The External Landforming Processes.

These processes occur at the earth’s surface. They are divided into two groups: Denudation and deposition.

Denudation processes.

These wear down the landforms reducing them to an almost level plain called a peneplain. The high fold mountains, basalt lava plateaus, escarpments, and all other landforms we have already discussed, are gradually being reduced in height by denudational forces. These are weathering, erosion, mass wasting and transport.

Erosion is the wearing away of rock material of the earth’s surface by agents such as wind, ice, running water and waves.

Mass wasting is the down hill movement of rock debris and soil under the force of gravity. This takes place particularly on steep slopes. On such slopes, material tends to move downwards without any agent involved.

Transport involves the carrying away of eroded or weathered material by running water, wind and so on.

Deposition process:

Deposition is the complement of denudation. It is the laying down of transported materials by rivers, waves, glaciers and wind.

Weathering and its effects.

Weathering is the breaking down or disintegration and decay of rocks at or near the surface of the earth. It takes place slowly that we can not notice it. We can only see its effects on the landforms. The broken particles may be transported by agents of transportation to other places.

Types of weathering.

There are three types of weathering: Physical weathering(mechanical) weathering, chemical weathering and biological weathering.

1. Physical or mechanical weathering.

This is the disintegration of rocks into small particles. It does not involve any chemical change.

The original rock remains the same, for example, when a piece of chalk is broken into two, the smaller pieces still remain chalk, only they are now smaller.

Physical weathering takes place in several ways(processes):

- exfoliation(temperature changes)

- unloading or pressure release

- crystal growth

- frost action

- block disintegration.

Exfoliation.

The rocks of the earth’s crust are poor conductors of heat. Semi-arid and arid regions of East Africa have high day and low night temperatures. Rock surfaces in these regions alternately heat and cool.

This causes the outer parts of the rocks to expand during the day and to contract during the night. Alternative expansion and contraction of rock set up powerful internal stress in the top few centimetres of the rock. The stresses produce fractures which cause the surface rock peel off as thin sheets or slakes. This process is called exfoliation.

See diagram below:

Exfoliation leads to the development of smooth, rounded domes called exfoliation dome.

Unloading or pressure release:

This process results from both extensive and intensive erosional activity on rock surfaces. When overlying weight of rocks is removed by denudation, the new exposed surface then expands to adjust and in the process, cracks develop on the surface, such cracks develop parallel to the surface of the rocks.

Crystal growth.

Salt minerals in rocks have also been considered to cause disintegration of rocks under certain conditions.

Different salts like common salt(sodium chloride) gypsum (calcium sulphate) and sodium carbonate enter rocks in solution form. On drying and crystallizing , they expand. In this way they produce stresses in the rocks which may break them up.

This process is also called salt weathering or salt crystallization. It is common in arid and coastal areas of East Africa. For example, the lake Magadi region and North Eastern Kenya.

Frost action.

In some highland regions of East Africa water often enters cracks and joints of rocks. When temperature falls to or below freezing point, it may freeze and expand in volume by about 99 and a tremendous pressure is applied to the sides of the cracks. Repeated freezing and melting, causes the cracks to widen and deepen. Over a long period of time frost action breaks down rocky outcrops into angular fragments. The process by which rocks break up by frost action is known as frost shattering. In steep slopes, rock fragments fall at the bottom collecting there in a pile to form a scree. This process is common on Mt. Kenya, Mt. Kilimanjaro and Rwenzori.

Block disintegration.

This type of mechanical weathering occurs in rocks which are well- jointed. The heating by day causes expansion and the cooling by night causes contraction. This sets up powerful internal stresses in the rock which cause it to break down large rectangular – shaped blocks. This break down is facilitated by action of water which helps to enlarge the joint.

See diagram below:

Chemical weathering.

This is the decomposition of rocks due to chemical reactions between rock minerals, water and atmospheric gases like carbon dioxide and oxygen. Rocks can also decompose when their minerals react with organic acids from decaying vegetation. The reactions change the original composition of rocks producing new types. The rocks decay and breakdown slowly into smaller fragments. Chemical weathering consists of several processes such as:

Solution: This is the first stage of chemical weathering. In this process, water dissolves soluble materials which are carried away in solution. For example common salt(sodium chloride) gypsum(calcium sulphate), calcium chloride etc. But solution as a weathering process is important in the removal of certain residual products of other weathering processes notably hydrolysis and carbonation.

Hydrolysis. This process involves a chemical reaction between rock minerals and water. For instance, hydrogen ions from water react with the ions of the rock minerals, the rock begins to disintegrate. Hydrolysis is a major process in the decomposition of felpars which are important minerals in igneous rocks. The felpars break down to produce clay minerals, potassium carbonate and silica.

