening them and eventually causing the rock to break down e.g. at Hells Gate near Naivasha.

    7. Slaking/Rain Water Action
      • Breaking up of sedimentary rocks due to alternate wetting and drying.
      • When it rains, the rock absorbs water and swells.
      • When dry season comes, the rock loses water and the outer surface shrinks.
      • The process is repeated and the minerals become loosely attached to another e.g. in Kenyan Coast at Tudor and Miritini areas.
  1. Chemical Weathering
  • Weathering involving changes in the chemical composition of minerals making up rocks

    Processes
    1. Solution
      • Break up of rocks as a result of dissolving of minerals in water without chemical change in them.
      • Rain water falls on rocks with soluble minerals.
      • The minerals are dissolved and carried down in solution.
      • The rock gets weakened and crumbles.
    2. Carbonation
      • Weathering caused by reaction of calcium carbonate in rocks with rain water containing a weak carbonic acid.
      • Common in temperate regions.
      • Rain water absorbs small quantities of carbon dioxide forming a weak carbonic acid.
        H2O+CO2 → H2CO3
      • The weak carbonic acid falls on limestone rocks reacting with calcite forming calcium bicarbonate.
        CaCO3+H2CO→ Ca (HCO3)
      • Calcium bicarbonate is removed from the rock in solution.
    3. Hydrolysis
      • Weathering caused by reaction of hydrogen ions of water and ions of rock minerals.
      • Igneous rocks are greatly affected.
    4. Oxidation
      • Weathering in which minerals in rocks combine with oxygen in the presence of moisture to form new minerals.
      • Rocks containing iron are affected.
      • Ferric oxide is formed on the rock surface which appears as a soft brown or red earth which can be scooped by hands.
    5. Hydration
      • Weathering in which hygroscopic minerals in rocks take up water causing them to swell and expand causing disintegration of rock due to internal stress.
  1. Biological Weathering
  • Weathering of rocks due to action of living organisms on them.

    Processes
    1. Action of plants

      Mechanical
      • The roots grow bigger into the cracks and joints of rocks widening them.
      • With time the rock separate into blocks (wedging mechanism).
      • The widened joints and cracks also provide passages for moisture and air to penetrate deeper into cracks facilitating hydrolysis and solution to act at deeper levels.
      • Burrowing animals dig and break up small bits of rock from the main rock mass and bring them to the surface.
      • By digging they also provide passages for other elements like gases and moisture to reach rocks that are deep.
      • Large herds of animals such as cattle, zebra etc. pound the rocks with their hooves as they move resulting in resulting in mechanical breakdown of rocks.
      • People break up rocks by using explosives in mining by exploding bombs on the ground and during building of houses and construction of roads.

      Chemical
      • Plants rot on rock in the presence of moisture and produce organic acid
      • It reacts with some minerals within the rock causing decay.
      • Animals excrete on rocks and release chemical substances which react with some minerals in rocks causing them to break up.
      • Chemical substances released from the industries to rivers cause the water to act on rocks over which it flows.
      • Gases such as CO2 emitted from motor vehicles and industries are
      • Absorbed by rain and acids such as carbonic or sulphurous which react with minerals causing rock to decay.

Significance of Weathering

Positive

  1. Leads to soil formation which is important for agriculture.
  2. Produces other natural resources such as clay used in pottery, brick making, etc.
  3. Weathered rocks form beautiful scenery for tourist attraction e.g. Hells Gate and crying stones of Kakamega.
  4. Weakens rocks easing their exploitation by quarrying and mining

Negative

  • May weaken the earth’s crust resulting in unstable foundations of buildings and roads and eventually lead to their collapse.


Mass Wasting

  • Movement of weathered material down slope under the influence of gravity

Factors Influencing Mass Wasting

  1. Degree of slope
    • Movement of weathered material is faster on steep slopes than on gentle slopes due to the influence of gravity.
  2. Climate
    • Weathered material in areas receiving heavy rainfall move faster since wet materials have less cohesion.

  3. Nature of the material
    • Material saturated with water is more likely to move down slope as its heavy.
    • Mass wasting is more likely to occur in areas where the weathered material is deep.
    • Weathering is more likely where massive rocks lie on weak rocks such as clays, shale than where fine materials lie over weak rocks.
  4. Vegetation
    • Surfaces with vegetation experience less mass wasting because it binds weathered material together.
  5. Tectonic movements
    • Earth movements such as earthquakes, volcanic eruptions or faulting cause large and widespread mass wasting.
  6. Human activities
    • Explosives used in mining and quarrying shake the ground initiating downward movement of materials.
    • Mining and quarrying also interferes with the stability of the surface by loosening it making it easy for the loosened materials to move down slope.

Types of Mass Wasting

  1. Slow Mass Wasting
    • Slow but steady movement of soil or loose rock debris down slope.

    Processes
    1.  Soil Creep
      soil creep.PNG
      • Slow and steady movement of soil and other fine materials along a very gentle slope.

        Causes
      • Alternate heating and cooling causing expansion and contraction of particles causing them to change their positions.
      • Alternate wetting and drying of soil whereby when it’s wet its compact and when dry the particles are loosened and tend to move away from each other.
      • Trampling and burrowing of animals.
      • External forces e.g. shaking by earthquakes, explosives, heavy vehicles, etc.
      • Ploughing down hill
      • Freezing of soil water causing it to expand which lifts particles at right angles to the slope in a process called heaving.
    2. Solifluction
      • Movement of saturated soil, gravel and weathered rock down a moderate slope.
      • Common in mountainous and very cold climates
      • Thawing occurs during spring causing top soil to become saturated.
      • Saturated soil begins to creep over the subsoil which still remains frozen(permafrost).
    3. Talus Creep
      talus creep.PNG
      • Slow and gentle movement of the mass of broken rock particles which accumulate at the base of cliffs (scree) downhill.
    4. Rock Creep
      rock creep.PNG
      • Slow movement of individual rocks which lie on clay at a very low speed down slope in the presence of moisture.
  1. Rapid Mass Wasting
    - Type of mass wasting involving large amounts of weathered material moving suddenly and fast down slope.
    1. Mud Flow
      mud flow.PNG
      • Movement of oversaturated weathered material inform of liquid down slope.
      • It occurs mainly in dry areas after heavy rains.
    2. Earth Flow
      earth flow.PNG
      • Movement of saturated earth material on hill sides down slope.
    3. Land Slide
      • Sudden slipping of large quantities of loosened surface rock or soil down a slope.
    4. Slump
      slump.PNG
      • Erosion occurs on the weak rocks at the base of a cliff undercutting the weak rock.
      • The overlying rocks break off causing the overlying rocks to slide down hill rotating around a curved plane.
    5. Debris Slide
      • Sudden downhill movement of accumulated rock debris and other loose material downhill as a whole
        debris slide.PNG
    6. Debris fall
      debris fall.PNG
      • Sudden free fall of debris from a vertical or hanging cliff to the base of the slope.



    7. Rock Slide
      rock slide.PNG
      • Sliding down of masses of rock a steep slope along a bending plane, joint of fault.
    8. Rock fall
      rock fall
      • Falling or rolling of individual rocks or boulders down a steep slope or a cliff.
      • Most rapid of all mass wasting.
    9. Avalanche
      • Sudden slipping and falling of a large mass of snow, ice and loose rock materials down a mountain side.
        avalanche.PNG
    10. Rain Wash
      • Type of mass wasting involving removal of weathered materials by rain water.
      • When rains come, the first drops scatter soil particles that have been loosened by drying.
      • The increasing downpour then washes large quantities of loosened soil downhill.

      Types

      1. Sheet wash
        • Uniform removal of soil from a large area.
        • Rainfall with uniform drops fall on loosened soil on a land with uniform slope.
        • The water from the rainfall then flows down slope.
        • As it does so, it uniformly sweeps all the loose soil from the surface. Its common around L. Baringo and Marigat.

      2. Gulleying
        gulleying.PNG
        - Removal of soil through wide and deep channels called gullies.
        • Rain falls on an even slope
        • The water irregularly runs down slope along specific channels called rills.
        • The channels are widened and deepened by the water to form gullies.
        • Neighbouring gullies are widened and the ridges between them are reduced to form earth pillars.
      3. Splash erosion
        • Removal of soil by rain drops scattering loose particles and carrying them down slope by runoff.

Effects of Mass Wasting On Physical and Human Environment

Positive

  1. Make the soil to become fertile where soil from fertile areas is deposited.
  2. Leads to formation of new land forms such as scars, depressions, lakes, rock pillars, etc.

Negative

  1. Soil creep may destroy walls built across the slope when creeping soil exerts pressure on them.
  2. Decrease soil fertility where fertile soil moves down slope.
  3. Makes the ground prone to soil erosion especially where scars have formed.
  4. Hinders transport and communication by blocking railway lines making maintenance to be costly.
  5. Hinders mechanisation of agriculture e.g. gulleying does not allow movement of vehicles and machinery on farms.
  6. Leads to destruction of property and loss of live by burying people in their houses and stones falling on escarpments along roads causing accidents.
  7. May Cause Rivers to change their courses e.g. mud flow.


Hydrological/Water Cycle

hydrological cycle.PNG

  • Endless interchange of water between the sea, atmosphere and land.

Processes in Which Circulation Is Carried Out

  1. Evaporation
    • Changing of water into water into water vapour when it’s heated by solar radiation.
    • Evapotranspiration: Combined loss of water from the soil through direct evaporation and transpiration by plants.
  2. Cooling
    • Reduction of water vapour temperature as it rises into the atmosphere when it expands due to reduced temperature and pressure.
  1. Condensation
    • Turning of water vapour into tiny water droplets which form clouds when cooling continues below dew point.
  1. Precipitation
    • The process in which the earth receives moisture from the atmosphere.
    • It occurs when droplets formed by condensation combine forming heavier drops which fall on the ground as rain or may become frozen to form snow, hail, sleet, etc.
  1. Surface runoff
    • Some of the water from precipitation that flows on the surface into valleys, ponds, lakes, etc.
  1. Infiltration
    • Entry of water into the ground through pores, joints and cracks in rocks.
  1. Percolation
    • Downwards and sideways movement of water that has entered into the ground.
  1. Overland flow
    • Surface runoff makes the overland flow.
    • River water flows back to the oceans where evaporation takes place again and water cycle is repeated.

Significance of Hydrolological Cycle

Positive

  1. Provides water to man from precipitation and underground water.
  2. Provides rain to man who is useful in agriculture.
  3. Atmospheric water is important in regulating heat loss from the earth by absorbing terrestrial radiation and reflecting it back to the earth keeping the lower atmosphere warm.

Negative

  1. May lead to shortage of water when evaporation rate exceeds precipitation.
  2. May lead to decreased agricultural production as a result of excessive evaporation causing weathering of crops.
  3. May lead to flooding when excessive evaporation cause increased rainfall.
  4. May lead to shortage of rainfall if there is less evaporation due to low temperature.


Action of Rivers

  • A river is a mass of water flowing over the land in a definite channel.

Work of a River

  1. Drain excess water from the land.
  2. Sculpturing land through erosion and transportation 

River Erosion

  • Removal by river water of materials from the sides and bed of the river channel.

