WEATHERING

disintegration or alteration of rock in its natural or original position at or near the Earth's surface through physical, chemical, and biological processes induced or modified by wind, water, and climate. During the weathering process the translocation of disintegrated or altered material occurs within the immediate vicinity of the rock exposure, but the rock mass remains in situ. Weathering is distinguished from erosion by the fact that the latter usually includes the transportation of the disintegrated rock and soil away from the site of the degradation. A broader application of erosion, however, includes weathering as a component of the general denudation of all landforms along with wind action and fluvial, marine and glacial processes. The occurrence of weathering at or near the Earth's surface also distinguishes it from the physical and chemical alteration of rock through metamorphism, which usually takes place deep in the crust at much higher temperatures. Weathering involves physical, chemical, and biological processes acting separately or, more often, together to achieve the disintegration and decay of rock material. Physical weathering causes the disintegration of rock by mechanical processes and therefore depends on the application of force. Disintegration involves the breakdown of rock into its constituent minerals or particles with no decay of any rock-forming minerals. The principal sources of physical weathering are thermal expansion and contraction of rock, pressure release upon rock by erosion of overlaying materials, the alternate freezing and thawing of water between cracks and fissures within rock, crystal growth within rock, and the growth of plants and living organisms in rock. Rock alteration usually involves chemical weathering in which the mineral composition of the rock is changed, reorganized, or redistributed. The rock minerals are exposed to solution, carbonation, hydration, and oxidation by circulating waters. These effects on the mineral decomposition are added to the effects of living organisms and plants as nutrient extraction to alter rock. Several factors control the type of weathering and the rate at which rock weathers. The mineralogical composition of a rock will determine the rate of alteration or disintegration. The texture of the rock will affect the type of weathering that is most likely to occur. Fine-grain rock will usually be more susceptible to chemical alteration but less susceptible to physical disintegration. The pattern of joints, fractures, and fissures within rock may provide an avenue for water to penetrate. Thus, shattered and fractured rock masses are more likely to undergo weathering than are monolithic structures. Climate will also control the type and rate of weathering by affecting the likelihood of freeze-thaw cycles and chemical reactions. Chemical weathering is more likely to occur and to be more effective in humid tropical climates, and disintegration of rock from freeze-thaw cycles is more likely to take place and to be more effective in sub-Arctic climates.