Effects of Weathering


Differential Weathering: Because some minerals and some rock types are more resistant to weathering than others, the more rapid removal of less resistant rock strata is one principal cause of highs and lows on the Earth's surface, especially recognizable in landforms such as cliff and bench topography and valley and ridge topography. (see landforms and crustal structure) Differential weathering can also be recognized on the outcrop scale, for example where a vertical foliation in a schist or gneiss presents alternating bands rich in quartz and feldspar vs. micas and ferromagnesian minerals, the surface may develop a subtly ridged appearance as in the Manhattan Schist in Central Park.

Granular Disintegration: Chemical weathering generates clays. Some clays swell upon being wetted. Iron oxides also swell when hydrated. This swelling can pry apart the crystals in coarse-grained rocks. Granular disintegration of igneous and metamorphic rocks like granite, diorite, and gneiss yields masses of slightly weathered grains termed "grus." Granular disintegration of sandstone simply yields sand.

Weathering Pits: Water lying in small depressions in the surface of exposed bedrock like granite induces more rapid granular disintegration of small weathering pits. The pits can develop overhanging rims if they often only partially fill with water.

Spheroidal Weathering: Bedrock typically is fractured into discrete blocks by joints and sometimes faults (see geologic structures). As weathering proceeds, the smaller blocks are completely reduced to sediment while central portions of the larger blocks still remain. As weathering proceeds, edges and corners of blocks weather more rapidly than flat surfaces because of the increased surface area compared to the mass of rock being weathered, so the blocks become rounded. Isolate rock knobs, called Tors in England and inselbergs in Africa, may form by exhumation of remnant "core stones" by erosion of the surrounding sediments derived from the already deteriorated rock.