Account for the occurrence of mechanical weathering in East Africa. 

The candidates are expected to define the term Mechanical weathering

Mechanical weathering is the process that involves disintegration or breakdown of rocks into smaller fragments without any chemical changes in the composition of the rock.

Mechanical weathering occurs under the conditions of temperature fluctuations which may lead to alternate heating and cooling or freezing and thawing as well as by biotic factors like the action of roots of plants or activities of living organisms that break the rock into rock fragments without changing mineral composition of the rock.

The areas in East Africa where physical weathering dominate include:-Central Tanzania, North Eastern Kenya around Turkana, North-Eastern Uganda in Kaabong, Moroto, Kotindo districts and on upper slopes of high mountains of East Africa like Kilimanjaro, Rwenzori and Kenya at an altitude above 4500m

Physical weathering processes that dominate East Africa include

• Exfoliation which is the peeling off of surface layers of the rock as a result of differential expansion of the surface layers of the rock due to alternate heating and cooling.
• Block disintegration which is the breakdown of rock into rectangular shaped blocks when jointed rocks are heated or cooled leading to expansion or contraction.
• Granular disintegration which is the breakdown of rocks into grains due to differential heating and cooling.
• Salt crystallization which is process where saline solutions in rock cracks and joints begin to crystallize causing stress on the rock resulting into its disintegration.
• Aridity shrinkage which is the process by which non porous rocks like clay which absorbs water in the rainy season and expand while during the dry season they lose water and crack leading to their disintegration.
• Action of roots of plants that break the rock into rock fragments.
• Unloading/ pressure release while is the process by which newly exposed rock due to mass wasting expand and break due to release of weight caused by pressure.

Candidates should bring out factors for occurrence of mechanical weathering in East Africa

• 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 the rock lead to peeling or breaking of the rock through a process 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 water and crack resulting into rock disintegration.
• In Semi- Area areas there is limited cloud cover which leads to  temperature  fluctuations  e. hot/very hot during the day, cool/cold during the night leading to expansion and contraction, resulting  into breakdown of rock    .
• 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 rock.
• Jointed rocks such as limestone in Turkana, Kotido and Moroto areas result into block disintegration where rocks break into regular blocks when heated and cooled
• Mineral composition- Rocks having different minerals absorb heat and loose heat at different rate, when heated or cooled respectively resulting into granular disintegration.
• Colour of the rock- dark coloured rocks absorb a lot of heat in Semi – Arid areas when heated leading to their disintegration.
• Differences in rock hardness: Soft rocks such as limestone in semi-arid areas like Turkana, Kotido easily break d own when exposed to rapid cooling and contraction due to bot temperatures during day and cold during the night
• Relief:
• Steep slopes tend to experience a lot of soil erosion that expose the rock 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 a form of mechanical weathering
• Limited vegetation cover in semi-arid areas like Turkana, Central Tanzania, Moroto exposes the rock to extreme temperatures (hot during day and very cold during night) resulting into rapid expansion and contraction of the rock leading to disintegration.
• Biotic factors:
• Movement of heavy animals like elephants, antelopes, zebras in Kidepo National Park help to break the rock into small particles.
• Burrowing animals such as squirrels, Hog and insects like locusts, termites, ants destroy the limited vegetation thus exposing the rock to physical weathering processes like exfoliation due to temperature extremes.
• Effect of plants that are adapted to semi-arid conditions. Such plants like the cacti, acacia and the baobab grow very long roots that t penetrate deeper underground breaking the nearby rocks. Some plants such as thorn bush grow on rocky surfaces in Turkana and Kaabong thus creating cracks which eventually widen and disintegrate into regular blocks.
• Human activities like mining and quarrying for example gold extractions in Kotido involves excavations and break down of rocks into smaller pieces. Overgrazing by pastoral tribes in Turkana, Kotido exposes the rock surface to extreme temperatures leading to expansion and contraction resulting into rock disintegration
• Time: The more the time, the more the mechanical weathering and rock disintegration. Over time new rock surfaces are exposed to mechanical weathering.