Factors Influencing River Erosion

  1. River volume
    • A river with a large volume has a greater kinetic energy to erode than one with a small volume.
  1. Slope of land
    • A river flowing on a steep channel has greater velocity and therefore more energy to erode its channel than one flowing over gentle or flat land.
  1. Rivers load
    • A river with large, rough and heavy load e.g. tree trunks and boulders erodes more than one with light, fine and smooth materials e.g. sand.
    • A river carrying more load erodes more than one with less load as it has more abrasive tools.
  1. Nature of bed rock
    • Erosion is faster where a river flows over soft bed rock and less where it flows over hard rock.

Processes/Ways of River erosion

  1. Solution/Corrosion
    • River water dissolving soluble minerals and carrying them away.
  1. Hydraulic Action
    • Erosion by the force of river water when it thrusts itself into cracks and joints of rocks on the sides of the channel dislodging lumps.
    • Also by pushing air into the cracks, compressing it increasing pressure which widens the cracks eventually dislodging lumps.
  1. Abrasion/Corrosion
    • Abrasion is scratching of the bed and banks by materials are carried away by the river.
    • Corrosion is hurling of rock fragments carried by the river against rocks which weaken and eventually break them.
  1. Attrition
    • Hitting against one another of rock fragments carried by river water breaking one another into smaller pieces.

 

 

 

 

Types of River Erosion

  1. Vertical Erosion
    vertical erosion.PNG
    • Erosion in which the river cuts downwards into its channel.
  1. Lateral Erosion
    lateral erosion.PNG
    • Erosion in which the river erodes the sides of the channel.
  1. Headward Erosion
    headward erosion.PNG
    - Erosion in which a river cuts back at its source.
    1. Where there is a water fall.
      • The river undercuts at the base of a waterfall.
      • The rock above the undercut cliff collapses.
      • The position of waterfall shifts upstream.
    2. Where gulleying or soil creep occurs where there is a spring causing its position to shift upstream (spring sapping).

Resultant Features of River Erosion

  1. Stream Cut Valleys
    - Valleys with V, open V or U shaped cross sections along the river channel.
    stream cut valleys.PNG
    • In the source region a river cuts itself a channel which starts as a gulley.
    • The channel is deepened by vertical erosion resulting into a v-shaped valley.
    • In the middle stage lateral erosion widens and deepens the valley resulting in a more open v-cross section.
    • In the old stage lateral erosion creates a very wide channel with a U-shaped cross section.
  1. Gorges
    • Narrow, deep, steep-sided valley.

    Ways/modes of formation
    • Where a river flows along a fault or a section of soft rocks eroding the channel vertically through the soft rocks or fault.
    • By headward erosion at a water fall when the river’s erosive activity is increased due to increased gradient causing the river to undercut at the base of the water fall, then the rock above the undercut base collapses causing the waterfall to shift upstream resulting in a gorge below the water fall.
    • Where a river flows across a plateau with alternating horizontal layers of hard and soft rocks eroding them resulting in a gorge with stepped sides called a canyon e.g. Grand canyon on R. Colorado in USA.
    • Due to river rejuvenation when the river’s erosive activity is renewed causing the river to vigorously erode deep into its channel.
    • Where a river maintains its course across land which is being uplifted gradually.
  2. Rapids
    • A section of the rivers course where the bed is suddenly steepened causing the water to suddenly flow swiftly.

    How they are formed
    1. Where a less hard rock lies below a soft rock and the soft rock is eroded more resulting in a steep slope.
      rapids formation - a.PNG
    2. Where a water fall has been eroded by headward erosion reducing its height.
      rapid formation - b.PNG
    3. Where resistant rock dips downstream and is unevenly eroded.
      rapid formation - c.PNG
    4. Water Falls
      • A place on a rivers course where a river bed is vertical or nearly vertical.

      Formation
      1. Where a river descends over a sharp edge of a plateau encountering a sharp drop.
      2. Where a river descends a cliff into the sea.
      3. Where a river descends a fault scarp.
      4. Where a river descends a sharp edge of a plateau.
      5. Where a river is blocked by lava flow causing water to accumulate on the upstream side and a water fall forms at the point of overflow.
      6. Where a resistant rock lies across a river with a less resistant one on the downstream side and the less resistant one is eroded faster causing a rapid to be first formed, then a waterfall.
        water falls.PNG
      7. Pot Holes
        • Circular depressions on a river bed.
        • Form where a river flows over shallow depression and develops strong circulating currents which cause the load to scratch the bed in circular motion.
          pothole.PNG
      8. Interlocking Spurs
        interlocking-spurs.PNG
        • Highland projections which appear as they fit together.

        Formation
        • Where In the youthful stage, a river flows around spurs undercutting the outer bank more than the inner bank causing the bends to be more pronounced making the spurs to appear as if to fit together.
        • The outer bank becomes river cliff/bluff and the inner bank slip off slope.

River Transportation

  • River carrying away materials that its water has eroded from the channel.

Factors Influencing River Transportation

  1. Rivers Volume
    • A river with large volume of water has more energy and therefore greater carrying ability than one with a small volume.
  2.  Gradient
    • A river flowing on a steep channel has greater ability to transport than one on a gentle slope because it flows fast due to gravity
  3. Rivers Load
    • Small and light particles are transported over long distances while heavy materials are transported for a short distance.
    • Dissolved load is carried all the way to the rivers mouth.
    • Small amount of load is transported for a long distance while large amounts of load collide reducing the speed and therefore rivers ability to transport causing some of the load to be dropped along the way.

Processes/ways of River Transportation

  1. Suspension
    • River transportation of light and insoluble materials in form of a mixture.
  2. Saltation/Hydraulic Lift
    • River transportation of large particles through a series of jumps and hops.
    • Materials are lifted by force of moving water and pushed for a short distance and land back on the river bed by gravity.
    • The process is repeated causing the load to be transported downstream.
  3. Traction
    • River transportation of heavy materials like boulders by rolling them by the force of water.
  4. Solution
    • River transportation of load in solution form.
    • Load transported by suspension, Saltation and traction is called clastic load while that by solution is called dissolved load.

Deposition

  • Laying down of some of the load carried by the river when energy decreases.

Factors Influencing Deposition

  1. Gradient
    • When gradient reduces the river’s speed decreases and hence its energy is reduced causing it to drop some of the heavy load.
  2. Rivers Volume
    • When rivers volume decreases its energy also decreases causing it to deposit heaviest load then lighter ones.
  3. Obstacles
    • Obstacles such as swamp vegetation and rock outcrop reduce the river’s speed and also trap some of the load thereby facilitating deposition.
  4. River Bed Width and Depth
    • Where a rivers channel becomes wide and shallow there is less water per unit area and hence the river has lower capacity to transport so deposition of excess load begins.

Resultant Features of River Deposition

  1. Alluvial Fans and Bajadas
    alluvial fan and bajada.PNG
    • Fan shaped deposits of alluvium.

    Formation
    • The river flowing through a narrow channel enters a plain from a higher ground and suddenly spreads out.
    • There is a sudden loss of velocity causing the river to scatter alluvium all around to form an alluvial fan.
    • Alluvial fans merge to form a continuous feature called bajada or piedmont fan.
  2. Meanders and Oxbow Lakes
    • Meanders are loop-like bends in a rivers course.
    • Oxbow lake is a horse shoe shaped section of a former river.

    Formation
    • In mature stage river flows sluggishly due to reduced gradient.
    • It meets an obstacle and flows around it.
      meanders and oxbow lakes-a.png
    • Erosion is greater on the outer bank and deposition on the inner bank causing the river to form loop like bends.
      meanders and oxbow lakes-b.png
    • Erosion continues on the outer bank (bluff) narrowing the land between the two outer banks forming a pronounced meander e.g. on rivers Yala, Nzoia and Tana.
      a pronounced meander.PNG
    • During the floods when the river has more energy it cuts across the narrow land.
    • The former bends are cut off by deposition to form an oxbow lake e.g. Kanyaboli on R.Yala and Shakababo on R.Tana.
      oxbow lake.PNG

Flood Plains

flood plains.PNG

  • Wide gently sloping plain of alluvium on the floor of a river valley.

Formation

  • A river meanders.
  • There is erosion on outer bank and deposition on the inner bank.
  • The process continues and layers of alluvium deposited on inner bank join to form a plain e.g. Nzoia and Yala flood plains.

 

River Braids

river braids.PNG

  • Network of diverging and converging channels along a rivers course.

Factors Favouring Formation of Braids

  1. River must be carrying large load.
  2. Reduced gradient on the section.
  3. Reduced amount of water such as in dry season or arid conditions.
  4. Presence of obstacles such as rock out crops.

Formation

  • River flows sluggishly due to low gradient.
  • Deposits of alluvium are laid on river bed.
  • The deposits raise the river bed causing the channel to be subdivided into channels or distributaries.

Natural Levees

natural leaves.PNG

  • Raised river banks which are made of alluvial materials.

Formation

  • River floods and spills over its banks.
  • Deposition of coarse materials near the banks and fine materials are carried further on the flood plain.
  • Coarse materials accumulate raising the banks above the general level of the flood plain.

Effects of Levee Formation

  1. Creation of differed tributaries and confluences.
    • Differed tributary: Tributary blocked from joining the main river by levees.
    • Differed confluence: New point where the differed tributary joins the main river downstream.
  2. Destructive flooding.
    • Due to the river bursting its banks during the flood season due to the bed being raised above the general level of the flood plain.
    • Due to differed tributaries flowing into the flood plains.
    • Because the river channel has become narrower and shallower due to deposited alluvium.

Estuaries

  • Broad channel at the mouth of a river where the river enters the ocean as a whole.
  • Some are deep and narrow because sediments are carried away by ocean currents while others are wide and shallow due to sediments covered by water e.g. on R. Congo and Gabon.

Deltas

  • Low lying tract of alluvial deposits formed at the rivers mouth.

Ideal Conditions for Formation of A Delta At A Rivers Mouth

  1. Large load such as from a large catchment area where erosion is taking place actively.
  2. The rivers course to be free from obstacles such as swamps so as not to filter sediments before they reach the mouth.
  3. Low speed at the point where the river is entering a sea or lake for deposition to take place.
  4. The rate of deposition should be higher than the rate of erosion by sea or lake currents.

How a Delta Forms

  • The speed of the river is checked by sea or lake.
  • Heavy load is first deposited.
  • Lighter load is carried further into the sea causing that part of the sea to become shallower.
  • The part is colonised by plants making it swampy but firmer.
  • Plants trap more alluvium making the delta to grow in height.
  • The river builds levees making it narrower.
  • The river burst its banks and small channels branch off the main river and carries water into the sea or lake (distributaries).

Types of Deltas

  1. Marine: Type formed at sea.
  2. Lacustrine: at a lake.
  3. Inland Delta: Deltas which form along a rivers course before it reaches the lake or sea.

    Formation
    • The velocity of the river is checked on entering a relatively flat swampy land.
    • The river builds up levees.
    • The river bursts banks forming distributaries.
    • Alluvial deposits are spread over vast areas when river floods e.g. Niger and Okavango deltas.
  1. Arcuate Delta
    arcuate delta.PNG
    • A delta with a convex shoreline on the seaward end due to strong currents spreading materials over a wide area on seaward side.
    • Has many distributaries e.g. Tana and Rufiji deltas.
  1. Birds Foot Delta
    birds foot delta.PNG
    • Type of a delta with a pattern resembling the foot of a bird.
    • Has few distributaries.
    • Formed on a river carrying large quantities of fine alluvium into water where there is low wave energy e.g. Omo and Mississippi deltas.
  1. Estuarine Delta
    estuarine delta.PNG
    • Delta which has formed on an estuary.

