Earth Weathering and Erosion
Weathering refers to the disintegration and decomposition of rocks. It involves no transportation of the broken material. Thus, weathering involves simply the breaking or crumbling down of the rocks in situ.
Three Types of Earth Weathering:
Mechanical or Physical Weathering
In this, the rocks are broken down into progressively smaller segments and the chemical composition of the rocks remains unchanged. It is prominent in hot and dry/moist climatic regions because of high diurnal range of temperature.
This type of weathering takes place in different ways:
Frost Action Weathering: In cold climatic region, where water fills the pores, cracks and crevices in rocks and freezes, it expands and exerts a bursting pressure. Thus, the rocks are ruptured and fragmented.
Thermal Expansion and Contraction Weathering: In the area of hot deserts, the tremendous diurnal range of temperature brings the expansion and contraction of the surface rocks, leading to their disintegration into smaller pieces.
Exfoliation Weathering: This is the expansion by unloading process. Unloading occurs when large igneous bodies are exposed through the erosional removal of overlying rocks and the subsequent reduction in pressure.
On being exposed to the surface they expand slightly in volume. This leads to breaking of thick shells like an onion’s layers from the parent mass just lying below.
It changes the basic properties of the rocks. Since most of the chemical changes occur in the presence of water, this type of weathering is more potent in hot and humid regions.
Principal processes of chemical weathering are:
Solution Weathering: Here the rocks are completely dissolved. It leads to the evolution of Karst Topography where the water dissolves the rock structure of limestone, salt, gypsum, chalk, etc.
Oxidation Weathering: The presence of dissolved oxygen in water, when comes in contact with mineral surface, leads to oxidation (esp. in rocks containing iron).
Hydration Weathering: Most of the rock forming minerals absorbs water. This not only increases their volume but also produces chemical changes resulting in the formation of new minerals which are softer and more voluminous.
Carbonation Weathering: Water combining with Carbon dioxide produces carbonic acid which dissolves several elements of minerals and the rock is weakened and broken into pieces.
Plants and animals also contribute to weathering through various activities. Man is perhaps, the most important agent of weathering today. Cultivation, mining and transportation are some of the activities of man leading to weathering of rocks.
Plants also contribute to weathering as the penetration of roots into rocks loosens the joints. Decaying organic matter combines with rain-water and acts as a mild acid on the rocks, thus helping in weathering.
Erosion means wearing down of the earth’s surface. It involves removal of rock material from higher areas.
As it involves transportation of rocks, erosion is performed by mobile agents such as streams, glaciers, winds, waves and the underground water.
Each agent of erosion tends to erode rocks from the higher areas and tends to deposit the eroded and transported matter elsewhere, usually in the lower areas, thus transforming die uneven surface of the earth into an even surface. The erosional and depositional activity results in the formation of a variety of land form features.