- Felpars + water carbon dioxide

Clay minerals + potassium carbonate + silica

Carbonation. This process occurs when carbon dioxide in the air combines with rainwater to form a weak acid called carbonic acid. Carbonic acid dissolves rock compounds particularly lime stone dolomite to form other features. For example, it combines with calcium carbonate to produce calcium hydrogen carbonate which is removed in solution by water. The chemical reaction involved is:

H₂O + CO₂ → H₂CO₃

(water) (carbon dioxide) (carbonic acid)

CaCo₃ + H₂ Co₃ → Ca(HCO₃)2

(limestone) (carbonic acid) (Calcium Bicarbonate)

Carbonation is common in limestone areas of East Africa such as, the coast of Kenya and Tanzania and Nyakasura in Fort Portal (Uganda).

Oxidation. The process occurs when rock minerals absorb additional oxygen to produce oxides or hydroxides. Oxidation of minerals takes place in association with water in which atmospheric oxygen has been dissolved. It is most active in the zone above the water table.

Oxidation is responsible for deep weathering on hot and humid tropical regions. It causes rock decay and break up particularly of sedimentary rocks such as clay containing iron compounds. Oxidation can be detected by changes in colour. A good example of this process is when ferrous oxide(blue or grey) changes into ferric oxide(red or brown).

Hydration. Certain rock minerals absorb water and expand causing internal stress and finally fracturing of the rock. A good example is the conversion unhydrated calcium sulphate to hydrated calcium sulphate(gypsum):

Caso₄ + 2H₂O → Caso₄ . 2H₂O

(unhydrated) (water) (gypsum)

Another example is of haematite, an iron oxide, absorbs water to form limonite(yellowish iron ore).

2Fe₂ O₃ + 3H₂O → 2Fe. O₃. 3H₃O

(haematite) (water) (limonite)

Reduction. This is the opposite of oxidation. It involves the removal of oxygen from rock minerals and addition of hydrogen to them. It takes place in water logged areas like swamps where the pore spaces in rocks are filled with stagnant water. An example of reduction is when reddish or yellowish coloured ferric iron compounds change to grey blue ferrous oxides. It is this process which gives swampy soils(clays) their characteristic is grey colour.

3. Biological weathering.

Biological weathering involves the disintegration or break down of rocks by living organisms. The living organisms responsible for rock breakdown include: plants, man and other animals like bacteria, moles, rodents, rabbits and so on.

Biological weathering takes place in several ways(processes).

Respiration.

Respiration by soil organisms such as bacteria, fungi, earthworms, burrowing animals and the roots of other plants increase the level of carbon dioxide in the soil. The carbon dioxide produced combines with soil water to form a weak carbonic acid which dissolves rock minerals.

Burrowing and churning.

Burrowing is the break down of rock materials by burrowing animals such as squirrels, moles, rabbits and others. Besides, earthworms breakdown rocks by eating them. Termites, earthworms and burrowing animals mix rock particles and expose fresh rock materials to agents of weathering such as water, sunshine and air. The process of mixing rock materials is called Churning.

The action of plant roots.

The roots of plants , particularly trees penetrate the cracks in rocks. As the plants grow, their roots get bigger and widen the cracks, leading to the rock breakdown. Besides, the roots of some plants produce substances like humic acids which cause rock decomposition.

Man’s activities.

The activities of man such as cultivation, road construction and quarrying often cause the breakdown of rocks.

Factors influencing the rate and nature of weathering.

The main factors controlling the rate and character of weathering are; climate, relief, nature of the rock, time and plants and animals.

Climate:

This has great influence on weathering processes in the following ways;

Physical weathering process largely depend on temperature changes which fall under climate. Also chemical weathering processes largely depend on water as a medium. Therefore they are most active in areas with high temperatures and high rainfall which are the main attributes of climate.

Relief:

On gentle slopes, weathered material accumulates burying the unweathered surface. This reduces further weathering. On steep slopes, rocks are rapidly weathered because the weathered material is quickly removed and weathering rate becomes faster.

The nature of the rock.

The nature of the rock particularly mineral composition and rock hardness are important factors. For instance, dark coloured rocks heat up and expand faster than light – coloured rocks. A good example is basalt( dark- coloured) is weathered more rapidly than granite which is light coloured.

Time:

The longer the weathering duration, the deeper the depth of weathering and also the more advanced the stage of weathering would have reached.

Plants and animals:

Thick vegetation cover such as tropical forests act as protection against physical weathering and also help to slow down removal of the weathered layer. In deserts and some very high mountains, the absence of vegetation cover accelerates the rate of weathering. Plants and animals produce chemicals which cause chemical weathering of rocks.

