How are rocks affected by chemical weathering?



Dorothee Mertmann, Berlin

Weathering of rocks encompasses the totality of all reactions to exogenous influences, i.e. to the physical, chemical and biological conditions prevailing on the earth's surface or just below it. These express themselves in a progressive disintegration of the rock structure, through dissolution as well as restructuring and / or new formation of minerals. By processing the rock, weathering creates the conditions for erosion by various transport mechanisms (ice, water, wind, gravity) and the formation of new sediments. The climate is a decisive factor. Physical (mechanical) weathering predominates in arid and nival areas, whereas chemical weathering dominates in humid areas. Weathering is closely linked to soil formation processes. Weathering rates depend on various factors. The physical and chemical properties of the rock to be weathered, the duration and type of exogenous influences, climate, development of the soil and type of vegetation play a role in this (Tab.).

Physical weathering

The physical weathering (mechanical weathering) causes mechanical destruction of the raw material along fissures, crevices, micro-cracks and grain boundaries. It manifests itself in macroscopically visible crevices. A rock is broken up into smaller and smaller fragments or fragments. This is particularly encouraged by short-term fluctuations in temperature and precipitation. In addition, the local orientation and inclination of exposed rock surfaces to the effects of water and air play a role. Various mechanisms are to be mentioned: a) Frost splitting becomes effective with the increase in volume (approx. 9%) during the phase transition from water to ice in cracks and crevices of the subsoil, especially where there is enough water and the temperatures often fluctuate around freezing point. This is particularly the case in periglacial areas and in high mountains, where it can lead to falling rocks. Potholes on the road are also partly due to frost blasting. b) Temperature explosion (Insolation weathering) is caused solely by strong temperature fluctuations. Dark, little reflective rock surfaces are exposed to alternating expansion and contraction movements when exposed to strong sunlight and subsequent rapid cooling, which lead to surface-parallel desquamation (peeling, peeling), usually a few centimeters thick shells. With extremely rapid cooling, the tensile stresses can even be sufficient to form core cracks perpendicular to the surface in boulders. The alternation of heating and cooling results in different thermal expansion of dark and light minerals, depending on the chemical bond. This leads to tensions and movements at the grain boundaries of the minerals and thus to a loosening of the mineral compound, which is referred to as gravel. When sandstones are grilled, one speaks of sanding. c) salt blast or Saline weathering takes place through evaporation of salty water in pores or cavities, so that gypsum or rock salt crystallize out. The crystallization pressure mechanically loosens the structure further. Frequent alternation between moisture penetration and drying out makes salt blasting particularly effective. This process is also used for the gentle preparation of rock samples, e.g. to extract microfossils.

Biological weathering