WEATHERING

The in-situ alteration and fragmentation of material exposed to the atmosphere or hydrosphere. The general term ‘weathering’ can be subdivided into three types—physical, chemical and biological weathering—although there are interactions between them. The term has been criticised by Hall et al. (2012), who prefer it to be called rock decay, which avoids the implicit assumption that climate is the primary driver behind specific weathering processes. They argue that rock properties constitute the dominant control. Weathering is a fundamental process in the formation of, for example, colluvium, clay minerals, saprolite and soils in general. In breaking down rocks and sediments, it also prepares these materials for transport by Earth-surface processes acting as agents of erosion. It also plays a fundamental role in long-term landscape evolution, in the rock cycle and in biogeochemical cycles. Through the carbon cycle, it influences the role of carbon dioxide as a greenhouse gas. Weathering intensity of a rock type [Page 1167]or mineral, which is the extent to which it has been altered by weathering, can be measured by weathering indices, which may be based on physical or chemical properties. The type and processes of weathering, and weathering rates, are often closely related to the prevailing environmental conditions (e.g. climate and host material). Thus, weathering products and their weathering intensity can sometimes be used for either (1) palaeoenvironmental reconstruction, provided their age is known, or (2) relative-age dating, if their environmental history is known.