The movement of animals and insects such as termites and rodents promotes weathering by mixing and loosening partially of weathered rocks.

Man uses explosives, hammers, grinding machines and so on to break rocks down into smaller units.

v) Frost disintegration

In areas where alternate freezing and melting occurs during night and day respectively, water seeps into cracks in the rock and on freezing it widens the crack. In well jointed rocks this can be very significant in bringing about disintegration or ice wedging.

vi) Mechanical weathering by living Organisms

Mechanical weathering also results from action of both plants and animals e.g. the use of explosives, hammers, grinding machines etc. to break rocks down into smaller units mainly for use as building slabs, road-stones etc.

When roots of trees penetrate and grow into rock cracks and joints expand them and eventually break the rocks into smaller Iragments, physical weathering is said to have occurred.

Generally products of mechanical weathering are comparatively coarse and often angular while products of chemical weathering are finer than those of mechanical weathering. The fact that the residues of chemical weathering are normally clay or soluble minerals, has many implications. In an area where chemical weathering is dominant, rivers may only contain suspension or solution loads and corrosion may be non-existent. Where mechanical weathering is dominant, rivers will move large traction loads and corrosion may be the dominant process.

Factors controlling the rate and character of weathering

i) Rock Hardness

Rocks vary a great deal in hardness depending on their constituent minerals, nature of their cementation, the degree of their compression (which may in , part be a reflection of their age). Most igneous rocks are hard due to their mineral composition (e.g. feldspars and quartz) and because these minerals in the process of cooling and crystallization, are tightly bonded together.

4) Sedimentary rocks are generally softer although they may be composed of very hard minerals such as sandstone. A sandstone may largely be made up of quartz grains but will be quite weak because these are bonded by a soft cement such as iron oxide or calcium carbonate.

On the other hand if the cement happens to be very resistant, the rock may be extremely hard. Quartzite which comprises quartz particles cemented by silica is one of the hardest and most enduring of all rocks.

ii) Chemical Composition

This factor is of prime importance in influencing rock resistance to chemical decay. It can also have some effects on mechanical weathering e.g. a rock composed of variously coloured minerals is subjected to strain because the capacity of these minerals to absorb the sun's heat is not uniform and as such differential expansion and contraction is caused. In areas where insolation weathering is able to operate such as in continental deserts, rock colour may help to determine the amount of block and granular disintegration.

On the earth's surface as a whole chemical weathering is more active in the breakdown of rocks than is mechanical disintegration and in some regions its dominance is very pronounced. It is especially effective in the presence of water (which allows the formation of acids) and high temperatures. (Which speeds up the rate of chemical reaction) s

Resistance of rocks to chemical decay will depend on their composition, though other factors such as rock jointing and porosity (which permit entry of water, acids and oxygen), and the prevailing conditions all of which must also be taken into account.

Rock Jointing

This is a factor of utmost importance in all types of weathering because jointing has the effect of:

· Greatly increasing the surface area of rock available for attack by chemical processes.

· Allowing the entry of water, organic acids and oxygen.

· Providing lines of weaknesses which can be utilised by mechanical agents _: such as ice wedging and plant roots.

Weathering processes are aided by joints in a number of ways of which the following are only a few:

a) Chemical weathering by acidified rain water is

concentrated along joints

b) Joints aid the process of frost and ice wedging in

periglacial and glacial climates.

c) Processes of exfoliation almost entirely depend on the

existence of sheet jointing resulting from dilation.

N.B. Joints in rocks may be developed in three main ways:-

a) In igneous rocks tensile stresses are set up as a result of contraction and cooling.

b) In sedimentary rocks joints are produced by shearing and tensional forces set up during earth movements.

c) In crystalline rocks such as granite or gneiss, dilation occurs after a heavy overburden of rocks has been removed by denudation. As pressure release operates, the rock mass tends to recoil upwards and joints running parallel to the surface (sheet jointing) are formed.

iv) Climate

This has a great influence on weathering processes in the following ways:

a) Frost weathering occurs only where there are atmospheric cycles of freezing and melting of ice.

b) Insolation weathering requires a considerable diurnal fluctuation of temperature between very hot during day and cold at night.

c) All types of chemical weathering operate most effectively in very warm climates because the intensity of chemical reactions increase with temperatures but up to certain points only.

d) Chemical weathering is most active in wet climates because water is essential to processes such as hydration, hydrolysis and carbonation.

v) Relief

This factor is not generally taken into account in consideration of weathering, but it can exert a good deal of influence. The renewal of exposure of live rock is essentially to the continuation of mechanical weathering. Hence in areas of high relief and steep slopes, which favour mass transportation processes such as landslides, slumping, soil creep and solifluxion, the constant laying bare of rock surfaces is a common occurrence brought about by frost and insolation weathering processes.

vi) Plants and Animals

Thick vegetation cover such as tropical forests act as protection against physical weathering and also help to slow down removal of the weathered layer. In deserts and some very high mountains, the absence of vegetation cover accelerates the rate of weathering. Plants and animals play an important role in rock destruction by chemical decomposition through action of organic acid solutions. These acids develop from water percolating through decayed vegetation and animal matter.

