From the moment that rocks are exposed to the atmosphere, they are subject to weathering. This breakdown of the rocks is brought about by the effect of chemical, physical and biological factors.
Chemical weathering is mainly brought about by the action of carbonic acid that is produced wherever carbon dioxide and water mix, as in rainfall. Some rock minerals dissolve and are washed away. Others are altered by various chemical reactions, most of which occur when the rock surface is exposed to the atmosphere. All but the inert parts of rock are eventually decomposed and the rock crumbles as new minerals are formed and soluble material is released. Oxidation is particularly important in the formation of iron oxides, which give soils their red and yellow (when aerobic), or blue and grey colours (in anaerobic conditions).
Physical or mechanical weathering processes break the rock into smaller and smaller particles without any change in the chemical character of the minerals. This occurs on exposed rock surfaces along with chemical weathering but, in contrast, has little effect on rocks protected by layers of soil.
The main agents of physical weathering are frost, heat, water, wind and ice. In temperate zones, frost is a major weathering agent. Water percolates into cracks in the rock and expands on freezing. The pressures created shatter the rock and, as the water melts, a new surface is exposed to weathering. In hot climates the rock surface can become very much hotter than the underlying layers. The strains created by the different amounts of expansion and the alternate expansion and contraction cause fragments of rock to flake off the surface; this is sometimes known as the 'onion skin' effect. Moving water or wind carries fragments of rock that rub against other rocks and rock fragments, wearing them down. Where there are glaciers the rock is worn away by the 'scrubbing brush' effect of a huge mass of ice loaded with stones and boulders bearing down on the underlying rock.
Biological weathering is attributable to organisms such as mosses, ferns and flowering plants which fragment rock by both chemical and physical means, e.g. they produce carbon dioxide which, in conjunction with water, forms carbonic acid; roots penetrate cracks in the rock and, as they grow thicker, they exert pressure which further opens up the cracks.
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