Weathering of rocks

The earth’s crust constantly undergoes geological changes caused by internal forces, which create new features. Weathering is one such phenomenon which breaks up rocks. Weathering could be chemical, physical or mechanical in process.

Chemical weathering:

• Chemical weathering is the basic process by which denudation proceeds. It is an extremely slow and gradual decomposition of rocks due to their exposure to air and water.
• Air and water contain chemical elements which, even in small quantities, are sufficient to trigger chemical reactions on the surface layers of exposed rocks.
• Such reactions may entirely dissolve the constituents of rocks, thus loosening other crystals and weakening the whole surface.
• When the surface of a rock weathers, some of the loosened material is washed away by the process of erosion.
• In case soil cover exists, chemical weathering of the underlying rocks does not cease, on the contrary it is accelerated. This is because the soil absorbs water and thus keeps the rocks in contact with moisture.
• Rain water absorbs organic acids from the soil and thus becomes a strong weathering agent.

There are six forms of chemical weathering:

Hydration:

• The chemical combination of water molecules with a particular substance or mineral leading to a change in structure
• Soil forming minerals in rocks do not contain any water. They undergo hydration when exposed to humid conditions. Upon hydration, increase in the volume of minerals causes swelling of the rocks.
• Minerals lose their lustre and become soft. It is one of the most common processes and applies to secondary minerals such as aluminium oxide, iron oxide minerals and gypsum.

Hydrolysis:

• It is the most important process in chemical weathering.
• It happens due to the dissociation of H2O into H+ and OH- ions which chemically combine with minerals and bring about changes, such as exchange, decomposition of crystalline structure and formation of new compounds.
• Water acts as a weak acid on silicate minerals.

Solution:

• Some substances present in rocks are directly soluble in water.
• The soluble substances are removed by continuous action of water and the rock no longer remains solid but form holes, rills or rough surface and ultimately disintegrates into pieces or decomposes.
• The action is considerably fastened when water is acidified by the dissolution of organic and inorganic acids.

Carbonation:

• Carbon dioxide when dissolved in water forms carbonic acid
• Carbonic acid attacks rocks and minerals and brings them into solution
• The carbonated water has an etching effect up on some rocks, especially lime stone.
• The removal of cement that holds sand particles together leads to their disintegration.

Oxidation:

It is the process of addition and combination of oxygen to minerals. The absorption is usually from O2 dissolved in soil water and that present in atmosphere.

The oxidation is more active in the presence of moisture and results in hydrated oxides.

Reduction:

• It is the process of removal of oxygen and is the reverse of oxidation. It is equally important in changing soil colour to grey, blue or green as ferric iron is converted to ferrous iron compounds.
• Under the conditions of excess water or water logged condition (less or no oxygen), reduction takes place.

Physical and mechanical weathering

Mechanical weathering is the physical disintegration of rocks by the actual prising apart of separate particles. This can even happen with a completely fresh rock or a rock going through chemical weathering. It takes in the following four types:

Repeated temperature changes:

• In deserts, rocks are exposed to the sun during the day and are intensely heated.
• The outer layer of rocks expands faster than the inner cooler layer, which results in a strong pulling force; while at night the outer temperature is cold and tends to pull away the rock. This process is repeated for years and results in breaking of the rock particles.
• Fragments from large rocks outcrops the fall by gravity to the foot of the slope. Stress is larger at the slope which accelerates the process.

Repeated weathering and drying:

• Exfoliation is not confined to desert areas. Similar stresses may be set up in rocks by repeated wetting and drying of rocks.
• This mostly takes place in tropical regions and coastal areas.
• When the rock is wetted, the outer layer absorbs the moisture and expands when they dry the moisture gets evaporated leaving the rock to shrink.
• This repeated process results in breaking of rocks.

Frost action:

• In temperate latitudes, frost is a potential rock breaker.
• All rocks have cracks and space, in which rain water collects. When the temperature drops at night, the water accumulated in the rocks freezes.
• This causes the rocks to expand. Repeated freezing of this kind deepens and widens the cracks in the rocks resulting in the breaking of the rocks.
• On mountain peaks, this process creates sharp peaks.
• Biotic factors
• Small fragments of rock loosened by either chemical action or mechanical weathering lodge in cracks and crevices of the rocks and plants may sprout in them.
• As they grow, their roots penetrate the rocks below, usually along joints and other lines of weakness, prising them apart.