The movement of animals and insects such as termites and rodents promotes weathering by mixing and loosening partially weathered rock. This increases the access of oxygen and water to mineral particles and 'carrying organic matter down from the surface. Trees and other plants act as agents of weathering by the action of their roots which grow deep into the ground and open up joints.

Man uses explosives, hammers, grinding machines etc. to break rocks down into smaller units, mainly for use as building materials. Similarly man's activities such as deforestation, poor cultivation methods, overgrazing and open cast methods of mining help to expose the earth's surface to agents of weathering.

vii) Factor of Time

The longer the weathering duration, the deeper the depth of weathering and also the more advanced the stage of weathering would have reached.

Weathering in the Major Climatic Region

i) Equatorial Regions

High temperatures and excessive rainfall throughout the year promotes very active chemical decomposition. Except on steeper slopes, where mud flows and landslides occur, much of the weathered materials remain "in situ" especially in tropical forest areas where the root systems reduce surface creep.

Mechanical weathering is virtually absent in equatorial and tropical rain forest areas where solid rock is normally cushioned from atmospheric temperature changes by the layers of rotted rock, decaying vegetation and the dense vegetation cover. However, temperature changes may contribute to exfoliation of inselbergs and other exposed rock outcrops such as when rain suddenly cools a previously heated rock surface. But it should be noted that physical weathering is not a major factor where humidity is high and both annual and daily temperatures are low.

ii) Savannah regions

These are regions of alternating wet and dry climates. This means that both chemical and physical weathering are important. During the long dry season, the sparse vegetation of grass and shrubs give only limited protection to the ground against changes in temperature and humidity. By the end of the dry season, the ground surface resembles that of a desert especially in regions where annual bush burning is common. On bare surfaces daily temperature ranges can be as 30°C thus helping to promote exfoliation and block disintegration.

Chemical weathering is especially active during the wet season when humidity is higher, while the high evaporation rates of the dry season help in drawing mineral salts to the surface by capillary action. A result of weathering processes in savannah lands, is the formation of lateritic duricrusts and inselbergs.

Hi) Arid and Semi Arid Regions

It was formally believed that in arid areas chemical weathering was largely inactive due to lack of moisture and that mechanical weathering was at its peak.

These are regions of high daily ranges of temperatures producing alternate expansion .and contraction of the many bar-e rock surfaces which in turn causes block and granular disintegration and exfoliation. The general coarseness of detritus (weathered materials) in desert areas and the relative lack of decomposition" products such as clay points to the importance of physical breakdown of rocks.

However chemical processes which can act in the presence of minute quantities of water (moisture), assist greatly in exfoliation and the basal weathering of steep slopes and isolated rocks in desert. There are rocks that are normally susceptible to chemical attack e.g. limestone and dolomite but appear to be highly resistant under arid conditions.

Temperate Climate

Most of the main weathering processes are found in this region and in rocks in mountain areas on cliff faces. Oxidation affects rocks containing iron minerals. Carbonation is active in chalk and limestone country. Hydrolysis and associated processes of carbonation and solution operate on igneous rocks.

The important point here is that none of these processes acts very rapidly in temperate conditions. Chemical weathering is more active than mechanical weathering because it is aided by the more or less continuous soil and vegetation cover which favours infiltration of rain water and the generation of organic acids.

Moderate temperatures mean that the rates of chemical reactions are not fast and one never encounters deep rotting of rock as is the case with the tropical humid areas. In Africa, such temperate climates are found in the sub tropical areas of South Africa and North West Africa north of the Atlas mountains. Also found in this category of regions are the East African mountains such as Ruwenzori, Kenya and Kilimanjaro.

v) Arctic Regions

The dominant weathering process in this region is frost action which produces spreads of blocky debris together with large quantities of finer materials resulting from granular disintegration of larger boulders.

In this region weathering occurs in two main ways

a) On steep bare rock faces water can penetrate into cracks, freeze and cause ice wedging.

b) In the active zone above permafrost freeze and thaw cycles lead to the rapid breakdown of solid rock or weathered material being moved by solifluxion. The part of chemical weathering in glacial and periglacial climates is usually considered to be insignificant.