      Formation
    • The rivers load is deposited on the estuary when the speed is checked by sea.
    • The river cuts across in a single channel that may be bordered by levees e.g. on R.Volta in Ghana and on R. Zambezi.

Development of a River Profile

- Longitudinal section of a river from source to mouth.

  1. Youthful/ Torrent Stage

    Characteristics
    1. Steep gradient.
    2. The river flows very fast
    3. Vertical erosion is dominant
    4. Headward erosion is evident.

    Features
    1. V- shaped valleys
    2. Waterfalls
    3. Rapids
    4. Potholes
    5. Gorges
    6. Interlocking spurs.
  2. Mature/ Valley Stage

    Characteristics
    1. Low and almost regular gradient.
    2. The flow is less swift.
    3. The river is wider due to being joined by tributaries.
    4. Lateral and vertical erosion but lateral is more active.
    5. Deposition starts at some sections.

    Features
    1. Wider open v-shaped valley
    2. Meanders
    3. River bluffs/cliffs
    4. Slip off slopes
  3. Old/ Plain Stage

    Characteristics
    1. Very gentle/almost level gradient.
    2. Very slow flow of river.
    3. The main work of the river is deposition.
    4. Some lateral erosion occurs.
    5. Seasonal floods are common.

    Features
    1. Shallow broad flat bottomed u-shaped valley.
    2. Meanders
    3. Oxbow lakes
    4. Natural levees
    5. Differed tributaries
    6. Differed confluences
    7. Braided channels
    8. Flood plains
    9. Deltas
    10. Distributaries

River Capture/Beheading/Piracy/Abstraction

  • Diversion of head waters of one river into the system of an adjacent powerful river due to erosion.
  • The river that captures is called pirate.
  • The captured one is called victim.

    How it occurs
    • At first there are a powerful river and a weaker river flowing adjacent to each other.
      river capture.PNG
    • The powerful river erodes vertically and laterally than the weak river making it to flow at a lower level.
    • At the same time, it extends its valley backwards by headward erosion.
    • The stronger river eventually joins the valley of the weak river.
    • The headwaters of the weaker river start flowing into the valley of the stronger river e.g. R. Tano in Ghana was captured by the Black Volta River and R. Eyong was captured by Imo in S. Nigeria.
  • The remaining section of the beheaded river is called a misfit/beheaded river.
  • The dry valley between the elbow of capture and the new course of the misfit stream is called a wind gap.
    secondary divide.PNG

    elbow of capture.PNG

River Rejuvenation

  • Renewal of erosive activity of a river.
  • Happens in the old stage.

Causes

  1. Change in the Base Level
    • Base level is the lowest level to which a river can erode its bed.
    • Rejuvenation resulting is called dynamic rejuvenation

      Drop in sea level
    • The river mouth moves further seawards.
    • A steep gradient occurs between the old and the new mouths causing the river to starts to move swiftly.
    • Vertical erosion resumes extending back to the flood plain.

      Uplift of a section of land along the rivers course.
    • Faulting or folding may occur.
    • A section of land along a rivers course is uplifted.
    • The gradient is increased causing the river to flow swiftly and undercut through the uplifted section.
    • An antecedent gorge is formed.

      Unequal sinking of land along a rivers course.
    • The downstream side sinks more than the upstream one.
    • An increase ingredient occurs causing the river to flow swiftly
    • The river starts to undercut more vigorously than before.
  2. Increase in Rivers Discharge
    - Rejuvenation resulting is called static rejuvenation
    • The rivers discharge increases due to high precipitation or capture.
    • The rate of erosion becomes higher due to increased discharge.
    • The river starts to undercut more vigorously.
  3. Change in Rock Structure
    • A river passes a resistant rock and starts flowing over a less resistant rock.
    • The river starts eroding more vigorously into the softer rocks.

Features of River Rejuvenation

  1. Knick Points
    knick points.PNG
    • A sudden break of slope in a rivers profile as a result of change in sea level.
  1. River Terraces
    • Step like features formed when a river rejuvenates and cuts a new valley through the flood plain causing a plat form will form where the floor of the former flood plain was.
  1. Water Falls
    • Are formed when knick points are deepened e.g. Charlotte falls in Sierra Leone.
  1. Antecedent Gorges
    • Gorges which form where a river undercuts though a section of land that is being uplifted e.g. Turkwel gorge.
  1. Incised Meanders
    • Meanders that have been cut deeper into by a rejuvenated river.

 

Types

  1. Entrenched Meanders
    entrenched meanders.PNG
    • Formed from vertical erosion causing both valleys to be steep and symmetrical.
  2. Ingrown Meanders
    • Formed by lateral and vertical erosion causing one valley side to be steeper than the other and hence asymmetrical in cross section.
      Ingrown meanders.PNG
  3. Abandoned Meanders
    abandoned meander.PNG
    • Meanders abandoned during formation of oxbow lakes when the river takes a short-cut leaving an enclosed portion of land surrounded by an oxbow lake.

Drainage Systems

  • Main river together with its tributaries.

Types

  1. Accordant Drainage System
    • Drainage system in which a river flows according to slope and rock structure by following areas of weak lines.
  1. Discordant Drainage System
    • Drainage systems in which rivers don’t flow in accordance with the slope, rock structure and land forming processes.

      Types
    • Antecedent Drainage System
      Antecedent drainage system.PNG
      • Drainage system where a river maintains its course while the surrounding land is being uplifted.
    • Superimposed Drainage System
      • Drainage system which develops where a river maintains its flow over a new set of rocks after removing a former set of rocks.
  1. Back Tilted/Reversed Drainage System
    • Drainage system where direction of flow is reversed be due to capture, uplifting or down warping e.g. R. Kagera, Katonga and Kafu.

Significance of Rivers and Their Features

Positive

  1. Rivers are sources of water for domestic and industrial use.
  2. Rivers water is used for irrigation.
  3. They provide port facilities where they have rias and estuaries.
  4. Some rivers are used for transportation e.g. R. Congo and Nile.
  5. Some rivers are fishing grounds e.g. Tana.
  6. Rivers are dammed and used for H.E.P generation.
  7. Features formed by river action such as waterfalls, gorges and oxbow lakes are a tourist attraction.

Negative

  1. Rivers flood causing loss of life and property.
  2. Rivers may lead to drowning accidents especially when they are flooded.
  3. River water can be a medium of spreading diseases such as bilhazia and malaria.
  4. Some wide rivers are barriers to transport and communication.
  5. Some rivers also harbour dangerous wild animals which can kill humans e.g. crocodiles, hippos and snakes.


Lakes

  • A lake is a depression on the earth’s surface where water has accumulated.

Classification/Types of Lakes

According To the Nature of Water

  1. Fresh water lakes which contain fresh water.
  2. Salty lakes which have salty water.

According To the Mode of Formation of Depression They Occupy

  1. By Earth or Tectonic Movements
    1. Faulted or Rift Valley Lakes
      • During Rift Valley formation some parts of the rift valley floor sunk more than others.
      • A long narrow and deep depression formed.
      • Water from seepage and rain accumulated into these depressions to form lakes.
    2. Down Warped and Tilted Lakes
      downwarped lakes.PNG
      • Tensional and compression forces caused some parts of the earths crust to up warp while others down warped.
      • A shallow depression formed.
      • The depression may also be filled with water from rain or ground water.
      • In the case of L. Victoria Rivers Kafu, Kagera and Katonga were tilted eastwards and Nyando, Yala and Nzoia continued flowing west wards adding water into the depression.
      • Victoria is the second largest fresh water lake after L.Superior.
      • Has a maximum depth of 87m deep. Other examples of lakes are L. Kyoga and Wamala.
      • Playas/sebkha is a lake contained in an inland drainage basin in a desert formed when rain or flood water flows into a basin formed by crustal warping e.g. Chemchane Sebkha in Mauritania.
  1. By Vulcanicity
    1. Crater Lakes
      • Lake formed by water accumulating into a crater.
      • Are usually salty.
      • A crater lake formed on an explosion crater is called maar.
      • Examples are Lakes Mossoko in Tanzania, Paradise in Marsabit and Myungu in Uganda.
        crater lakes.PNG


    2. Lava Dammed Lakes
      lava dammed lakes.PNG
      • Formed as a result water accumulating on the upstream side of a lava barrier across a river.
      • Highly viscous lava erupts across a rivers course.
      • It solidifies and blocks the river forming a lava dam.
      • The rivers water accumulates behind the lava dam.
      • A narrow and winding lake is formed e.g. Lakes Bunyonyi, Mutanda and Bulera in Uganda.
  1. by Erosion
    1. Glacial Erosion
      1. Corrie/Tarn Lakes
        corrie lakes.PNG
        • Lake formed when water from melting snow accumulates into a corrie/cirque e.g. Teleki, Nanyuki and Hidden tarns on Mt. Kenya.
      2. Ribbon Lakes
        • Finger like on a glaciated valley.
        • Glacier erodes the floor of a u-shaped valley.
        • It over deepens some of its sections.
        • Elongated hollow results.
        • Water from melting ice accumulates into it forming a lake.
          ribbon lakes.PNG
    2. Wind Erosion
      • Lakes formed when ground water accumulates in a depression formed by wind deflation and abrasion.
      • Wind continuously erodes the earth’s crust by deflation and abrasion.
      • The water bearing rocks are reached.
      • Water oozes from the water table into the hollow or water from flash floods may accumulate in it to form temporary lakes called pans e.g. in Quattara depression between Egypt and Libya and Etosha pan in Namib.
    3. Solution Lakes
      • Lakes formed when rain or ground water accumulates in depressions formed in limestone rocks when rain water containing a weak carbonic acid dissolves limestone rocks e.g. Lakes Barber in Morrocco and Ojikoto in Namibia.
  1. by Deposition
    1. River Deposition
      • Formed when river deposition occur cutting off a section of a pronounced meander e.g. oxbow lakes Shakababo and Mukunguya at lower part of Tana.
    2. Wave Deposition
      wave deposition.PNG
      • Lakes formed when wave deposition occurs across a rivers mouth or where the coastline changes suddenly enclosing a body of calm water.
      • Waves break at an angle.
      • The long shore drift causes materials to be progressively arranged across a rivers mouth resulting in a body of calm water called a lagoon/sound.
  1. by Man
    1. Dams are Lakes formed when water accumulates behind dams constructed across rivers resulting into a large man made reservoir called manmade lake e.g. behind Seven Forks Dam and Lakes Volta in Ghana and Nasser in Egypt.
    2. Barrage is a bank of earth or stones built across a river to provide water for farming.

