Factors influencing Weathering and Mass Wasting
Weathering is the breaking, disintegration and decomposition of rocks on or below the earth's surface in situ. The breaking and disintegration occur as a result of physical or chemical processes. This process of weathering leads to production rock waste or debris.
Factors influencing weathering
i) Climate. Relief/Topography.
ii) Nature of rocks/chemical composition/ structure! texture.
(iii) Action of plants and animals/Biological organisms.
There are various elements of climate that weather rocks. These are temperature, precipitation, humidity, sunlight and cloud cover. These elements affect weathering by controlling the amount of solar energy and water that interact with the earth's surface. The results weathering vary from one place to another depending on the climate.
Weathering varies from place to place depending on the nature of the slope. On steep slopes the weathered layers are removed quickly, exposing a new layer to further weathering. On gentle slopes, the removal of the weathered layer is slower. Weathering is faster on steep slopes than on gentle slopes,
(Ill) Nature of the rocks
This depends on two factors.
(a) Structure of the rock
Lines of weakness play an important role in influencing chemical and physical weathering. The joints and bedding planes influence the freeze and thaw actions e.g. on Mt. Kenya. The combined effect of temperature change and chemical decomposition along the joints results in pressure release. This leads to outer parts of rocks peeling off in a process known as exfoliation.
(b) Mineral composition
The mineral composition in the rocks varies from one rock to another and this leads to the variation in the rate of weathering.
Minerals expand when heated at different rates according to their composition and the size. This difference in expansion leads to stress in the rocks and eventually the rock disintegrates. Different minerals are known to absorb heat at different rates depending on their colour.
(c) Color difference
Dark color minerals absorb more heat hence expand more causing internal stress in the rock leading to break up.
(IV) Action of plants and animals
Thick vegetation cover for example in the tropical rainforest vegetation acts as a protection against physical weathering. The vegetation slow down the removal of the weathered layer. The absence of vegetation therefore accelerates the rate of weathering. Plants and animals play a significant role in chemical decomposition through the action of organic acid solutions. The action of animals such as termites and rodents helps in weathering by loosening and partially mixing weathered rock. Tree and plants act as weathering agents by the action of their roots, which grow deep into the ground and open up joints.
Weathering can be classified into;
1. mechanical /Physical
3. Biological (Combination of physical and chemical) weatherings.
1. mechanical /Physical
This is disintegration or breaking down of the rocks into smaller fragments without causing changes in the chemical compositions of the rocks. It occurs in all regions where there are extreme temperature changes such as deserts, high mountains and the arctic region. The main processes of physical weathering include;
(b) Granular disintegration.
(c) Block disintegration.
(d) Frost action (frost shattering)
(e) Pressure release (unloading)
(g) Crystal growth.
This is the peeling off or scaling of layers of rock due to alternate expansion and contraction of the outer layer in a rock. This is caused by temperature differences.
It is common in arid areas where, during the day the rock surface heats up rapidly and then cools at night. Rocks are poor conductors of heat and so there is little or no Heat that is transferred to the inner layer. The minerals of the outer layer expand at a higher rate than those of the inner layer. This causes strain to result between the two layers and with time the outer shell will develop cracks and later peel off. Exfoliation is common in homogenous rocks_ Once the material has peeled off and rolled away, a smooth dome shaped rock is left behind. - It is called an exfoliation dome. Example along Mombasa — Nairobi road between Mtito Andei and Vol.
(a) Granular disintegration -Where the rock is of different chemical composition the change in temperatures result in the individual minerals expanding at different rates. This causes internal stress and eventually the rock will crumble and
disintegrate into smaller pieces (grains). E.g In the case of a granitic rock which contains the minerals quartz, mica and feldspar. These minerals being of different colours- quartz glassy, mica dask brown and feldspar white, expand and contract at different rates. This sets up stress in the rock resulting in break up to form grains of sand hence the process is referred to as granular disintegration.
(c) Block disintegration
This process is effective in areas that experience great diurnal temperature ranges. It is very common in the arid areas.
In this process, a well jointed rock is subjected to intense heating during the day and cooling during the night. The rock minerals expand due to the heating and contract as a result of cooling. The rock joints enlarge due to this alternating expansion and contraction of the rock mass. When this occurs repeatedly over a long time, the rock mass eventually break into blocks along the joints hence, the name block disintegration. e.g. Mudanda rock in Tsavo East.
