Weathering processes

The surface of the continental crust is exposed to the atmosphere, making it vulnerable to physical, biological and chemical processes. Physical weathering is a mechanical process which fragments rock into smaller particles without substantial change in chemical composition. When the confining pressure of the crust is removed by uplift and erosion, internal stresses within the underlying rocks are removed, allowing expansion cracks to open. These cracks may then be prised apart by thermal expansion (caused by diurnal fluctuations in temperature), by the expansion of water upon freezing and by the action of plant roots. Other physical processes, for example glacial activity, landslides and sandblasting, further weaken and break up solid rock. These processes are important because they vastly increase the surface area of rock material exposed to the agents of chemical weathering, i.e. air and water.

Chemical weathering is caused by water—particularly acidic water—and gases, for example oxygen, which attack minerals. However, many of these reactions are catalysed by bacteria (biological weathering). Chemoautotrophic bacteria, for example Thiobacillus, derive their energy directly from the weathering of minerals (see Section 5.4.2). Some ions and compounds of the original mineral are removed in solution, percolating through the mineral residue to feed ground-water and rivers. Fine-grained solids may be washed from the weathering site, leaving a chemically modified residue, the basis of soils. We can view weathering processes —physical, biological and chemical weathering usually occur together —as the adjustment of rocks and minerals formed at high temperatures and pressures to Earth surface conditions of low temperature and pressure. The miner-alogical changes occur to regain stability in a new environment.

0 0

Post a comment