 

Significance of Lakes

Positive

  1. Fresh water lakes provide water for domestic and industrial use.
  2. Fresh water lakes also provide water for irrigation e.g. Naivasha for horticultural farms around it.
  3. Manmade lakes and some other lakes e.g. Victoria (Owen falls) are used for generation of H.E.P.
  4. Lakes are used for transport.
  5. Some lakes contain valuable minerals e.g. trona at L. Magadi and salt at L. Katwe in Uganda.
  6. Many lakes have fish which is a source of food and employment to fishermen and traders.
  7. Lakes are also a tourist attraction by providing recreational facilities and being habitats for wildlife.
  8. Some lakes are sources of rivers e.g. Victoria for White Nile and L.Tana for Blue Nile.
  9. Lakes modify the climate of surrounding areas by sea breezes and convectional rainfall.

Negative

  1. Lakes are habitats for disease vectors e.g. mosquitoes and snails which transmit Malaria and bilhazia.
  2. Lakes may cause flooding due to excessive rainfall or when dams break leading to loss of life and property.
  3. Lakes are habitats for dangerous animals like crocodiles, hippos and snakes which kill humans.
  4. Lakes cause drowning accidents to people in time of storms.


Oceans, Seas and their Coasts

  • An ocean is a large and extensive body of saline water occupying a basin between continents while a sea is a large body of saline water on the margins of continents.

Nature of Ocean Water

  1. Ocean water is salty
    • Due to abundant sodium chloride which rivers dissolved from land, from rocks that the water is in contact with and volcanic materials on the ocean floor?
    • Ocean water has high salinity in areas where there is addition of little water and high rate of evaporation leading to high salt concentration e.g. Dead Sea and lower where there is low temperatures and addition of fresh water from rivers, rain or snow melts e.g. Baltic Sea.
  2. Surface water is warmer than that at the bottom except in Polar Regions where a thin layer of cold water may overlie warmer water.
  3. Ocean water is a habitat for living organisms
    Planktons are plants and animals occupying ocean surface.
    1. Phytoplankton are ocean plants e.g. algae.
    2. mZooplankton are ocean animals e.g. lobsters, jelly fish, crabs, etc.

      Types
      1. Nektons are all forms of fish.
      2. Benthos are ocean creatures which live only at the bottom of margins of continents where sunlight reaches Sea floor e.g. snails, starfish and sea anemones.
  4. Ocean water is pollutedg. by industrial effluents, pesticides and herbicides carried by rivers and runoff to the sea.
  5. Ocean topography is composed of several features
    ocean topography.PNG
    1. Continental shelf- Relatively flat part of the continent covered by ocean water.
    2. Continental slope- Steeply dipping surface between continental shelf and the ocean basin proper.
    3. Abbysal plain- Almost level area of the ocean where sediments are deposited.
    4. Mid ocean ridges- Range of hills which are submerged formed by volcanic and seismic activities.
    5. Sea Islands- pieces of land surrounded by water.
      1. Continental islands- Ones rising from continental shelf.
      2. Oceanic islands-Ones which rise from the sea floor e.g. Canary and Cape Verde.
      3. Coral islands-Ones made of coral.
    6. Deep sea trenches - narrow steep sided submarine valleys on the ocean floor.
    7. Guyots- submerged atolls forming an underwater mountain.
    8. Sea mount- a volcano which doesn’t rise above the sea floor.
  6. A portion of ocean water moves
    • There are two types of movements namely:

      Vertical Movements
    • Movement of ocean water from surface to bottom and vice versa.

      How they occur
    • Cold polar water sinking before moving horizontally towards equator.
    • Ocean currents converge
    • When ocean water sinks at lower depths after ocean currents converge.
    • When ocean water rises to the surface in a process called upwelling.
      vertical movements.PNG

      Significance of vertical movements
    • Carries nutrients for sea animals by upwelling.
    • Oxygenation of water vital for fish survival.

      Horizontal Movements
    • It occurs in the following ways:
      1. Ocean Currents
        - An ocean current is a large mass of surface ocean water which is moving in a particular direction e.g.
        • Mozambique- warm
        • Canaries -cold
        • Benguela-cold
        • Atlantic drift-warm
        • Gulf stream drift-warm

        Factors that influencing formation of ocean currents
        • Wind by blowing over water causing a mass of surface ocean water to move in its direction forming drift currents.
        • Rotation of the earth by causing deflection of ocean currents.
        • Shape of land mass by influencing current direction and causing it to flow following the coastal outline.
        • Differences in temperature by causing cold polar water which is dense due to low temp moves towards the equator passing on the ocean floor and warm water of the tropics to move towards the poles passing on the surface.
      2. Tides
        • Periodic rise and fall in the level of ocean and other large water bodies.
        • Occurs when the moon and to some e the sun exert gravitational pull on the water bodies on the earth.
          tides.PNG
        • Moons gravitational pull is exerted on the earth causing the water on that side A to bulge resulting in high tide 1
        • Some water flows from sides C and D to side B to occupy space created by the moons pull resulting in high tide 2 and low tides 1 and 2 at C and D.

          Rotation of the Earth
        • It brings any longitude under the influence of 2 high and 2 low tides in a lunar day.
        • Similar tides occur at an interval of 12hrs 26 minutes.
        • A lunar day is time taken by the earth to complete one rotation with respect to the moon (24 hrs 52 min)
        • Lunar month is time taken by the moon to complete one revolution around the earth (27.3 days)
        • The moon is always ahead of the earth by 52 minutes due to its revolution e.g. if Nairobi is opposite the moon at 6pm the following day the high tide will be at 6.52pm.
          high and low tides.PNG
        • Tidal range is the difference between the highest level reached by high tide and lowest level reached by low tide.

          Types of tides
        • Caused by relative positions of the moon and the sun from the earth.
        • Sometimes the moon and the earth are nearer or farther from each other due to their elliptical orbits.
          1. Spring Tides
            spring tides.PNG
            • In which the highest and lowest tides occur.
            • Occurs when the sun, moon and the earth are in a line (syzygy position) and pulling in the same plane causing pulling force to be greatest.
          2. Neap Tides
            • In which high tide is lower than normal and low tide is higher than normal.
            • Occurs when the sun, moon and earth form a right angle and pulling water to themselves.
              neap tides.PNG
          3. Perigian Tides
            perigian tides.PNG
            • In which tidal range is 20% higher than normal.
            • Occur when the moon is nearest to the earth (perigee position) causing pulling force to be greatest.
          4. Apogean Tides
            • In which tidal range is lower than normal.
            • Occur when moon is farthest from the earth (apogee position) causing pulling force to be weakest.
          5. Diurnal Tides
            • 1H1L in a lunar day
          6. Semi Diurnal Tides
            • 2H2L in a lunar day which may rise or drop at the same level.
            • Occur in most of Pacific Ocean.
          7. Mixed Tides
            • 2H2L in a lunar day where one pair may fluctuate in level while the other remains constant.
      3. Waves
        • A wave is a moving ridge of water on the sea.
        • It’s formed when wind blows over an open water body causing oscillation of water particles.

          Parts of a wave
          parts of a wave.PNG
          - Crest
          - the top of a wave.
          - Trough - the bottom of a wave.
          - Wavelength - horizontal distance between two successive crests.
          - Height - difference in height between crest and trough.
        • When a wave reaches the shore, the water particles below the surface start touching the ocean floor causing it to break.
        • There is forward movement of water to the beach which is called swash/send.
        • There is backward movement of water to the sea due to gravity called
        • The rest flows at the bottom back into the sea in a water current called undertow.

          Types of waves
          1. Constructive Waves
            • Waves in which swash is stronger than backwash resulting in deposition.
          2. Destructive Waves
            • Waves in which swash is weaker than backwash resulting in erosion.





          Wave Erosion

          Processes of Wave Erosion
          1. Abrasion
            • Scratching of ocean floor by materials carried by the back wash.
          2. Corrasion
            • Hurling of pebbles and rock fragments against the rocks causing some particles to break off.
          3. Attrition
            • Rock fragments dragged up and down by the swash and backwash hitting against each other becoming smaller in size. It provides tools for abrasion and corrosion.
          4. Hydraulic Action
            • Removal of materials from the coast by action of the force of moving water.
              1. Direct wave force
                - Large amounts of wave water crush against a rock face weakening and eventually breaking of the rock.
              2. Compressed air action
                1. Waves crush against a rock.
                2. The force of water pushes air into cracks compressing it and exerting pressure causing them to widen.
                3. Wave retreats causing trapped air to expand resulting in sudden pressure release causing cracks to expand further.
                4. The process is repeated several times causing the rocks to shatter.
          5. Solution
            • Some soluble minerals in rocks dissolve directly in water and are carried away in solution leaving cavities in rocks.
          6. Corrosion
            • Some minerals such as limestone reacting with sea water which has dissolved carbonic acid.

          Factors influencing wave erosion
          1. Waves must have strong backwash and a weak swash
          2. Slope -The coast that slopes steeply into the sea favours erosion.
          3. Load-large amount provides more abrasive tools. Angular shaped load is more effective in abrasion.
          4. Amount of water in a wave - the larger the amount the greater the hydraulic force.

          Features Resulting From Wave Erosion
          1. Cliff and Wave Cut Platform
            - Cliff – A steep rock face which borders the sea.
            - Wave Cut Platform - A fairly flat part of the shore formed when a cliff retreats inland.
            • Breaking waves erode rock surface of a steep coast cutting a notch.
              notch.PNG
            • Erosion continues causing the base of the rock to be undercut resulting into an overhanging rock.
              over hanging cliff.PNG
            • Undercutting continues causing the overhanging rock to eventually collapse forming a cliff.
              wave cut platform.PNG
            • The process is repeated and a fairly flat part of the shore is formed between the new and the former cliff.
          2. Bays and Headlands
            bays and headlands.PNG
            - Bay 
            – Piece of sea water jutting into the land or a curved inlet of sea.
            - Headland - a piece of land jutting into the sea.
            • At first there is a coast with hard and soft rocks.
            • Soft rocks are eroded more by wave action to form sea inlets called bays.
            • Resistant rocks called headlands are left sticking into the sea. A big bay is called a gulf.



          3. Caves, Blow Hole and Geos
            caves,blow holes and geos.PNG
            - Cave 
            - Natural cylindrical tunnel like chamber extending into the cliff or into the side of a headland.
            • A small hollow form on a weak area of the cliff after limestone is acted upon by carbonation.
            • Corrosion and direct dissolving act on the hollow extending it into the cliff forming a cave.

            Blow Hole/ Gloup - Vertical hole formed on the side of cliff bordering the land.
            • Formed when a cave reaches the surface some distance inland as a vertical pit.
            • It’s called a blow hole because when the waves break water is forced out of the hole.  

            Geos - Narrow sea inlet formed when the roof of a cave between the blow hole and the sea collapses.                          
          4. Natural Arch, stack and stump
            Natural arch, stack and stump.PNG
            - Natural arch 
            – Opening from one side of a headland to the other.
            • Formed when a cave extends into the head land to the other side.
            • Or when caves which have developed on both sides of headland join each other.

            Stack - Pillar of rock left standing on the seaward side.
            • Formed when continuous wave erosion causes the roof of the arch to collapse.

            Stump - The base of stack left when it collapses as a result of erosion at the base.

          Wave Transportation
          - Types of load moved by waves are such as shingle, sand, mud and other objects dumped into the sea.

          How the sea acquires its load
          1. Materials brought by rivers and wind.
          2. Products of weathering.
          3. Materials brought by rivers and wind.
          4. Debris from volcanic eruptions in the sea or on land bordering the sea.