(d) Frost action/frost shattering/freezing and thawing/conger fraction.
This process results from the action of ice. It is common in areas that are subject to alternate freeze and thaw, areas to where temperatures can fall below zero and later rise. Such areas are high mountainous regions in the tropics or temperate regions which experience snow fall in winter.
- When the temperatures are high, water from melting ice collects in small cracks of rocks. When the temperatures fall the water freezes in cracks expanding rocks joints. Repeated freezing and thawing causes the rock to break into angular blocks of rock fragments. This process is called congerifraction. Frost action is very significant in the formation of glacial erosional features like cirques, ar-etes and pyramidal peaks.
(e) Pressure release (unloading)
There are some rocks that lie deep below the surface.and are covered by layers of soil and other rocks. When the overlying layers are removed by denudation processes like erosion and mass wasting, the newly exposed rocks will have the weight that was pressing on them removed_ The removal of the weight above this rock releases pressure from it, leading to expansion of the rock. The process is evidenced by the curving and eventually pull out of rock shells of the outer rock.
The process also referred to as sheeting is common in granitic rocks. This process has led to the formation of granitictors in Seme Location in Kisumu, in Maragoli and in some parts of Machakos.
(f) Slaking/rain water
This process is effective in areas that experience a wet and dry season.
Once it rains the surface rocks absorb water and this causes them to swell. When the dry season sets in, the rocks loose water and the outer surface shrinks_
A repeat of the wetting and drying weakens the rock and it starts to crack and then breaks up. The process,.commonly affects sedimentary rock that are rich in mental clay. E.g East of Tudor and Miritiniares
(g) Crystal growth
It involves expansion and fracturing of a rock.
It occurs in dry areas. The dry atmosphere draws out the amount of moisture together with dissolved minerals from the rock is interior through capillary action. Once the moisture gets to the surface of the rock in evaporates due to high temperatures. This leaves behind salt crystals deposited in the cracks. increase in the salt deposits increase pressure in the cracks. With time the crevice widens leading to the rock breaking down. e.g. at Hell's Gate near Naivasha.
2. Chemical Weathering
• This is decomposition of rock due to chemical change.
• Chemical weathering is facilitated by high temperatures, moisture and atmospheric gases.
Types of chemical weathering
(i) Carbonation - (Calcium carbonate) to form calcium bicarbonate which is removed in solution.
(ii) Hydration -Occurs in rocks which have minerals that absorbs water when the minerals absorb water the rocks expands causing stress to the rock. This leads in decomposition e.g. Calcium Sulphate.
(iii) Oxidation - Occurs in rocks rich in iron. The iron reacts with oxygen from the air to form a new mineral (ferric oxide). The new mineral which is formed is unstable and crumbles easily.
(iv) Hydrolysis - common in rocks rich in Silica. Hydrogen in water reacts with minerals in rocks.
The reaction contribution to formation of new minerals resulting to decomposition of rocks/rock decay.
(v) Solution - Affects rocks with soluble minerals e.g. rock salt. After dissolving weathering occurs and minerals are carried in solution form.
3. Biological weathering
- This occurs due to plant, animal and human activities.
Plant roots penetrate into joints of rocks exerting pressure. This contributes to eventual disintegration.
Lichens, mosses that grow on rocks keeps rocks surface moist thereby facilitating chemical weathering.
Dead plant tissues produces organic acids which reacts with minerals in rocks leading to rock decay.
Borrowing animals break the rocks as they excavate the holes. Animals produce uric acids which reacts with rocks leading to disintegration.
Human activities - includes cleaning of forests, mining, road construction.
Significance of weathering to physical and human environment
(i) Weathering is very important because it leads to the formation of soil; soil is important to man for agricultural activities.
(ii) Weathered rocks offer beautiful sceneries for tourist attraction. A good example is the weathered rocks at Hell' Gate and the Crying Stone of Kakamega.
(iii) Weathering process breaks up rocks making mining easier. Weathering process is also known to produce valuable minerals e.g. bauxite, which is an aluminium ore a weathering product.
(iv) Weathering reduces the size of rocks providing suitable materials for construction e.g. rock blocks and sand.