Weathering of Rocks

All rocks which are exposed on the Earth's surface (high moun­tain peaks, deserts) are decomposed to a certain degree. The process of rock disintegration by the direct influence of local atmospheric conditions on the Earth's surface is called weathering. This phenomenon is often referred to in geology because weathering is an active process. It takes place in the upper layers of the Earth's crust.

The main cause of physical weathering is the change in tempera­ture that takes place with the succession of day and night. This phenomenon can best be observed in the deserts and high moun­tains where the changes in temperature are common.

During the day under the influence of heat, rocks expand whereas at night they begin to contract. As rocks are generally com­posed of different minerals, their expansion and contraction do not occur uniformly. As a result of this rocks crack. At the beginning

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these cracks or fissures are hardly noticeable but gradually they become wider and deeper until the whole surface of rock is finally transformed into gravel, sand or dust.

In the regions of a moderate or cold climate, where the tem­perature in winter goes down to below 0 (zero), the decomposition of rocks is greatly facilitated by the action of water. When water freezes it increases in volume and develops enormous lateral pressure. Under the action of water, rocks decompose to pieces of varied forms and sizes.

The decomposition of rocks under the direct influence of heat and cold is called physical weathering.

Rocks are subjected not only to physical decomposition but also to chemical weathering, i.e. to the action of chemical agents, such as water, carbon dioxide and oxygen. In a general way, chemical weathering is an acid attack on the rocks of the Earth's crust, in particular an attack on the most abundant minerals — quartz (sand) and aluminosilicates (clays). Only few minerals and rocks are resistant to the action of natural waters. The solvent action of water is stronger when it contains carbon dioxide. Water causes more complex and varied changes. With the participation of oxygen and carbon dioxide up to 90 per cent of rocks is transformed into soluble minerals, which are carried away by the waters.

Organisms and plants also take part in the disintegration of rocks. Certain marine organisms accelerate the destruction of rocks by making holes in them to live in. The action of plants can often be even more destructive. Their roots penetrate into the fissures of rocks and develop the lateral pressure which fractures and destroys rocks.

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