          - Waves transport load by a process called long shore drift.
          - Long shore drift is progressive dragging of materials along the beach as a result of waves breaking at an angle.
          • Waves break at an angle.
          • Swash pushes materials up the beach at an angle.
          • Backwash brings them back at right angle to the edge of water.
          • Process is repeated causing materials to be progressively dragged along the beach.
            wave tranportation.PNG

          Factors Influencing Wave Transportation
          1. Strength of waves
            • Strong waves carry large quantities of load over a long distance while weak waves carry small quantities of load over a short distance.
          2. Tides
            • Tides cause waves to break farther inland causing materials that were not in contact with breaking waves to be moved about.
          3. Ocean currents
            • Ocean currents cause movement of materials from one part of the ocean to another e.g. coconut fruits from southern part of Africa to Gulf of Guinea by Benguela current.
          4. Gradient of the shore
            • On gentle coasts transportation of materials is favoured by long shore drift while on a steep coast they bounce off cliffs and remain floating.
          5. Orientation of coast line.
            • Transportation by long shore drift is favoured where coast is aligned obliquely to the direction of breaking waves while on transversely aligned coast swash moves materials back and forth along the same line.
          6. Nature of the load.
            • Lighter materials such as sand are carried over long distances while heavy load is transported over a short distance.

          Deposition
          - Process in which materials transported by waves are laid down on the shore.

          Factors Influencing Wave Deposition
          1. Load
            factors influencing wave deposition- load.PNG
            - Deposition occurs in selective manner:
            • Boulders are deposited at farthest end of land because they are swept towards the land by powerful swash during high tide followed by pebbles.
            • Then sand and finally mud because the weak backwash brings them back towards the sea as they are light.
          2. Waves
            - Waves must have a strong swash and a weak backwash in order to cause excess load to be left behind on the shore.
          3. Gradient of the shore
            - The coast must be sloping to reduce the velocity and hence the energy of waves so that depositing occurs.
          4. Depth of Water
            - Deposition takes place where water is shallow for waves to come into contact with ocean floor and break the cyclic motion of water.

          Features Resulting From Wave Deposition

          1. Beaches
            • Gently sloping mass of accumulated materials such as sand, shingle and pebbles along the coast.
            • Formed by constructive waves during a relatively calm weather when backwash is weakest resulting in materials accumulating at the shore.
            • During storms destructive waves destroy beaches creating other minor features such as:                         
              1. Beach cusps
                beach cusps.PNG
                • Horn like projections of sand and gravel which gives the coast line a series of curves.
                • Waves break at right angles.
                • Powerful swash in form of eddies scour depressions moving coarse materials to either side forming head like projections called cusps leaving finer materials forming bay like inlets.
              2. Beach Ridges
                beach ridges and beach bems.PNG
                Beach Ridges
                - Low ridges of coarse sand, boulders and shingle deposited roughly parallel to the shore formed by waves approaching the coast at right angles.
              3. Beach Berms
                • Narrow terrace of shingle thrown up the beach by storm waves formed where tidal range is high.
              4.  Beach Rock Shells
                • Masses of sand, shells and pebbles cemented together by calcium carbonate forming projections above the beach.
          2. Spits
            spits.PNG
            • Low lying ridge of sand, shingle and pebbles with one end attached to the coast and the other projecting to the sea.
            • Movement of materials by long shore drift is halted causing deposition due to coast changing its direction towards the land e.g. across estuary or entrance of a bay.
            • The process continues and the accumulation grows towards the sea.
            • Waves carry sand to the inner end of the spit creating a hook like feature e.g. at the mouth of R. Senegal.
          3. Tombolo
            • Spit that grows out from the coast into the sea and joins an island e.g. Ras Hafur in Somalia and Ngomeni on Kenyan coast.
              tombolo.PNG
          4. Bars
            • Ridge of sand, shingles and mud which lies almost parallel to the coast.

              Types
            • Bay bar – Bar which forms across the entrance of a bay.
              bay bar.PNG
            • Offshore bar - Bar which forms off a very shallow coast line.
              offshore bar.PNG
            • Wave is forced to break off shore due to water shallowness.
            • They throw up a ridge of materials off the coast.
            • Between the bar and the coast there will be a shallow lagoon colonised by marsh plants.
          5. Cuspate foreland
            • Broad triangular shaped deposits of sand or shingle projecting from the mainland into the sea.
            • Two spits grow towards each other at an angle.
              cuspate foreland.PNG
            • A triangular feature called cuspate barrier develops which encloses a lagoon.
            • The lagoon is filled with mud and sand to form the foreland.
              cuspate foreland 2.PNG
            • Vegetation grows on the marsh and with time dries up e.g. ‘A’ Laree in Malagasy.
          6. Dune Belts
            Dune belts.PNG
            • Belt of low lying mounds of sand found on extreme landward side of the beach above the high tide level.
            • Sand on the beach dries up during the high tide.
            • It’s picked by onshore winds and deposited at a distance away from the reach of breaking waves.
            • It collects behind obstacles like grass or other vegetation and gradually builds up forming a dune.
            • The dunes may be covered with vegetation to form marshes.
          7. Mud Flats and Salt Marshes
            - Mudflats - Platform of mud consisting of fine silt and alluvium deposited in sea inlets such as bays and river estuaries.
            - Salt marshes - Vegetation such as grasses and mangrove that grows on a mudflat
            • Fine silt and river alluvium are deposited in sea inlets by tides.
            • A platform of mud builds up and is colonised by vegetation forming a swamp called salt marsh.
            • The dense network of plants roots trap more mud and alluvium causing the mudflats to extend seawards.

Factors Determining the Type of Coasts

  1. Wave action
    • Wave erosion makes a coast to have erosion features while deposition causes depositional features.
  2. Tidal currents
    • Where tidal range is high more surface area is exposed to wave action.
  3. Nature of rocks
    • Weak rocks are eroded to form bays (inlets) while resistant ones are left standing to form headlands.
  4. Alignment of coast
    • There is more erosion on exposed coasts while deposition occurs where the coast is obliquely aligned to the breaking waves.
  5. Change in sea level
    • Fall in sea level leads to emergence and rise to submergence.

 

Types of Coasts

According To the Alignment of Coast

  1. discordant/transverse/irregular coast
    • Coast which lies transversely to the coast line.
    • Has a large number of inlets and receives heavy rainfall because winds blow onshore e.g. Mombasa.
  1. Concordant coasts/regular/longitudinal coasts
    • One which lies almost parallel to the coastline.
    • Almost straight and lacks inlets and receives little rainfall due to winds blowing offshore e.g. Lamu.

According To Features Present

  1. Submerged Coasts
    • Coasts where a part of coastal land lies under the sea.

      Causes of submergence
    • Rise in sea level e.g. when large quantities of melt water were released to the sea causing its level to rise due to climate change at the end of ice age.
    • Sinking of coastal land and a part of the sea floor.

      Types

      Submerged Highland Coasts
      • Found where submergence occurs on a coast characterised by steep slopes.
      • Characterised by drowned features.
        1. Ria Coast
          ria cost.PNG
          • A Ria is a drowned river mouth.

            Characteristics

          • Funnel shaped
          • U-shaped in cross section.
          • Deeper and wider on the seaward side and shallower and narrower inland e.g. the Kenyan coast at Kilindini and Mtwapa.
        2. Fiord/Fjord coast
          fiord coast.PNG
          • A fiord is a submerged glaciated valley.
          • Ice deepens and widens glacial valleys until the floor lies below the sea level.
          • When the ice retreats sea water flows in drowning the valley forming inlets called fiords.

            Characteristics

          • Deeper inland.
          • Shallower at the sea ward end due to terminal moraine deposited when glacier was melting.
        3. Longitudinal/Dalmatian Coasts
          • Coast where ridges and valleys lying parallel to the coast line are drowned.
          • Valleys form inlets called sounds while ridges form islands.

      Submerged Lowland Coasts
    • Found where submergence occurs on a coast characterised by gentle slopes.

      Types
      1. Estuarine Coast
        • Coast characterised by broad shallow estuaries and mud flats which are visible at low tide.
        • Wider and shallower than rias e.g. coastlines of Guinea and Senegal.
      2. Fjard Coast
        • Coast characterised by numerous inlets formed by submergence of glaciated rocky lowland coasts.
        • Have numerous islands and are deeper than rias e.g. S.E. coast of Sweden.
  1. Emerged Coasts
    - Coast where part of seafloor has become permanently exposed.
    Causes of Emergence
    1. Decrease in sea level due to decline in the source of water e.g. waters being held up in a glacier instead of it flowing back as rivers to the ocean.
    2. Uplift of the coastal land by faulting, folding or isostatic adjustment.

    Types
    1.  Emerged Highland Coasts
      • Found where emergence occurs on a coast characterised by steep slopes.
      • Characterised wave action features which are isolated on land e.g. raised beaches, raised cliffs, raised wave cut platforms and raised arches.
    2. Emerged Lowland Coasts
      • Found where emergence occurs on a coast characterised by gentle slopes.
      • Characterised by exposed depositional features e.g. spits and offshore bars which are found on land and a coastal plain formed as a result of a part of continental shelf becoming exposed.
  1. Coral Coasts
    • Coasts composed of coral rocks which are exoskeletons of marine organisms called coral polyps.
    • They live in colonies/groups, feed on plankton and extract lime from the sea and build shells for protection.

      Conditions Necessary for Coral Growth
    • warm water(25-29◦C)
    • Saline and clear water.
    • Sunlight should penetrate at least to a depth of 50m to allow plankton growth.
    • Plentiful supply of plankton which they feed on.
    • Shallow water.

      Types of coral reefs
      1. Fringing Reefs
        fringing reefs.PNG
        • Platform of coral formed when coral polyps start building a reef near the shore.

          Characteristics
        • Flat or concave shaped
        • Higher on the seaward side
        • Outer edge falls steeply into the sea
      2. Barrier Reefs
        barrier reefs.PNG
        • Platform of coral formed a long distance from the shore.
        • Formed when polyps start to grow and extend seawards where conditions are favourable.

          Characteristics
        • Its coral is joined to the shore.
        • Its outer edge falls steeply into the sea.
      3. Atoll Reef
        atoll reef.PNG
        • Coral ring formed around a submerged island.

          Characteristics
        • Circular in shape.
        • Encloses a fairly deep lagoon.

          Theories of Formation

          Darwin’s Theory
        • Fringing reef develops around an island.
        • The island starts to sink.
        • Coral continues to grow upwards to keep pace with rising sea level and seawards because there is more food and water is clear.
        • The reef extends great distance away from the land to become barrier reef.
        • The island continues to sink becoming completely submerged.
        • The barrier reef forms a ring of coral called atolls.

          Murray’s Theory
        • Fringing reef grows on a submarine hill.
        • It disintegrates due to wave attack.
        • Coral fragments accumulate on the seaward end.
        • Polyps start building on it upwards where there is more food and clear water to form barrier reef.
        • The barrier reef forms a ring of coral called atolls.

