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Weathering may be described as the disintegration or decomposition of rocks in situ by natural agents at, or near, the surface of the Earth. Weathering changes hard massive rock into finer material. For this reason, weathering is often described as the first essential phase in the denudation of the landscape. as it prepares rock materials for transportation by the other agents of land erosion, including mass movement of material down slopes. Rock weathering is also an important prerequisite to the formation of soils.
Two general types of weathering are normally recognized: physical (mechanical) weathering involves rock disintegration without any change in the chemical constituents of the rock; in chemical weathering, on the other hand, some or all of the minerals in the rocks suffer decay or alteration by such I agents as water, oxygen, carbon or various organic acids.
Physical WeatheringThe main factors responsible for physical weathering are temperature changes; the crystallization of water into ice or other crystal growth; the pressure-release mechanism; and the mechanical action of animals and plants.
The thermal expansion of rock has long been cited as an important cause of rock cracking and disintegration. The theory is that rocks are poor conductors of heat; given strong diurnal heating, the outer layers of the rock warm up considerably, but do not transmit heat to the inner layers. This should lead to the setting up of stresses in the rock, causing fracturing parallel to the surface. This process has been termed exfoliation.
The explanation of the large-scale mechanical exfoliation of rock may lie in the availability of parallel curved structures created by the pressure-release (unloading) mechanism. Both mechanical and chemical weathering processes further weaken the joints, the layers thereby peeling off in sheets. Hence it is probably best to conclude that chemical weathering and pressure release ally with temperature changes to produce rock disintegration in arid areas.
Ice crystal growth operates as a physical weathering agency. When water turns to ice, it undergoes a nine per cent increase in volume. In the sub arctic zones of the world, or at high elevations, temperatures frequently oscillate about the freezing point, and the consequent alternations of water between its liquid and solid state cause pressures in rock crevices sufficiently powerful to disrupt many rocks. Rapid disintegration of such rocks is achieved, creating a mass of frost-shattered debris. This process is sometimes referred to as freeze-thaw disintegration.
The crystallization of salts can cause the disintegration of rocks under certain conditions. A number of salts, such as sodium chloride (common salt), calcium sulphate (gypsum) and sodium carbonate may enter rocks in dissolved form. On drying and crystallization they expand and set up a disruptive effect. Crystallization has been observed to occur against pressures as great as 47 bars. The tensile strength of many rocks is as low as 20 bars, thus crystal growth can inevitably cause splitting.
Salt crystallization produces cavernous weathering, of which the small-scale honeycombing of rock surfaces is a well-known example.
The role of plants and animals as agents of physical weathering is fairly limited. Tree roots can occasionally be shown to have forced apart adjacent blocks of rock. Worms, rabbits and other burrowing creatures may help in the excavation of partially weathered fragments of rock. However, organisms do play an important part in soil formation, continuing the process of weathering a stage further.
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