The rocks break and undergo decay under the influence of atmospheric agencies like winds, sun, frost, water and organisms and produce soil. This phenomenon is called ”weathering”. Weathering and erosion together lowers the mountains and produce sediments. The weathering may be defined as the process which tends to break and decompose rocks in place. It includes two processes: (1) disintegration or physical, and (2) decomposition or chemical decay. In nature these two processes work simultaneously. Following processes are involved during weathering.
Erosion is a process which includes the destruction of existing rocks and removal of the product from the site of destruction. Transport is the important aspect of erosion. It is usually done by water, wind or ice.
The combined effect of weathering and erosion is called ”Denudation”. It involves the general wearing down the earth surface.
It is mainly the physical weathering process in which large sheets of rocks peel off from an outcrop. In exfoliation, reduction in pressure due to removal of the overlying rock plays an important part. As each layer breaks off, it releases weight from the underlying mass. As a result its outer layer expand and separate from the rockmass. Exfoliation is continuously seen in homogeneous rocks like granite.
If an outcraft of jointed rock is subjected to chemical weathering, the rounded boulders are produced. This process is called ”Spheroidal weathering”. The spheroidal weathering resembles exfoliation except that it takes place on a much smaller scale. In spheroidal weathering, the hydration which is accompanied by increase in volume, plays an important part. As the minerals in the rock weather to clay, they increase in size because of the addition of water into their structure. This increase in volume exerts an outward force due to which the concentric layer of rock break loose and fall.
Types of Weathering:
(1). Physical Weathering:
Physical Weathering or disintegration involves application of mechanical forces. In physical weathering a rock breaks into smaller pieces without any chemical change. Thus by breaking into smaller pieces, physical weathering increases the amount of surface area for chemical attack. The principal agents of physical weathering are: (i) frost, (ii) heating and cooling, and (iii) organism
- Frost: When water freezes, it expands about percent of its volume. The liquid water enters into cracks of the rocks. Upon freezing it expands and exerts great force on the walls of cracks. As a result, the rocks breaks into pieces. This process is called Frost Wedging. By this process angular fragments of rocks are broken off from the high mountain ranges. These rock fragments roll down the hill slope and accumulate at the base to form ”Talus Deposits”.
- Heating and Cooling: It is believed that delay cycle of temperature change weakens rocks, particularly in hot dry regions. Heating a rock causes it to expand and cooling causes it to contract. The repeated expansion and contraction tend to develop cracks in rocks. In desert areas the coarse grained rocks like granite, disintegrate soon into their constituents crystals and become deserts sediments as a result of temperature variation.
- Organisms: Plants, burrowing animals and men play an important part in disintegrating the rocks. Plant roots grow into rock joints. As the roots grow longer, they exert mechanical pressure and wedge the rock apart. The burrowing animals such as earth-worms, ants and rodents also contribute to disintegration of rocks. Men also break rocks by making road cuts, buildings, quarrying, mining and cultivating the land.
(2). Chemical Weathering
Chemical Weathering or decomposition is a process in which rocks are broken down by chemical decay of minerals. During chemical weathering, the chemical reactions act on rocks which changes their minerals to unstable forms. The principal agent of chemical weathering are Water and Organisms.
Water is the main agent of chemical weathering. Although water in pure form is inactive, it becomes a powerful chemical agent when a smlll amount of oxygen and carbon dioxide are dissolved in it. Rain water usually contains these gases.the main reactions involved in the chemical weathering are:
- Oxidation: The oxygen present in the water in dissolved state oxidizes some minerals. Mafic minerals like pyroxenes, amphiboles and olivines contain some iron. During weathering the oxygen unites with the iron producing insoluble iron oxides. Limonite and Hematite are the very common products of oxidation which impart red and yellow colours to soils.
- Hydration: In hydration water molecules combine chemically with minerals to produce new compounds. The formation of gypsum (Calcium sulphate+water) from anhydrite (calcium sulphate) is good example of hydration. Carbonation frequently operates along with hydration. The orthoclase felspar, a mineral which is abundant in granite, is decomposed and converted into Kaolin by hydration and carbonation. Other hydrous silicates formed by hydration of the primary silicates, such as pyroxenes, amphidoles and olivine are chlorites, serpentines, talcs and zeolites.
- Carbonation: Carbon dioxide dissolves in water to form carbonic acid. Carbonic acid is an effective weathering agent. Granite is the most abundant continental rock. Under the influence of carbonic acid it weathers into clay. The granite consists mainly Quartz and Potassium Felspar.
- Quartz is very resistant to chemical weathering, hence it remains unaltered. The reaction between Carbonic Acid and felspar is as under;
- 2K Al Si3O8 + H2O+H2CO3 –> K2CO3+Al2Si2O5(OH)+4SiO2aqu
- The product of this reaction are potassium carbonate and clay minerals. The Potassium Carbonate being soluble is removed by ground water. While clay minerals accumulates near the surface.
The weathering products of important silicates minerals are shown in following table.
|Mineral||Residual Product||Material in solution|
|Felspar||Clay Minerals||Silica, K++, Na+, Ca+++|
|Hornblende||Clay Mineral, Limonite, Hematite||Silica, Ca++, Mg++|
|Olivine||Limonite, Hematite||Silica, Mg++|
Organisms are another cause for the chemical weathering. Many dead organisms produce organic acids as they decay. These acids increase the solvent power of water. For example, the solubility of silica, alumina and iron is much grater in the presence of organic acids.