Weathering of Rocks

Weathering is the process by which exposed rocks are disintegrated and decomposed in situ (i.e their original position). Weathering begins as soon as rocks come in contact with one or more than one elements of weather on the surface of the earth. The weathered material (i.e. disintegrated and decomposed) lie in situ (i.e. at its original position) In this process no transportation or movement of material is involved other than its falling down under the force of gravity.

Types of Weathering

There are three types of weathering:

  1. Physical Weathering
  2. Chemical weathering
  3. Biotic weathering

Physical Weathering

When the rocks are broken up into smaller fragments without any chemical change in their composition, it is called physical weathering.

Block Disintegration

The successive heating and cooling causes expansion and contraction of the rocks. In hot desert regions, day temperatures are very high while nights are very cold. This causes successive expansion and contraction of the rocks which tend to enlarge the joints. Finally the rocks disintegrate into smaller blocks. This process is known as block disintegration.

Exfoliation

Rocks are generally poor conductors of heat. As a result of intense heating the outer layers of the rock expand rapidly while the inner layers remain almost unaffected by heat. Due to successive expansion and contraction, the outer layer of the rock subsequently peels off from the main mass of the rock in the form of concentric shells. The process is called exfoliation.

Frost Action

One of the most important physical weathering processes in cold climates is frost action, the alternate freezing and melting of water inside the joints of the rocks, splits them into fragments. This is because conversion of water into ice increases the volume of water. In cold regions rocks are disintegrated into small particles through this process. It is called frost action.

Chemical Weathering

Chemical change in the rocks through formation of new compounds or formation of new substances is called chemical weathering.

Oxidation

This is the process in which atmospheric oxygen reacts with the rock to produce oxides. Greatest impact of this process is observed on ferrous minerals. Oxygen present in humid air reacts with iron grains in the rocks to form a yellow or red oxide of iron. This is called rusting of the iron. Rust decomposes rocks completely with passage of time.

Carbonation

This is the process by which various types of carbonates are formed. Some of these carbonates are soluble in water. For example, when rain water containing carbon dioxide passes through pervious limestone rocks, the rock joints enlarge due to the action of carbonic acid. The joints enlarge in size and lime is removed in solution. This type of breakdown of rocks is called carbonation.

Hydration

This is the process by which water is absorbed by the minerals of the rock. Due to the absorption of water by the rock, its volume increases and the grains lose their shape.

Solution

This is the process in which some of the minerals get dissolved in water. They are therefore removed in solution. Rock salt and gypsum are removed by this process.

Biotic Weathering

Biotic weathering is carried out by plants, animals and humans.

Plants

Plants contribute to both mechanical and chemical weathering. The roots of the plants penetrate into the joints of the rocks. They grow longer and thicker. In this manner they exert pressure on the rocks and the rock joints are thereby enlarged and break into smaller fragments.

Animals

Burrowing animals like earthworms, rats, rabbits, termites and ants breakdown the rocks. These disintegrated rocks can easily be eroded or removed by wind, etc. 

Humans

Human beings play a very important role in weathering of various rocks. Man breaks a large amount of rocks in the course of his activities, like agriculture, construction of houses, roads, etc. He quarries for mining minerals, thus helps in weathering by breaking, weakening and loosening the rocks.