Describe the weathering processes taking place in the Lake Victoria basin of East 

Candidates  are expected to

• Define weathering
• Give the climatic conditions/characteristics of Lake Victoria basin
• Identify the types of weathering processes taking place in the Lake victoria Basin and describe how they take place.

Weathering is the integration/breaking and decomposition/decay/rotting of rocks in situ or near the earth’s surface

• Lake Victoria Basin is located in the Southern/South-Eastern part of Uganda, South-western Kenya and Northern Tanzania including the Islands such as Kalangala (or draw a location map of Lake Victoria Basin)
• Lake Victoria Basin has the following major climatic conditions which influence weathering process
  • Heavy rainfall is received (i.e. between 2000- 2500mm)
  • Rainfall is double maxima/bimodal/two seasonal peaks
  • It is received throughout the year with no distinct dry season
  • Temperatures are uniformly hot through the year(i.e. between 24⁰C-27⁰C). the diurnal temperature range is small between 2-3⁰
  • Relative humidity is usually high of 80% dense cloud cover experienced.

 

The above climatic conditions have led to the dominance of chemical weathering processes which include the following:

•  Solution

This is where soluble mineral components in rocks are dissolved by water e.g. rock salts are carried in solution leaving behind joints, cracks/widened hollows in rocks.

• Oxidation

This is the reaction between oxygen and mineral components in rocks such as iron, aluminum facilitated by the hot temperatures and existence of water. This has led to the formation of laterites for example on the flat-topped hills of Buganda, Rocks containing Iron (ferrous state) change to ferric state.

• Hydrolysis 

This is the reaction between the hydrogen ions from water and mineral ions from rocks to form new compounds. It the major process in of decomposition of feldspars in igneous rocks e.g.  granites.

• Carbonation.

This involves rain water dissolving atmospheric carbon dioxide to producing carbonic acid  This reacts with calcium and magnesium carbonates in the rocks to form, new compounds.

• Hydration

Is a process in which certain minerals absorb water and expand causing internal stress and fracturing of the rock. Examples include the conversion of hematite to limonite, mica in sedimentary rocks like sandstone.

•  Spheroidal weathering.

Is the swelling/expansion of the outer shell of a rock mass by penetration of water forcing them to peel successively away and loosen.

• Reduction.

Also known as gleying occurs in swampy/water logged areas at the shores of lake Victoria. It involves anaerobic bacteria absorbing the limited oxygen leaving behind hydrogen that reacts with the rocks to form new compounds i.e. the clay rich rocks turn into blue greyish colour.

 Biological weathering.

• Plants use their roots to extract nutrients from rocks and therefore cause a change in the composition of those rocks . Also the decaying of plants and animals results in the formation of humus, which mixes with rain water and forms humic acid that further decomposes
• The roots of plants/trees can force apart joints and cracks in rocks as they grow and enlarge
• Human activities have led to disintegration/decay of rocks such as quarrying, application of fertilizers, garbage/sewage disposal, industrialization etc.
• Burrowing animals such as rodents also break surface rocks as they create passages in rocks through which water vapour, gases pass to react chemically with
• Bacteria existing in leguminous plants absorb nitrogen to produce nitric acid which reacts with parent rock to form new compounds.

However physical weathering processes also occur in the lake Victoria Basin especially the South Western shores in Uganda and North Western shores in Tanzania where the following climatic conditions exist.

• Low and seasonal rainfall, long dry seasons, hot temperature, large temperature range.
• The above conditions have led to the following physical weathering processes:

• Exfoliation

Is the peeling of surface layers of exposed rocks as a result of alternating heating and cooling. This causes the outer layer of the rock to expand during day because of hot temperatures and contract at night due to low temperatures. This causes rock stress and strain shattering the surface rock layers. For example the Bismarck rock at Mwanza.

• Block disintegration. Is the breaking of homogeneous jointed rocks into blocks due to alternating heating and cooling.
• Granular disintegration; Is the breaking up of heterogeneous rocks into smaller grains due to alternating heating and The different mineral components in the rocks expand and contract at different rates causing stress and strain leading to disintegration.