Chapter : 6. Natural Resources
Weathering
Weathering :
It is the process of breaking down of rocks into small, fine mineral particles. It may occur due to physical, chemical or biological means.
Physical weathering : It involves pulverisation of rocky matter caused by physical (climatic changes such as heating, cooling, wetting-drying, frost action) and mechanical forces (abrasion by rain and hail, rolling stones, wave action, wind action)
Sun : Under the influence of solar radiations, the rocks heat up and expand. At night, these cool down and contract. Since all parts of the rocks do not expand and contract at the same rate, cracks appear in the rocks and ultimately the large rocks break up into smaller pieces.
Water : Water infuences the formation of soil in two ways : -
It gets into the cracks in the rocks formed due to uneven heating of different parts of the rocks by the sun. On freezing, the water expands in rock crevices and breaks the rocks.
Flowing water wears away even hard rocks over long periods of time. Fast flowing water generally carries various-sized particles of rocks downstream. On the way, these moving rock particles rub against other rocks. The resultant abrasion forms still smaller particles. The water takes these particles of rocks and deposits them down its path. In this way, soil is found in places far away from its parent rock.
Wind : Strong winds influence the formation of soil by continuosly rubbing against rocks and eroding them. These also carry sand from one place to other.
Living organisms (biological weathering) : Lichens, mosses (bryophytes) and other plants also influence the formation of soil. The lichens live on the rocks and produce acids. The latter corrode the surface of rocks to form thin layer of soil. Other small plants, e.g., mosses later grow on such surfaces and cause the rocks to break up further. Roots of trees sometimes enter the cracks and provide anchorage. As the trees grow, roots also grow bigger and force of cracks to widen. The whole process of weatherning of rocks involving living organisms is called biological weathering.
Chemical weathering of rocks involves a number of chemical processes such as hydrolysis, hydration, oxidation and reduction. For instance, complex compounds present in the rocks are broken down by the action of carbonic acid present in water or by acidic substances derived from the decomposition of organic matter in soil. The end products of chemical weathering are silica, hydrated oxides, inorganic salts etc.
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