WEATHERING IN EAST AFRICA:

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1 WEATHERING IN EAST AFRICA: Weathering is a denudation process which involves the physical breakdown and chemical decomposition of rocks in situ (one place) on the surface or near the earth s surface. Types of Weathering: Mechanical/physical weathering. Chemical weathering. Biological weathering. Mechanical weathering: This is the physical breakdown/disintegration of rocks into small particles mainly due to changes in temperature or influence of animals, plants and man. It is common in semi arid areas such as karamoja region (Moroto, Kotido, Kabong and Nakapiripiriti), Ankol- Masaka dry corridor, Semuliki and Albert flats in Uganda.Turkana region, Chalbi desert of Kenya. Chemical/physical weathering processes: Block disintegration. It involves the breakdown of rocks into rectangular blocks. During day the temperatures are hot and at night temperatures drop. Such temperature difference leads to expansion of rocks during day and contraction at night. This is followed by creation of cracks which are widened and rock pieces fall down. It results into the formation of granitic tors/inselbergs. Examples include Kachumbala and Bismarck rock in Mwanza. Exfoliation. During day the temperatures are hot and at night temperatures drop. Such temperature difference leads to expansion of rocks during day and contraction at night. This gradually leads to the cracking of the upper layer on the rock and results into peeling off of the broken layer (screes) forming an exfoliation dome.

2 Exfoliation domes are found at Mubende, Nakasongola and Serengeti. Major are the Hells Gates at Naivasha and Mudanda rock along Nairobi-Mombasa highway. Granular disintegration. It operates on rocks with different minerals. For example granite rocks contain mica, quartz and feldspar which are coloured differently and therefore have different heat absorption capacities. Such rocks breakdown into rock particles called granules. Examples are found in Bubende, Nakasongola and Turkana land of Kenya. Aridity shrinkage/wetting and drying. Some rocks such as clay absorb water during rainy seasons and expand. During the dry season water evaporates and they contract. This contraction leads to strains leading to the cracking of the rocks. Aridity shrinkage is common in swamps or areas with clay such as Masai-Mara plains in Kenya and Tanzania, Nakasongola and Tororo in Uganda. Pressure release/unloading. This takes place mainly on granite or metamorphic rocks. Removal large quantities of overlying rocks expose the underlying rock to heat making it to expand and crack. The exposed surface layer bends and starts peeling off. However this process is called spalling or sheeting. Examples can be found in the abandoned quarries like Mattuga along Bombo road and Walukuba in Jinja. It is also found in Nakasongola and Mubende inselbergs. Frost shattering/wedging. This takes place in areas where temperatures fall below o o c. It occurs on the glaciated mountains such as Mt Kenya, Rwenzori and Kilimanjaro. During day, snow/ice melts and water enters into cracks.at night, it is very cold and therefore the water freezes into ice and therefore expands in volume. The expansion of the ice exerts great pressure on the crack walls leading to rock disintegration. Chemical Weathering Processes: Is the decomposition of rocks due to chemical reaction between rock minerals, water and atmospheric gases such as oxygen and carbon dioxide. Chemical

3 weathering is common in areas that experience hot temperatures and heavy rainfall which facilitate chemical weathering processes. Such areas include, Lake Victoria basin, western Uganda, Kigezi highlands, the east African coast (Mombasa), Kenyan highlands, western Kenya, southern Tanzania and Mbeya. Processes of Chemical Weathering: Carbonation. This common in limestone areas like Nyakasura, Tanga and Tororo. Rainwater in the atmosphere combines with carbon dioxide in the air to form a weak solution of carbonic acid. It is the carbonic acid that reacts with calcium bicarbonate which is then removed in solution form. This process led to the formation of stalagmites, stalactites and pillars in limestone regions. Hydration. This refers to the addition of water in a rock. Some minerals absorb water and in the process, expand giving rise to new compounds. The rock is stressed up and so disintegrates to form new rocks. For example when calcium sulphate absorbs water it changes into limonite. Solution. This is where rocks are dissolved in water and carried away in solution form. Leaving behind widened hollows or cracks. This process is common in sedimentary rocks like limestone (Caco3) and rock salt (Calcium chloride) which are soluble in water. Examples are found at Nyakasura, Katwe and Fort portal in western Uganda and Tororo. Hydrolysis. This involves reaction of hydrogen ions and some minerals within rocks. For example hydrogen combines with metal ions resulting into clyays, carbonates, and potassium. Hydrolysis is common in broad valleys of Buganda, Naigombwa in Iganga and Bukoba in Tanzania. Oxidation. It involves reaction of oxygen and iron or aluminum compounds. Reduction. This is the removal of oxygen from a substance and addition of hydrogen to it. It occurs in saturated conditions where ferric hydrate is reduced to

4 hydrogen sulphide. The addition or removal changes the chemical composition of the rock thus weathering. Chelation. This involves Base Exchange between plants and rocks which cause changes from either side. For example plants secrets a fluid through their roots which react and cause rock disintegration. Plants also extract/absorb mineral salts from rocks. This is common in forested regions and savanna woodland areas such as Timu, Morongole in Karamoja, Miombo woodland in Tanzania and Turbo forests of Kenya. Biological weathering. This is the breakdown of rocks caused by living organisms such as burrowing animals, man, trees and grass. Importance of weathering: Landforms such as granitic tors, stalactites, stalagmites, pillars and caves are tourist attractions which generate foreign exchange. It favours soil formation through the breakdown of rocks which promote crop cultivation. Weathering helps to expose minerals along Kimberly rocks in Mwadui exposed diamonds in the volcanic plug. Weathering has led to landslides on the steep slopes of glaciated mountains which cause destruction of property. Leads to formation of infertile soils which discourage crop cultivation. Leads to the formation of rugged landscape in limestone regions composed of clints, grikes and arenas. These limit agriculture and settlement in Tororo, Tanga and Nyakasura. Weathering on steep slopes encourages soil erosion thus discouraging crop cultivation in Kigezi highlands and on the slopes of Mt. Kilimanjaro.

5 Factors Influencing Weathering: Climate. Hot temperatures and heavy rainfall increase the rate of chemical weathering. For this reason it is dominant in the equatorial climatic regions. Hot temperature during day and cold conditions at night increase physical weathering processes such as exfoliation and block disintegration. Nature of the rocks in terms of hardness, chemical composition and presence of joints determines the rate and character of weathering. Hard rocks take long to break as compared to soft rocks. Living organisms like plants and animals contribute to weathering. For example burrowing animals like squirrels break down into soils. Relief. On steep slopes there is intense soil erosion such that whatever soils formed is removed down the slope hence exposing the rock to further weathering agents. Time factor. For example the longer the rock is exposed to agents of weathering the more weathered it is likely to be.