          Daly’s Theory
        • During ice age there was withdrawal of water causing global fall in sea level.
        • Coral growth was retarded by low temperatures.
        • Waves pounded coral reefs and islands and flattened them to the same level as the sea.
        • At the end of ice age temperatures began to rise again favouring the growth of coral once again.
        • More water was added to oceans causing polyps to continue to grow upwards to keep pace with the rising sea level.
        • They were permanently exposed on the surface to form atolls

Significance of Oceans, Coasts and Coastal Features

Oceans

Positive

  1. Influence climate by contributing the bulk of precipitation, warming effect in cool season and cooling effect in hot season by breezes and ocean currents.
  2. Used for transport by means of boats, steamers and ferries.
  3. Tourist attraction by being site for recreation e.g. swimming and sport fishing and marine life in parks such as in Mombasa and Lamu.
  4. Oceans are a source of fish which is a source of food, income and employment.
  5. Source of fresh water when its water is distilled.
  6. Tropical seas have mangrove forests with mangrove trees which provide strong building and fencing posts and tannin for tanning leather and also habitat for marine life which is a tourist attraction.
  7. Source of salt which crystallizes naturally after water evaporates in constricted bays in hot climates.

Negative

  1. Tsunamis from oceans flood the neighbouring coastal areas causing great loss of life and property.
  2. Oceans may also flood the neighbouring coastal areas as a result of rise in sea level caused by melting of glaciers caused by global warming.
  3. Also harbour dangerous animals such as sharks and crocodiles which may attack and hurt or kill people.
  4. Drowning accidents when there is breakdown of vessels or ship wrecking.

Coasts and Coastal Features

Positive

  1. Fiords, rias and lagoons favour development of deep and well sheltered harbours.
  2. Fiords are also a good breeding ground for fish as their shallow continental shelf encourage growth of plankton which is food for fish.
  3. Coral rocks are used locally as building materials.
  4. Coral rocks are also a source of coral limestone for cement manufacture.
  5. Features such as coral reefs, caves, cliffs and fiords are a tourist attraction.

Negative

  1. Some emerged coastal lands have infertile soils unsuitable for agriculture for being covered by sand, gravel and bare rock.
  2. Deposited sands, bars and coral reefs are barrier to transport as they can cause ship wrecking if vessels hit them.


Action of Wind and Water in Arid Areas

  • An arid area is a land which is deficient of moisture leading to scanty or no vegetation.

Action of Wind in Arid Areas

Wind Erosion

  • Physical weathering is the initial process then it’s followed by wind erosion.
  • Wind is more effective in tropical deserts due to:
    1. Presence of loose unconsolidated dry masses of mud, sand and gravel.
    2. Occurrence of strong tropical storms.
    3. Absence of vegetation leading to high wind velocity due to little frictional force.

Factors Influencing Wind Erosion

  1. Wind speed- wind with high velocity has more energy to erode than with low velocity.
  2. Load- angular shaped load provide more effective abrasive tools than one which is round shaped.
  3. Nature of surface- Wind erosion is faster where the surface consists of unconsolidated materials.

Processes/Ways in Which Wind Erodes Deserts

  1. Abrasion
    • Materials carried by wind such as sand grains scratching rock surfaces across the path of wind.
    • Greater close to the ground because it’s where heavy and more effective abrasion tools are lifted and carried.
  2.  Deflation
    • Removal of unconsolidated materials such as sand and dust rolling and lifting or scooping and blowing away.
  3. Attrition
    • Sand grains carried by wind knocking against each other causing each other to become smaller and rounded in shape.

Resultant Features of Wind Erosion

  1. Millet seeds
    millet seeds.PNG
    • Sand grains which have been rounded to the shape of millet seeds by wind attrition.
  2. Ventifacts
    ventifacts.PNG
    • Boulders, stones and pebbles which are flattened by wind abrasion one or more sides due to changes in wind direction.
    • Dreikanter - Ventifact with three wind faceted surfaces formed when wind is blowing in different directions.
  3. Mushroom Block
    mushroom block.PNG
    Mushroom shaped rock in desert landscape.
    • Wind abrasion acts on a rock with uniform hardness.
    • It’s intensely undercut at the base and top part is slowly polished by light and less effective abrasive materials.
    • There results a rock with broad smoothed rounded top and a narrow base resembling a mushroom
  4. Rock Pedestal
    rock pedestal.PNG
    • Irregular rock pillar with a broad top and a narrow base found in the desert landscape.
    • Wind abrasion acts upon rock with alternating hard and soft layers.
    • Soft layers are eroded more than hard layers leaving hollows and protrusions.
    • There is more undercutting at the base where there is more abrasion.
    • There results an irregular rock with a narrow base.
  5. Deflation Hollows
    Deflation hollows.PNG
    • Shallow depressions found in desert landscapes formed by deflation.
    • Wind scoops and blows away unconsolidated materials such as dust and sand in a desert.
    • Circulating wind deepen the depression.
    • The hollow reach the water table forming an oasis.
    • If the depression doesn’t reach the water table flash floods water may accumulate into them to form temporary lakes called pans e.g. Etosha pan in Namib.
  6. Zeugen (Singular zeuge)
    zeugen.PNG
    • Ridges on a ridge and furrow desert landscape.
    • First there is a landscape with horizontal alternating layers of hard and soft rocks.
    • Weathering opens joints and cracks on the top resistant layer.
    • Abrasion erodes the opened joints while deflation carries away the eroded materials.
    • The process continues and with time causes a ridge and furrow landscape to develop. The ridges are zeugen.
  7. Yardangs
    yardangs.PNG
    • Narrow elongated rock ridges about 6m high on a desert landscape.
    • At first there is a surface with vertical alternating hard and soft rocks lying parallel to wind path.
    • Wind abrasion acts more on soft rocks and deflation carries away worn out particles.
    • The undercut rocks are left standing forming ridges called Yardangs.

Wind Transportation

Factors Influencing Wind Transportation and Deposition

  1. Wind velocity: when speed decreases strength also decreases and its ability to transport so wind starts to deposit materials.
  2. Wind direction- Winds blowing from different direction converge and cause load to collide causing some of it to be deposited.
  3. Nature of desert surface:
    • Wind transportation is more efficient on bare surfaces and hence less deposition there.
    • Water surfaces such as oasis and moist surfaces impede transportation through friction causing wind to deposit materials.
    • Less transportation on surfaces with vegetation as it reduces wind speed and also binds sand particles together.
  4. Obstacles- Objects such as rock masses, land forms and vegetation block and reduce wind speed causing deposition.
  5. Changes in weather conditions such as sudden showers halts transportation and causes deposition by washing down suspended materials.
  6. Load- Heavy load is deposited before light load when wind energy decreases. When many materials are transported by wind they collide causing each other to be deposited.

Processes/Ways in Which Wind Transports Load

  1. Suspension
    • Wind lifting and holding particles such as dusts by air currents and transporting them over long distances.
  2. Saltation
    • Wind transportation of heavy particles by a series of jumps and hops.
    • They are rolled.
    • They collide.
    • Bounce and get lifted.
    • Transported short distance ahead.
  3. Surface Creep
    • Wind transportation of heavy particles such as gravel and pebbles by pushing and rolling along the desert.

Resultant Features of Wind Deposition

  1. Sand Dunes
    - Dune - Low ridge of sand accumulated by wind deposition.

    Type
    1. Barchans
      barchans.PNG
      • Crescent shaped mound of sand in a desert.
      • Sand accumulates around an obstacle lying on the path of wind.
      • Deposition continues making the mound to grow bigger.
      • Wind blows sand over leeward side creating smooth gentle windward slope.
      • Wind eddies act on the leeward slope making it to be steep and concave in shape.
      • Side wind move the sand grains on the sides forward creating the two edges which are curved e.g. in Sahara and Arabian Deserts.

        Characteristics
      • Crescent/moon shaped
      • Smooth gentle windward slope
      • Steep concave leeward slope
      • Horns or 2 curved edges
      • Occurs individually or in groups
    2.  Seif Dunes
      seif dunes.PNG
      • Ridge shaped mounds of sand lying parallel to the path of prevailing wind.
      • Wind blows between barchan dunes.
      • Wind eddies move sand towards the sides.
      • Sand accumulates on the sides resulting into ridge shaped mounds e.g. in Namib Desert.
    3. Transverse/Wake Dunes
      transverse dunes.PNG
      • Wave like mounds of sand in a desert which lie at right angles to the prevailing wind.
      • Less strong winds blow over sand from one direction.
      • The wind concentrates larger grains of sand into series of transverse ridges.
      • Wind may continue pushing the sand causing it to accumulate on the leeward side to form wake dune e.g. in W. Sahara.
  2. Drass
    • Biggest sand features in a desert with surface resembling a plateau and with a height of up to 200m.
    • Barchans and Seif dunes may form on such features e.g. in E. Sahara desert.
  1. Loess
    • Fertile soils with great thickness of about 100m formed from deposition of dust from deserts.
    • Dust from deserts is carried beyond to wet areas.
    • It’s washed down by rain causing its deposition.
    • It accumulates into layers.
    • Deposition continues and the layers are compacted forming sedimentary rocks.
    • The sedimentary rocks wither to form fertile soils which favour cultivation e.g. Temperate lands of Europe along Rhine valley from Sahara dusts and along Huang He valley in N. China from dust of Gobi desert.

Action of Water in Arid Areas

  • Receives short occasional rains causing flash floods which erode transport and deposit large loads of materials produced by weathering.
  • Water action is short lived.

Resultant Features of Water Action in Arid Areas

  1. Wadis
    wadis.PNG
    • Wide deep steep sided dry valley in a desert
    • Strong surface runoff and flash floods form rills.
    • Rills are enlarged into gullies.
    • Flash floods flow into gullies widening and deepening them to form wadis.

      Characteristics
    • Wide and deep
    • Steep with cliff like walls
    • flat floor
    • Dry (lack permanent drainage)                
  2. Dry River Valleys
    • Valleys in arid areas through which streams flow during the wet season and dry up in dry season e.g. in Turkana, Wajir and Mandera.
  3. Mesas and Buttes
    messas and buutes.PNG
    Mesas
    - Extensive table like residual hills found in arid areas.
    Buttes - Smaller blocks of table like residual hills found in arid areas.
    • First there are sedimentary rocks occurring in layers with a resistant one on top and a less resistant below.
    • Weathering breaks the hard cap.
    • Then sheet floods break the surface and carries materials away.
    • Large outstanding blocks are left which are called mesas.
    • Mesas may be eroded farther to form smaller blocks called buttes.




      Features in an Inland Drainage Basin
      features in an inland drainage system.PNG
    • Gently sloping rock surface formed at the edges of desert uplands.
      • A steep/scarp slope of a highland is eroded by sheet flooding reducing its height.
    • The process continues causing the scarp slope to shift its position upwards.
    • The gently sloping surface of 6-7◦results at the foot of the upland.
  4. Playas/sebkha
    • Extensive inland drainage basin in a desert formed by deflation or crustal warping or a small fluctuating salty lake contained in an inland drainage basin in a desert formed when water from torrential outpours flows into the basin by multiple temporary streams e.g. Chemchane sebkha in Mauritania.
  5. Peripediment
    • Zone of thick alluvial deposits at the edge of playas in form of alkaline crust of mud, sand and gravel formed when streams deposit a lot of materials at the edge of the playa. Materials dry up leaving a hard salty crustal surface called Salina/salar g. in Arizona desert in U.S.A.
  6. Pediment
    pediment.PNG
  7. Peneplain
    • Low level plain formed when pediments are eroded to form a low level plain.
  8. Pediplain
    pediplain.PNG
    • Extensive low and gently sloping lands common in deserts.
    • Pediments surrounding a highland are extended by sheet erosion.
    • With time the highland is reduced to a residual hill like Inselbergs.
    • The hill is eventually eroded forming a continuous plain(Pediplain)
  9. Inselbergs
    • Prominent residual rocks in a desert.
    • Formed by extension of pediments into upland areas.

