Soil genesis, concept of the weathering

Soil genesis

Soil genesis or the soil formation mainly depends on the weathering of the rocks which is mainly by the breaking down of the rocks due to the action of the geological agents like water, air, wind , glacier etc.

Concept of the weathering of the rocks

Weathering of the rocks

Generally, rock weathering is the phenomenon of the interface between the atmosphere and the lithosphere. The weathering occurs when the rocks or the minerals comes in the contact with the water surface or the air and is disintegrated physically and decomposed chemically.

Factors affecting the weathering

1. Climate
2. Topography
3. Structure, texture and the mineral composition of the rocks
4. Vegetation Cover
5. Time

1. Climate

Climate includes all the metrological elements like temperature, moisture containing humidity, precipitation, pressure, the wind etc. The climate determines whether chemical decomposition or the mechanical disintegration will predominate and the speed with which these processes will operate.

The physical disintegration is noticed in the tropical and the arid region where the chemical decomposition can be easily noticed in the humid i.e the moist climate and the low-lying areas. The weathering type and the degree depend on the season and the temperature of the region. The various range of the temperature causes the surface exposed rocks to expand or the contract. Such thermal expansion or the contraction of the rocks causes the mechanical disintegration of the rocks. The alternate freezing and heating of the rocks cause the shattering of the rocks. Since the temperature is directly related to the rate of the chemical reaction, the rocks are weathered by the continuous process of heating and the freezing or the warmth and the humidity.

2. Topography

The relief is the gradient of an area, in other words, it is an angle of the inclination of the two specific points divided by the horizontal distance. The topography controls the degree of the weathering through the local relief. The erosion or the transportation of the surface materials increases with the relief which causes the more exposure of the rocks and the weathering of such rocks.

3. Structure, texture and the composition of the rocks

The structure, composition affects the degree of weathering on the rocks. The cracks, fissures on the rocks lead to the fast disintegration of the rocks. When the water impasses from the rocks and then it is followed by the heating and cooling of the temperature, it results in the breakdown of the rocks and also enhances its chemical composition.

Regarding the effect due to texture, the hard rocks are less weathered than the soft rocks. The soft rocks are more and easily weathered. Similarly, the fine-grained rocks are more weathered than the coarse-grained rocks.

Similarly, the mineralogical composition is also one of the factors for the weathering of the rocks. The silicates forming rocks have different stability. Example: Quartz is most stable in most of the inert oxides and persists all the temperature. Muscovite is less stable than quartz. The basic and the ultrabasic rocks are easily weathered in a short period than the acid rocks.

4. Vegetation Cover

The area with less vegetation is more weathered than the vegetation covered area. The roots of the plants pass through the fissures and the cracks and lead to the fragmentation of the rocks. Also, the humic formed by the decay of the plants causes chemical decomposition of the rocks.

5. Time

The more time exposed to the weathering the larger volume of the rocks gets affected.

Different process of the weathering and their role in the environment

The weathering is the process by which the rocks are broken down and are decomposed by the process of the external agents like wind, water, glacier, etc. In a work, we can say that weathering is the destruction and the synthesis process. The weathering occurs on the rocks when they are exposed to the surface of the earth or comes in contact with the atmosphere.

Types of the weathering

1. Physical weathering

Physical weathering refers only the physical disintegration of the rocks without undergoing the chemical change. The physical forces like temperature, pressure, frost etc. cause the fragmentation of the rocks. It does not lead to the formation of the new substances but promotes a fine state division which is necessary for the formation of the soil.

Agencies of physical weathering

1. Temperature

In the humid region, the high temperature accelerates the weathering. As the rock is the poor conductor of the heat; the high temperature extends up to the certain region and the greater expansion of the rock at the surface produces the strain which causes the layer to be break off. This causes the mineral to loosen the crystals and crumble the rock. The crumbling of the rocks in the dry climate due to the free radiation temperature changes at the sunset and the sunshine which is called exfoliation.

2. Freezing and thawing

The rocks which get porous during the time are disintegrated by this process. In cold and temperate climate where precipitation is abundant, the rocks are rolled down the slope as they get enlarged by thawing, freezing and finally the weathering takes place.

3. Glacier

The glaciers have the greater grinding power for the disintegration of the rocks. To some extent, it is also involved in the transportation of the rocks.

4. Water

Water aids in the disintegration, transportation and the deposition of the clay, slit and the gravel. The water load of the clay, silt, and the gravel deepen and the wide valleys grid the rocks for the soil formation.

5. Wind

The wind movement along with the soil particles causes the abrasive action on the rocks and minerals contained in the rock. The wind makes the action of the sea waves stronger in the coast and causes weathering.

6. Chemical weathering

The chemical weathering involves the process of decomposition of the previous rocks and the formation of the new substances from the original form. In this process, certain minerals disappear either in some portion (partially) or completely. The formation of the secondary minerals can take place with the total differentiation with the parent minerals.

References:

Keller, E.A. Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company, Bell, and Howell Company, 1985.

Mahapatra, G.B. Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.