Chemical weathering

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Chemical weathering

Chemical weathering

The chemical weathering of rocks is conducted in a variety of ways. Although no rocks-forming minerals are absolutely chemically inert, some minerals are soluble far more readily altered than others. Many minerals are soluble in rainwater, which contains carbon dioxide and acts as a weak carbonic acid. This is particularly important in the decomposition of limestones; the rainwater converts the calcium carbonate into calcium bicarbonate, which is soluble and can be taken away in the groundwater:

CaCO3 + H2O + CO2 --> Ca(HCO3)2

(Calcium carbonate) --> (calcium bicarbonate)

The acid of rainwater is supplemented by other acids, particularly those derived from plants and peat.

Hydration is a process by which certain types of mineral expand as they take up water causing additional stresses in the rock. Many of the decomposition of products of rock-forming minerals are subject to hydration, thereby accelerating the disintegration of the rock. In itself, hydration is a mechanical effect, but it occurs intimately with hydrolysis in such a manner that it is difficult to draw any hard and fast line between mechanical and chemical weathering. Hydrolysis is a reaction very important in the decomposition of feldspars, which are present in many igneous rocks, leading to their disintegration down to basic clay minerals. The reaction can be expressed in the words as: crystalline feldspatic rocks plus water clay, sand and carbonates in solution. The process is accelerated by the presence of carbonic acid in rainwater.

The process of oxidation occurs when minerals in freshly exposed rocks take up additional oxygen. Deep-buried clays are often blue or grey in color as long as air is excluded from them, but on exposure they are oxidized and turn red or brown as ferric compounds are formed.

Plants may help to promote chemical weathering by providing humic acids. In addition, the process of carbonation in soil and weathered rock is accelerated by the presence of many small animals, which through respiration can increase concentration of carbon dioxide in the ground to many times the atmospheric content. An interesting specialized case of weathering is the action of lichens in extracting iron from certain rocks, especially granites, and concentrating it at the surface. Factors affecting weathering vary from place to place depending on rock types and climates.

Dealing first with the igneous group, on the whole, basic rocks such as much olivine, augite and lime-rich feldspar, weather more rapidly than acid rocks such as granite. Another general rule is that dark-coloured minerals are more susceptible to chemical weathering than light-coloured minerals.

Structural weaknesses in igneous rocks are principally joint planes; the number of these determines the number of avenues open to weathering agencies. Intrusive igneous rocks tend to develop regular horizontal and vertical joints, creating angular blocks, which become rounded during the course of weathering. This type of change is sometimes known as Spheroidal weathering.

Sedimentary rocks vary considerably in their resistance to weathering. Conglomerates and sandstones usually consist of pebbles or smaller particles of quartz, which is not normally affected by chemical weathering. Silica, of which quartz is one form, is extremely durable, as witnessed by the resistance of flint, which survives as beach or river sediment long after the surrounding chalk has been disintegrated. The most durable rocks are therefore silica-cemented sandstones called quartzites, which are probably the most resistant rock on the Earth's surface.

Clays and shales are not generally susceptible to chemical weathering except through the impurities they contain, such as ferrous oxides. However, clays and shales are usually bedded or laminated and contain some water, thus being susceptible to freeze-thaw action and other physical processes, and readily reduced to their original fine-grained texture. Calcareous rocks- that is, the limestones are very susceptible to the action of carbon dioxide and humic acids. They are usually high permeable, since the well pronounced jointing systems present a large area available for weathering.

Of the metamorphic rocks, granulites (coarse quartz-feldspar rocks) and gneisses weather slowly in much the same way as granite, as their composition is somewhat similar. Schists, containing layers of mica, are more susceptible to weathering, as the layers allow weathering agents to penetrate more readily. Similarly, in slates the cleavage helps weathering to cause flaking of the rock.

Climate affects the relative importance of the different weathering processes by controlling rates of operation. Broadly speaking, chemical weathering predominates in tropical or temperate humid climates, and physical weathering in cold or dry regions. But this statement needs some qualification, which may be illustrated in the context of four climatic regions.

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