      Types
      Bonhardt - Steep isolated round topped mass of rock rising steeply from desert surface.
      • Dissection of plateau by streams producing steep sided valleys.
      • The plateau is further eroded forming remnant hills e.g. Sugar Loaf Mountain in Rio de Janeiro in Brazil.

      Castle kopje – Residual rocks in a desert found in groups.
      • Formed from break down of Bonhardt with closely spaced joints.
      • Or deep withering of a plateau edges.
      • Weathered rocks are then removed by water reducing plateau into Inselbergs e.g. Nzambani rock in Kitui.

Significance of Deserts and Desert Features

Positive

  1. Loess soils are used for agriculture because they are very fertile e.g. in Huang He valley and Ukraine.
  2. Loess soils in Europe and china have dug in caves which are inhabited during winter to provide warmth.
  3. Desert features are a tourist attraction e.g. rock pedestals, Yardangs, Zeugen and sand dunes.
  4. Oasis in deflation hollows are sources of water for domestic use.
  5. Oasis water is also used for irrigation such as of date palms.
  6. Deserts are good sites for testing military weapons, military training and experimenting ground for aircraft because they are sparsely populated.
  7. The scarce vegetation in deserts such as shrubs can be used in livestock keeping e.g. goats, camels etc.
  8. The hot sun in deserts can be harnessed to provide electricity for lighting, pumping of water, etc.
  9. Seasonal streams can be dammed to supply water to surrounding areas e.g. Kigombo dam in Mbororo in Taita which supplies water to Voi town.

Negative

  1. Some desert features can prevent physical development e.g. sand dunes can bury roads and it is difficult to construct bridges across wadis.
  2. Sand dunes can cover oasis and settlements.
  3. Sand dunes may destroy rich agricultural land.
  4. High temperatures, shortage of water, unreliable rainfall and lack of transport and communication infrastructure discourage settlement.


Under/Ground Water

  • Water that exists beneath the earth’s surface in pore spaces in soil and rocks.

 

Sources of Ground Water

  1. Rain Water
    • Some rain water which percolates and is trapped after meeting an impermeable rock.
  2. Melt Water
    • Water that infiltrates into the ground when snow melts during spring and summer.
  3. Surface Water
    • Water from rivers, seas, swamps, oceans, lakes and ponds that seep into the ground.
  4. Magmatic/Plutonic Water
    • Water trapped in rocks beneath surface during vulcanicity

Factors Influencing Existence of Ground Water

  1. Precipitation
    • For ground water to exist precipitation must exceed evaporation.
    • Light rain falling over a long period of time infiltrates more than heavy short lived downpour.
  2. Slope
    • On flat and gently sloping areas rain water has ample time to infiltrate because it remains in one place for a long time.
    • On steep areas there is low infiltration since a lot of water turns into runoff because of getting less time to percolate.
  3. Nature of Rocks
    • There is a greater possibility of existence of ground water where there are permeable surface rocks.
    • Ground water exists where impermeable rock overlie an impermeable one so that when water infiltrates and percolates underground it’s trapped by impermeable rock and accumulates above it.
      - Aquifer - permeable rock which is permanently saturated with water.
      - Permeable rocks - Rocks which allow water to pass through them.

      Types
    • Porous - Those with pores/airspaces between rock grains through which water passes e.g. sandstone, limestone and chalk.
    • Pervious - Ones with cracks fractures and joints through which water enters and passes e.g. granite, limestone and chalk.

      - Impermeable rocks
      - Ones which don’t allow water to pass through them.

      Types
    • Aquifuge - Impenetrable impermeable rocks e.g. gabbro, shale and slate.
    • Aquiclude - Porous rocks which absorb water and expand narrowing air spaces between grains preventing water to percolate downwards e.g. clay.
  4. Vegetation Cover
    • Plants break the speed of rain drops causing drops to hit the ground gently giving rain water ample time to percolate.
    • On bare surfaces most of precipitation flows away as run off.
  5. Level of Saturation of Ground
    • Infiltration is more on dry ground because it has wide open air spaces while and less on a ground whose air spaces are saturated with water.

Water Table

water table.PNG

  • Water that infiltrates and percolates into the ground fills air spaces creating zones of saturation whose upper levels are called water tables.

Zones of Saturation

  1. Zone of Non-saturation
    • Zone of permeable through which water passes but doesn’t remain in the pores for a long time.
    • Well sunk to this zone doesn’t contain any water.
  2. Zone of Intermittent Saturation
    • Zone which during the rain season the rocks are saturated with water while during the dry season they are unsaturated.
    • Temporary water table - Upper level of ground water in the zone of intermittent saturation.
    • Zone of non-saturation and that of intermittent saturation are called vadose zone.
  3. Zone Of Permanent Saturation
    • Zone where pores spaces are permanently filled with water.
    • Permanent water table - Upper level of ground water in the zone of permanent saturation.
    • Presence of ground water leads to formation of springs, wells boreholes and artesian basins.

Springs

  • Place where water flows out naturally onto the earth’s surface along a slope.

Ways/Modes Formation

  1. Hillside Spring
    hillside spring.PNG
    • Type formed where a permeable rock lies above an impermeable one on a hill and water comes out at the junction of those two rock layers.
  2. Dyke Spring
    • Type formed where an igneous dike cuts across a layer of permeable rock.
    • Ground water on the upslope side is trapped causing water table on that side to rises.
    • A spring develops where the water table is exposed on the surface.
      dyke spring.PNG
  3. Vauclusian Spring
    • Type formed on a limestone hill or escarpment overlying an impermeable layer.
    • Limestone rock becomes saturated with water.
    • Water comes out of the ground where water table appears on the surface.
      vauclusian spring.PNG
  4. Valley Spring
    • Type formed where water table intersects the surface along the side of the valley.
      valley springs.PNG

Artesian Basins

artesian basins.PNG

  • Saucer shaped depression consisting of a layer of permeable rock sandwiched between two impermeable rocks and the whole system forms a syncline.

    Characteristics
  • One or both ends are exposed on the surface on a rainy area or beneath a lake.
  • Water enters at the exposed end or ends.
  • With time the permeable rock is saturated with water and becomes an aquifer e.g. between Chad and Egypt across Quattara depression.

Artesian Well

  • Well sunk into the aquifer of an artesian basin from which water will come out without being pumped.

    Ideal Conditions for Formation of an Artesian Well
  • Aquifer to be sandwiched between impermeable rocks to prevent evaporation and percolation.
  • Aquifer to be exposed in a region which is a source of water e.g. rainy area or lake.
  • Aquifer to dip from the region of water intake.
  • Mouth of the well to be at a lower level than the intake area to develop hydraulic pressure which will force water out.

    Problems Associated With Artesian Wells
  • Water may be hot due to high temperatures.
  • Water may be salty because of water taking long to percolate through rocks thus dissolving large quantities of mineral salts.
  • Water may fail to come out naturally when water is drawn faster than it’s being replaced in the source region and necessitating pumping.

    Significance of Underground Water
  • Source of rivers and their tributaries.
  • Source of water for domestic and industrial use e.g. wells, springs, boreholes and oasis.
  • Used in agriculture e.g. oasis water is used for irrigation of date palms.
  • Influences settlements due to the availability of water e.g. in deserts people settle near oasis and spring line settlements in limestone areas.
  • Hot springs are a tourist attraction and their water is trapped and pumped into houses through pipes for heating during winter e.g. in Iceland.
  • Underground streams help in keeping some lakes fresh e.g. L. Naivasha.


Action of Water in Limestone Areas (Karst Region)

- Karst region - Region where the surface and the ground is covered with limestone rocks.

- Karst scenery - Unique features in a Karst region resulting from the action of water e.g. Shimoni caves at the coast of Kenya.

  • When calcium carbonate is exposed a weak carbonic acid formed by rain dissolving CO2 it’s dissolved to form calcium hydrogen carbonate which is removed from the rock in solution resulting in surface and underground features in such a limestone region.

Factors Influencing Development of Karst Scenery

  1. Surface rock and the rock below should be hard and well jointed for acid water to percolate and cause solution to happen e.g. limestone, chalk or dolomite.
  2. Climate should be hot to speed chemical weathering and humid for availability of rain which is a solvent.
  3. Should have a lot of vegetation to release CO2.
  4. Water table to be far below the surface so that the whole limestone rock is not dissolved and underground features fail to be formed.

Surface Features in Limestone Areas

  1. Grikes and Clints

    Clints - Blocks of limestone rock left standing when water infiltrates through the limestone rocks widening and deepening the joints.
    Grikes - Deep groves or gullies formed when rainwater infiltrates through limestone rocks widening the joints by solution.
    grikes and clints.PNG
  2. Swallow/Sink Holes
    • Deep vertical holes formed on limestone rocks when solution extends the grikes.
    • Referred to as swallow/sink holes because surface runoff or river water may disappear through them as a waterfall and come out of the ground as a vauclusian spring further downhill.
    • Vertical shaft from the surface of the sink hole down into the ground is called ponor.
      swallow holes.PNG
  3. Dry Valley
    • Steep valleys with no permanent streams on limestone surface at the section between the swallow hole and where the river emerges.
  4. Karst Window
    • Small outlet to the surface from a cavern formed when continuous carbonation at the surface causes the roof of the cave to collapse.
  5. Limestone Gorge
    • Deep steep sided river valley in limestone rocks formed when the swallowed river causes solution to continue underground causing the roof of underground water course to collapse.
  6. Karst Bridge
    • Small section left joining the roof between the karst window and gorge.
  7. Dolines
    • Elliptical hollow with gently sloping sides on the surface of a limestone region formed when several swallow holes collapse and merge.
      dolines.PNG
  8. Uvala
    • Depression which may be as wide as 1 km in diameter formed on the surface of limestone regions when several dolines collapse and merge.
      uvala.PNG
  9. Polje
    • Largest, shallow, elliptical and flat floored depressions on a limestone region formed when several uvalas collapse and merge.
    • May become a temporary lake or may be covered by a marsh.

Underground Features in Limestone Areas

underground features in limestone areas.PNG

  1. Stalactites
    • Finger like masses of calcite hanging vertically from the roof of a limestone cave or cavern formed by repeated evaporation of water and giving off of carbon dioxide from drops of water containing calcium bicarbonate hanging from the roof of the cave causing crystallisation of calcium bicarbonate into calcite.
  2. Stalactite
    • Stumpy rock masses of calcite which grow from the floor of a limestone cave upwards formed by repeated dripping of solution of calcium bicarbonate from the end of stalactite to the floor of a limestone cave then it spreads out and crystallizes.
  3. Limestone Pillars
    • Pillar like structures in limestone caves formed when stalactites and stalagmites grow towards each other, stalagmite grows until it touches the roof of a cave or when a stalactite grows until it touches the floor of the cave.
  4. Limestone caves
    • Underground chambers or cavities in limestone rocks.
      • Underground rivers dissolve limestone in horizontal joints forming a horizontal tunnel.
      • Surface water and underground water percolates through the joints into the tunnel enlarging it forming a phreatic cavee. cave below the water table.
      • The water flows out at the vauclusian spring lowering the water table causing the phreatic cave to become a limestone cave.
      • Continued solution from water percolating through the caves roof widens and lowers its floor to form a larger cave called limestone caverng. Carls band cave in New Mexico U.S.A.

