PHYSICAL OR MECHANICAL WEATHERING

PHYSICAL OR MECHANICAL WEATHERING Me­chanical weathering is the physical disintegration of a rock into smaller particles. It takes place without changing the rock's chemical composition. Although it is most rapid in sedimentary rocks, yet it does not spare even the harder granite and the marble. Several factors are responsible for mechanical weathering.

Temperature Mechanical weathering is common in deserts, cold or hot, under the influence of rapid changes in daily temperature. In deserts, rocks are exposed to the blazing sun during the day and are intensely heated. This results in expansion of outer layers. At nightfall, the temperature drops rapidly and the outer layers contract more rapidly than the interior, setting up internal stresses. The continuous expansion and contraction for several years cause the rocks to crack and split.
, Repeated Wetting and Drying Repeated wetting and drying of the surface layers of the rock results in development of stresses. Stresses so produced cause surface to split off. When rocks are wetted, the outer layers absorb a certain amount of moisture and ex­pand. When they dry, this moisture evaporates and they quickly shrink. Weathering by repeated wetting and - drying takes place especially in tropical regions such as Malaysia.

Frost Action In temperate latitudes or areas of cold climate, the alternate freezing and melting of water inside the cracks in rocks split them into fragments called frost. The conversion of water into frost or ice increase the volume of water. This phenomenon, also known as frost weathering, develops a strong force in widening the crevices in rock by physical destruction over a period of time. The magnitude of frost action is indicated by a continual increase in the formation of series over the mountain sides.

Biotic Factors The rocks are also destroyed by plants', and animals' activities. The long and tena­cious root fibres of the plants work down into the cracks of rock. The burrowing by earthworms, ants, rats, etc. makes channels through the rocks and con­tributes to their destruction. The quarrying, mining, deforestation and indiscriminate cultivation of land by man are other contributing factors. Such biological actions may be physical or chemical in nature.