Weathering is the in-situ (in place) breakdown of rock material. It differs from erosion which involves removal of material away from a site.
Weathering occurs by both chemical (decomposition) and mechanical processes (disintegration). Common chemical weathering processes are hydrolysis, dissolution, and oxidation. Mechanical weathering always involves fracturing--but that can occur by a whole host of causes. Chemical weathering tends to weaken rock, thereby making it easier to break. Likewise, mechanical weathering creates additional surface area that is prone to chemical attack. In this way, the two processes work together.
Weathering is controlled largely by climate. The more water available, the more likely that chemical processes can proceed. Additionally, if temperatures are warm, then chemical weathering can proceed even faster. Then, mechanical weathering can move more quickly also. In arid climates, however, weathering processes move very slowly. Mechanical weathering will be the dominant process in arid climates; however, because of its reliance on chemical weathering, it will also be quite slow.
Hydrolysis (also sometimes called hydration)causes most silicate minerals to turn into clays. An important exception is quartz which is extremely resistant.
Dissolution causes soluble minerals such as calcite to dissolve. Dissolution is the major weathering phenomenon involved in cave formation.
Oxidation causes degratdation of iron-rich minerals.
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