Erosion, Landforms, and Soils

Weathering is the in-place breakdown of rock at or near the surface, distinct from erosion, which involves the actual removal and transport of that broken material. Mechanical, or physical, weathering breaks rock into smaller fragments without changing its chemical composition, through processes such as frost wedging, in which water seeps into cracks, freezes, expands by roughly nine percent, and gradually pries the rock apart, and exfoliation, in which the release of confining pressure as overlying rock erodes away causes underlying rock to fracture in curved sheets parallel to the surface. Chemical weathering instead alters the mineral composition of rock, most commonly through the dissolution of soluble minerals by slightly acidic rainwater, oxidation of iron-bearing minerals producing the reddish-brown colors typical of rusted rock, and hydrolysis, in which water reacts with silicate minerals such as feldspar to form clay. Biological weathering, driven by living organisms, includes the mechanical prying action of plant roots growing into cracks and the chemical effects of acids secreted by lichens colonizing bare rock surfaces; in practice these three categories work together, with mechanical weathering typically increasing the surface area available for chemical attack.