10 FACTORS AFFECTING CHEMICAL WEATHERING

 Nature of the parent rock

Mineral composition of the parent rock: some rocks like those having calcium carbonate react with carbonic acids which are due to the combining of rainwater with carbon dioxide in the atmosphere to produce calcium bicarbonate by a process known as carbonation e.g. at Nyakasura. The calcium-bi-carbonate can easily be dissolved in water.

10 FACTORS AFFECTING CHEMICAL WEATHERING

Some rocks have minerals like feldspar which when mixed with water decompose to produce other mineral compounds like potassium hydroxide and aminocilic acids through the process of hydrolysis. However, in the absence of water, feldspar is a very hard element to weathering.

Some rocks have mineral compounds which react with oxygen in the presence of water to form new compounds or oxides through the process of oxidation e.g. ferrous rocks (rocks rich in iron compounds) are turned into brown or red ferric compounds or laterite soils.

Some rocks have minerals that can easily dissolve in water and the solution is carried away leading to the decomposition of the rocks through a process of solution e.g. limestone rocks, rock salts etc.

Jointing of the rock: the presence of joints or cracks increase the surface area for chemical reactions to take place and also allow water to penetrate to the deeper layers of the rocks to chemically weather the rock.

Permeability of the rock: when a rock is permeable, it allows water to penetrate and weather the deeper rock layers through the processes like carbonation, hydration and hydrolysis etc.

Climate:

The nature of the climate experienced in area determines the type of weathering as indicated below;

Rainfall or precipitation provides the water needed for chemical weathering to take place. Many areas in East Africa receive heavy rainfall amounts almost year throughout (equatorial climate). Other areas like the savannah regions receive moderate rainfall and hot temperatures and such humid conditions are conducive for chemical weathering to take place for most of the year.

Areas having hot temperatures for most of the year have physical weathering as the most dominant weathering process. However, most of the humid areas in East Africa have hot temperatures of over 20oC which increase the rate of chemical reactions thus promoting chemical weathering.

Relief:

chemical weathering is more dominant on gentle slopes and low-lying areas as water accumulates and percolates to chemically weather the rock than on steep slopes. However, erosion on the steep slopes exposes the rocks to chemical weathering.

Drainage:

leaching occurs on flat lands because of poor drainage i.e. rock minerals are dissolved and taken away in solution to deeper layers of the soil profile.

This leaves behind residual soils which are rich in iron, magnesium, and calcium compounds. The iron compounds are oxidized in the process of oxidization to form laterite soils. 

Poorly drained areas like flat plains have a high dominance of chemical weathering inform of hydrolysis, hydration, reduction and solution which help to decompose the rock. This is because of the stagnant water in valleys and other low lying areas.

Living organisms:

man’s influence; man may influence chemical weathering through a number of ways e.g.

  1. Emission of industrial gases in the atmosphere which increases acidity in rainy water which accelerates the rate of chemical weathering processes of carbonation.

  2. Dumping of industrial or domestic or agricultural influence on land or water which directly react or increase the activity in the environment thus increasing the rate of chemical weathering by carbonation etc.

  3. Man carries out activities that directly involve the breakdown of rocks e.g. mining, quarrying, road and other activities like agriculture which expose the underlying rocks to chemical weathering processes. Also, irrigation avails water that increases chemical weathering processes like hydration, hydrolysis and solution.

Vegetation: the dead decaying organic matter produce humic acids that assist in rock decomposition. These humic acids react with minerals in the rocks and eventually decompose.

Plant roots release mineral substances into the rock while extracting other mineral substances from the rock in a process known as chelation. This weakens the rock and it eventually breaks up.

Other living organisms like animals secrete acids that chemically decompose the rocks e.g. uric acids. Barrowing animals make holes through the soil e.g. moles, termites etc. and through these holes water penetrates to the deeper layers of rocks which aids chemical weathering through processes like solution, hydration, hydrolysis, carbonation etc.

Time:

 it takes time for the rock to undergo chemical weathering. The longer the time, the more the rock is chemically weathered and the shorter the time, the lesser the rock is chemically weathered.

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