 

 

Significance of Karst Scenery

Positive

  1. Features in karst scenery are a tourist attraction e.g. caves, gorges, stalactites, stalagmites, etc.
  2. Limestone rock is used in the manufacture of cement e.g. cement factory at Bamburi in Mombasa and Athi River.
  3. Limestone blocks are also used for building.
  4. Limestone regions are very good for grazing particularly sheep because the surface is dry.
  5. Large villages called spring line settlements form at the line of vauclusian springs due to the availability of water.

Negative

  1. Limestone landscape discourages settlement because the surface is rocky, soils are thin and unsuitable for agriculture, surface is rugged with features like grikes and Clints and the water supply is inadequate due to rivers disappearing into swallow holes.


Glaciation

  • Action of moving ice.

Glacier

  • Mass of ice moving outward from an area of accumulation.
  • Formed when snow accumulates on the surface, lower layers are compressed to a harder mass resulting in opaque ice due to air bubbles and accumulation continues compression lower layers squeezing out air forming glassy ice called glacier.

    Types
  • Cirque glacier - ice occupying a cirque.
  • Valley glacier - Ice confined within a valley
  • Piedmont glacier: Glacier formed when valley glaciers converge at the foot of the mountain.

Ice bergs - Large mass of ice floating in the ocean formed when an ice sheet moves to the sea e.g. in Arctic and N. Atlantic Ocean.

Ice sheet - Continuous mass of ice covering a large area on the earth’s surface.

Ice caps - Ice covering the mountain peak.

Snow line - Line beyond which there is a permanent snow cover.

Ways of Ice Movement

  1. Plastic Flowage
    • Movement of ice like a viscous liquid.
    • Great pressure is exerted at the bottom sides and centre causing some ice particles to melt slightly and move slowly down hill like a viscous liquid.
  2. Basal Slip
    • Movement of ice by sliding over the underlying rock.
    • Pressure is exerted on deepest layers of ice in contact with the rock beneath causing melting.
    • A film of water is created which acts as a lubricant between the ice and the rock causing ice to slip and slide over the rock and move down slope.
  3. Extrusion Flow
    • Movement of ice by spreading out.
    • Ice accumulates building to great thickness at the centre causing compression of layers of ice beneath.
    • The layers beneath are forced to spread out where there is less pressure.
  4. Internal Shearing
    • Breaking of ice into smaller pieces which move alongside one another.
    • Uneven movement causes ice to develop cracks on the surface.
    • The glacier breaks into smaller pieces which move alongside each other down slope.

Factors Influencing Ice Movement

  1. Gradient of the Land
    • Ice moves faster on steep slopes than on gentle slopes due to the influence of gravity.
  2. Season
    • Ice movement is faster in summer due to frequent thawing melting compared to winter when thawing is rare.
  3. Friction
    • Central parts of ice move faster than sides and bottom which are in contact with rock beneath due to friction.
  4. Thickness of Ice
    • Thicker masses of ice cause more pressure between them and rocks beneath which cause slight melting and therefore faster movement.

Glacial Erosion

Processes/Ways in Which Ice Erodes

  1. Plucking
    • Pulling away of parts of a rock at the base of glacier when the ice freezes into the cracks of a well jointed rock.
  2. Abrasion
    • Scratching of the underlying ground by stones and boulders carried by the ice as the glacier moves.

Factors Influencing Glacial Erosion

  1. Nature of Underlying Rock
    • Abrasion is more effective on soft rocks than hard rocks.
    • Well jointed and faulted rocks are more eroded than those which are not because cracks and joints enable water to enter rocks and freeze which facilitates plucking.
  2. Gradient of Slope
    • Glacier on steep slopes moves faster and has greater kinetic energy to erode than slow moving glacier
  3. Thickness of Ice
    • Thick ice is heavier and exerts greater pressure on rock debris making them to abrade the underlying rock more effectively.
  4. Availability of Debris
    • The more the rock debris the more effective abrasion will be since it acts as abrasive tools.
    • Too heavy debris makes erosion impossible since ice is not able to transport it but glides over it without acting on the rock below.

Erosion Features

On Glaciated Highlands

  1. Cirque
    cirque.PNG
    • Arm chair shaped depression on glaciated slopes of high mountains.
    • Snow accumulates into a shallow depression on the side of a mountain.
    • Freezing in winter and thawing in summer causes rocks to wither and break up resulting in enlargement of the hollow.
    • Accumulated ice advances by slipping down slope.
    • A deep crevice called bergshrund develops at the top of ice due to unequal movement.
    • Freezing occur deep down the bergshrund causing the back wall and sides to be steepened by plucking.
    • Plucked debris is carried forward scratching the floor of the basin deepening it forming the cirque, corrie or cwm.
    • Water from melting snow may accumulate in a cirque to form a tarn e.g. Teleki tarn.
  2. Arêtes
    • Narrow knife- edged steep ridge separating two cirques.
    • Formed when two cirques cut backwards on adjacent sides of a mountain leaving a narrow steep ridge separating them.
  3. Pyramidal Peaks
    • Sharp steep sided peak at the top of a mountain.
    • Formed when three or more cirques erode on mountain side towards each other leaving a sharp pointed rock separating them at the top of the mountain e.g. Corydon and Delamere on Mt. Kenya.
  4. Glacial Trough
    • Glacial Trough and Related Features
      glacial trough.PNG
    • Wide flat bottomed valley with steep sides on a glaciated highland.
    • Ice accumulates in a v-shaped valley.
    • Plucking and abrasion by ice occurs.
    • The v-shaped valley is deepened, widened and straightened to become a glacial trough.
    • Glaciated trough may be submerged to form a fiord.

  5. Truncated Spurs
    • Interlocking spurs of former river valleys which are eroded and straightened by valley glacier.

Erosion Features on Glaciated Lowlands

  1. Roche Mountonnee
    Roche Mountonnee.PNG
    • Rock outcrop with a long smooth gentle slope on the upstream side and a rugged steep slope on the downstream side found on glaciated lowland.
    • Formed ice acts on a rock on its way causing the side facing the upstream side to be polished by abrasion resulting into a smooth gentle slope and the downstream side is affected by plucking resulting in a rugged steep slope leaving a rock outcrop standing just above the surface.
  2. Crag and Tail
    crag and tail.PNG
    Crag - projection of resistant rock which protects a mass of softer rock on the downstream side of the glacier.
    • The ice moves over and around over the resistant rock eroding it slightly by abrasion.
    • Cracks develop on the upstream side causing the ice to move and pluck materials from the resistant rock leaving a projection of resistant rock with a steep rugged upstream side is formed. 

    Tail - elongated feature on the downstream side of the crag formed by formed by material deposited by the glacier on the downstream side and the weaker rock.
  3.  Depressional Lakes
    • Depressions filled with water from melting ice found in glaciated lowlands.
    • Formed when soft rocks are scooped out by moving ice sheet forming depressions which are filled with water to form a lake.

Glacial Deposition

  • Material carried by the glacier is called moraine.






    Types of Moraine
    types of moraine.PNG
    1. Ground/sub-glacier moraine - load carried at the base of the glacier.
    2. Englacial moraine - load within the glacier.
    3. Lateral moraine - load carried at the sides of the glacier.
    4. Medial moraine - load carried in the centre of the valley by glacier.
    5. Terminal/recessional moraine - load deposited at the point where a glacier melts.

Types of Glacial Deposits/Drift:

  1. Till - directly deposited by ice on melting in unstratified manner.
  2. Fluvial - materials deposited by water from the melting ice in stratified manner.

Causes of Glacial Deposition

  1. Amount of glacial drift
    • When ground moraine is too much the glacier glides over it leaving it behind.
  2. Weight of glacier
    • When more ice is added to a stationary glacier pressure is exerted at the base causing melting and the material which was embedded in the ice is dropped.
  3. Climatic change
    • During summer and spring ice melts depositing some materials the glacier was carrying.
  4. Friction beneath the ice
    • Friction between ice and surface reduces ice speed causing heavy materials to be deposited beneath ice sheets.
  5. Slope
    • Lowlands allow glacier to accumulate a lot of materials which are finally deposited by melting ice.

Features Resulting From Glacial Deposition

  1. Till Plain
    till plain.PNG
    • Extensive area of flat relief resulting from burying of former valleys and hills by glacial deposits.
  2. Erratics
    • Large boulders of resistant rocks transported by glacier from highland and deposited on the till plain.
  3. Drumlins
    drumlins.PNG
    • Long egg shaped hills deposited and shaped under an ice sheet of very broad glacier.
    • Glacier deposits boulder clay at the valley bottom due to friction between the bed rock and the boulder clay.
    • With more deposition large mounds of deposits are formed.
    • The moving ice streamlines the till that has been deposited irregularly resulting into the upstream sides of the till being steep but smoothed.
  4. Terminal Moraine Ridge
    terminal moraine ridge.PNG
    • Ridge like feature formed by extensive deposition of moraine along the edge of an ice sheet.
    • Ice remains stagnant for a very long time.
    • The ice at the edges of sheet melt and a lot of materials are deposited.
  5. Eskers
    eskers.PNG
    • Long winding ridge composed of gravel formed by glacial deposition.
    • Streams carrying large amounts of load flow fast in a sub-glacial tunnel parallel to the direction of moving ice.
    • When the ice melts the tunnels collapse causing streams to slow down and deposit much of the load forming a ridge.
  6. Kame
    kame.PNG
    • Isolated hill made of sand and gravel which have been deposited in strata by glacial water.
  7. Kame Terrace
    • Ridge of sand and gravel occurring in narrow lakes that exist between the glacier and an adjacent highland.
  8. Outwash Plains
    outwash plains.PNG
    • Wide gently sloping plain composed of gravel and sand formed by glacial deposition.
    • Formed when finer materials of terminal moraine are deposited in very thick layers over an extensive area forming a plain.

Significance of Glaciation

Positive

  1. Some outwash plains have fertile morainic soils suitable for agriculture e.g. Canadian prairies where wheat is grown.
  2. Water falls on hanging valleys are used for generation of H.E.P.
  3. Glaciated highlands are a tourist attraction especially during winter when sporting activities such as skiing and ice skating are carried out.
  4. Glacial lakes such as great lakes of N.America provide natural route ways and fish sources e.g. L.Superior and Huron.
  5. Glaciated mountains are catchment areas for permanent rivers.
  6. Sheltered water of fiords is a suitable bleeding ground of fish as natural harbours.
  7. Sand excavated from outwash plains and eskers is used for construction.

Negative

  1. Land in glaciated areas can’t be fully utilised for agriculture due to being marshy because of boulder clay deposits e.g. central Ireland.
  2. Infertile sands deposited in outwash plains make land unsuitable for agriculture.
  3. Numerous lakes formed as a result of morainic deposits reduce the land available for agriculture.
  4. Settlement and transportation in glaciated landscape is difficult due to ruggedness caused by glacial action.

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