Page | 1 Tribute to “ HANUMANTHHAPPA ” :- Siachin Martyr from 19 th Madras regiment : Salute to our National Hero CHAPTER THREE Weathering may be described as the disintegration or decomposition of rocks in situ by natural agents at, or near, the surface of the Earth. o Weathering changes hard massive rock into finer material. o 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. o Rock weathering is also an important prerequisite to the formation of soils (Chapter Eighteen). o A typical soil profile will reveal a gradation of weathering downwards to parent rock. Although the importance of weathering in the preparation of land surfaces is widely recognised, many weathering processes are only understood in outline. o It is convenient to explain individual weathering processes in isolation, but in reality several processes usually combine to cause rock weathering. o Two general types of weathering are normally recognised: 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 agents as water, oxygen, carbon or various organic acids. Physical Weathering The main factors responsible for physical weathering are temperature changes; the crystallisation of water into ice or other crystal growth; the pressure-release mechanism; and the mechanical action of animals and plants. o The thermal expansion of rock has long been cited as an important cause of rock cracking and disintegration. o Many travellers in the arid tropics, where daytime temperatures are at the most extreme, have reported hearing in the evening or night, sounds like rifle shots which they attribute to the cracking of rocks as they contract. o 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. o This should lead to the setting up of stresses in the rock, causing fracturing parallel to the surface.
Page | 2 Tribute to “ HANUMANTHHAPPA ” :- Siachin Martyr from 19 th Madras regiment : Salute to our National Hero o This process has been termed exfoliation, although we must note that this word should be more correctly applied to the pressure-release mechanism described below. Considerable doubt has been expressed by many geomorphologists as to the reality of this simple thermal expansion process. o In the Egyptian desert near Cairo, where granite columns have fallen on their sides, the parts fully exposed to the full blast of the sun are surprisingly the least weathered, whereas the shaded areas, in contact with moisture in the sand, are much more decayed.
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- chemical weathering