Account for the occurrence of physical weathering in East Africa.

A Candidate is expected to:

• Define physical weathering
• Identify areas of occurrence
• Identify process of physical weathering
• Explain factors for the occurrence of physical weathering while integrating specific physical weathering processes.

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
• Block disintegration involves breakdown of jointed rocks into rectangular shaped blocks due to heating during day causing expansion and cooling at night causing contraction.
• 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.

The factors that cause physical weathering

• Climate
• In areas of hot climate such as Turkana region in Northern Kenya, Central Tanzania, during the day it is hot, exposed rocks are heated leading to expansion of the rock, while during the night, temperatures rapidly fall leading to rapid cooling of rock resulting into Such alternate expansion and contraction of a rock lead to peeling or breaking of the rock through processes like exfoliation, granular disintegration, block disintegration etc. ·
• In Semi – Arid areas such as Central Tanzania, during the short rainy season, non-porous rocks like clay absorb water and expand while during the long dry season they lose wat..er and crack resulting into rock disintegration..
• In Semi- Arid areas there is limited cloud cover which leads to temperature fluctuations i.e. hot/very hot during the day, cool/cold during the night leading to expansion and contraction, resulting into breakdown of rocks.
• On high mountains of East Africa, the temperature fluctuations result into frost weathering which is the process by which water collects in cracks during the day and during the night water freezes, volume increases causing breaking of the rock
• Nature of parent rock.
• Jointed rocks such as limestone in Turkana, Kotido and Moroto areas result into block disintegration when rocks break into rectangular blocks when heated or
• Mineral composition- Rocks having different minerals absorb heat and lose heat at different rates when heated or cooled respectively resulting into granular disintegration.
• Color of the rock- dark colored rocks absorb a lot of heat in Semi – Arid areas when heated leading to their _I
• Differences in rock hardness. Soft rocks such as limestone in semi-arid areas like Turkana, Kotido easily break down when exposed to rapid cooling and contraction due to hot temperatures during day and cold during the
• Relief
• Steep slopes tend to experience to experience a lot of soil erosion that expose rocks to process of physical/mechanical weathering.
• Steep slopes tend to be affected by mass wasting that expose the rock to unloading/pressure release which is in form of mechanical weathering.
• Limited vegetation cover in semi-arid areas like Turkana, central Tanzania, Moroto exposes the rock to extreme temperature fluctuation resulting into rapid expansion during day and rapid contraction at night leading to rock disintegration.
• Biotic factors
• Movement of heavy animals like elephants, antelopes, zebra in Kidepo National Park lead to breaking of rocks
• Burrowing animals like squirrels, Hog and insects like locust, termites destroy vegetation thus exposing rocks to physical weathering due to temperature fluctuations.
• Effects of plants adapted to semi-arid areas like cactus, acacia grow strong roots into and break rocks
• Human activities such as quarrying cause disintegration of rock
• Time: the longer the time the more physical weathering occurs.