Weathering, Types of Weathering, Part 3

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The term weathering is generally applied to the combined action of all processes that cause rocks to disintegrate physically and decompose chemically because of exposure near the surface of the earth through the elements of weather. Among these elements, temperature, rainfall, frost, fog and ice are the most important ones.

In nature, generally both the processes of disintegration and decomposition act together at the same time and assist each other. The weathered materials lie in situ or 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

We can recognize three types of weathering:

Physical Weathering: It occurs when the rocks are broken up into smaller fragments without any chemical change in their composition. The term mechanical weathering is also used for physical weathering. Physical weathering takes place in different ways in different types of areas. They have been explained here with examples.

Block Disintegration: In hot desert regions, day temperatures are very high while nights are very cold. This high diurnal range of temperature 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.

Block Disintegration

Block Disintegration

Block Disintegration

Exfoliation: Rocks are generally poor conductors of 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 peeling of rocks in layers by this process is very similar to the peeling of successive layers of an onion. This process is known as exfoliation. Granite domes of Mahabalipuram, and those near Jabalpur on Madan Mahal Hill are good examples of exfoliation.

Frost Action: The alternate freezing and melting of water inside the joints of the rocks, splits them into fragments. This is because the conversion of water into ice increases the volume of water by 10 %. In cold regions rocks are disintegrated into small particles through this process of frost action.

Frost weathering

Frost Weathering

Frost weathering

Chemical Weathering: Chemical weathering is the chemical change in the rocks through formation of new compounds or formation of new substances. Chemical weathering involves four major processes:

Oxidation: In this process the atmospheric oxygen reacts with the rocks to produce oxides. 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. When rain water containing carbon di-oxide 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.

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. For example, feldspar is changed into kaolin through hydration. Kaolin on Vindhyan Hills near Jabalpur has been formed in this manner.

Solution: In this process 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 man.

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 and they exert pressure on the rocks thus, the rock joints are thereby enlarged and break into smaller fragments.

Animals: Burrowing animals such as earthworms, rats, rabbits, termites, and ants breakdown the rocks. These disintegrated rocks can easily be eroded or removed by wind. Hooves of animals break the soil and thus, assist soil erosion. According to the scientists, there is a possibility of occurrence of about 1,50,000 earthworms in an acre and they can convert 10 to 15 tonnes of rock mass into good soil and bring it to the surface.

Man: Man breaks a large amount of rocks in the course of his activities, such as agriculture, construction of houses, roads and quarries for mining minerals. Thus, helps in weathering by breaking, weakening and loosening the rocks.

Biotic Weathering

Biotic Weathering

Biotic Weathering