Processes of physical weathering in East Africa

Physical weathering is disintegration /break down of rocks into small fragments with no change in the rocks chemical composition in situ

Physical weathering is common in Semi-arid and arid areas such as Karamoja, Turkana land , Central Tanzania etc. as well as glaciated mountains such as Kilimanjaro, Kenya and Rwenzori.

The major processes of physical weathering include;·

• Exfoliation which is the peeling off of surface rock layers due to heating during day causing expansion and cooling at night causing contraction. Repeated expansion and contraction lead to peeling of rocks
• Block disintegration involves breakdown of jointed rocks into rectangular shaped blocks due to heating during day causing expansion and cooling at night causing contraction. Repeated expansion and contraction lead to peeling of rocks
• Aridity shrinkage or slaking occurs on non-porous rocks like clay which absorbs water during rainy season and expands, while during dry season lose water through evaporation and crack. Repeated alternative expansion and contraction eventually cause them to crumble into small elongated pieces.
• Granular disintegration refers to breakdown of rocks into tiny pieces/grains due to differential heating and cooling. This process affects rocks composed of different minerals (heterogeneous rocks) with different expansion capacity. When such rocks are subjected to alternative heating/expansion and cooling/contraction, internal stresses and strains build up within the rock and eventually cause rock to disintegrate into tiny/small pieces called granules.
• Frost shattering/freeze and thaw weathering takes place in well jointed rock in snowcapped mountains of East Africa such as Mount Kenya, Kilimanjaro and Rwenzori. On these mountains, ice melts (thaw) during day due to warm weather, and water seeps and collects in rock joints, cracks or pores. At night, temperature fall to zero degrees Celsius and water freezes, its volume expands by 10%. This expansion exerts great force on the rock walls which weaken the rock. Repeated action of freeze and thaw widen and deepen the joints and eventually causing the rock to disintegrate.
• Pressure release or unloading occurs when overlying rock layers, soil and other materials exerting pressure on granites below are removed by denudation forces such as erosion and mass wasting. The gradual removal of these materials reduces pressure and exposes rocks formally burried to alternative heating and cooling during day and night. Gradually the newly exposed granite expand and develop curved joints on the surface and eventually disintegrate into curved sheets. For example exposed granite in Apach, Serengeti and Rukwa

• Salt crystalization is a process by which saline solution in rock salt crystalizes, accumulate and expand leading to rock disintegration along joints and cracks. Salt crystalization mainly affects salt rock in semi arid basins. The occasional rain received percolates down into the rock and dissolves salt mineral to form salt solution. During the hot long season, the solution is brough to the surface by capillarity. Water evaporates and sallt crystals are left behind as tiny crystals which accumulate and overtime exertpressure on rock walls which eventually lead to rock disitergation.