CHEMICAL WEATHERING

WEATHERING
The term 'weathering' means the weakening, break­ing-up, rotting and the disintegration of rocks at or near the earth's surface. It starts as soon as a rock is exposed to the influence of weather. The disintegrated material, the products or the results of weathering, do not involve any motion except the falling down of the material by the force of gravity. Weathering is of two kinds: (i) chemical weath­ering, and (ii) physical or mechanical weathering.

CHEMICAL WEATHERING Chef!Ucal weathering is the basic process by which denudation take place. (The general wearing away of the land surface by external agencies or forces is known as denudation or degradation.) Chemical weathering is the extremely slow and gradual decomposition of rocks due to exposure to air and water. Air and water, main agents of chemical weathering, contain chemical elements which set up chemical reactions in the surface layers of exposed rocks. Such reactions may weaken or entirely dissolve certain constituents of the rock, thus loosening the other crystals and weakening the whole surface. The chemical reactions between rock and water are rapid if both the temperature and moisture are high as found in humid tropics.

There are four major chemical weathering processes. (a) Solution Solution is the most potent weathering
process in limestone regions because the rain water attacks and dissolves the calcium carbonate of which the rock is chiefly formed. However, limestone is not the only rock to suffer from solution. All rocks are subject to solution to some extent, although the process is much slower. The rate at which solution takes place is affected not only by the mineral composition of the rock but also by its structure. Sedimentary rocks have pore spaces in which air and water. can lodge and thus attack the rock. The density of joints or cracks in the rock is also crucial to weathering. This factor is very clearly seen in weathering of granite in Malaysia.
(b) Oxidation Oxidation is the reaction of oxygen in air or water with minerals in the rock. This results in decom­position of the rock and it starts crumbling.
(c) Carbonation The rain-water containscarbon~dioxide in solution. It has an acidic effect and it reacts with rocks to form new chemical substances. This is the process of carbonation and is noticed in lower humid latitudes.
(d) Decomposition by Organic Acids Bacteria present in rocks produce acids which, when dissolved in water, help to' speed up the weathering of the underlying rocks.