Chemical and Biological Weathering of Rocks

Rocks can be broken into smaller pieces by a natural process known as weathering. Weathering refers to the chemical alteration and physical disintegration of rocks by the actions of air, water, and organisms. Two principal types of weathering are chemical weathering and biological weathering.

The minerals that rocks are made of are subject to alteration by chemical weathering. Some minerals, such as quartz, resist this alteration quite successfully, but others, such as the calcium carbonate of limestone, dissolve easily. In any rock made up of a combination of minerals, the chemical breakdown of one set of mineral grains leads to the disintegration of the whole mass. In granite(made chiefly of the minerals quartz and feldspar), the quartz resists chemical decay much more effectively than does the feldspar, which is chemically more reactive and weathers to become clay Often granite surfaces are heavily pitted (marked with many small holes or depressions). In such cases, the feldspar grains are likely to have been weathered to clay and blown or washed away. The quartz grains still remain, but they may eventually be loosened too. So even rock as hard as granite cannot withstand the weathering process forever.

Three kinds of mineral alteration dominate in chemical weathering: hydrolysis, oxidation, and carbonation. When minerals are moistened hydrolysis occurs, producing only a chemical alteration but expansion in volume as well. This expansion can contribute to the breakdown of rocks. Hydrolysis, it should be noted, is not simply a matter of moistening: it is a true chemical alteration, and minerals are transformed into other mineral compounds in the process. For example, feldspar hydrolysis yields a clay mineral (silica)in solution(that is, dissolved in water), and a carbonate or bicarbonate of potassium, sodium. or calcium in solution. The new minerals tend to be softer and weaker than their predecessors. In granite boulders, hydrolysis combines with other processes to cause the outer shells to flake off.

When minerals in rocks react with oxygen in the air, the chemical process is known as oxidation. We have plenty of evidence of this process in the reddish color of soils in many parts of the world and in the reddish brown hue of layers exposed in such places as the Grand Canyon. The products of oxidation are compounds of iron and aluminum, which account for the reddish colors seen in so many rocks and soils. In tropical areas, oxidation is the dominant chemical-weathering process

Various circumstances may convert water into a mild acid solution, thereby increasing its effectiveness as a weathering agent. With a small amount of carbon dioxide. for instance. water forms carbonic acid, which in turn reacts with carbonate minerals such as limestone and dolomite(a harder relative of limestone). This form of chemical weathering, carbonation, is especially vigorous in humid areas, where limestone and dolomite formations are often deeply pitted and grooved, and where the evidence of solution and decay is prominent. This process even attacks limestone underground. contributing to the formation of caves and subterranean corridors. In arid areas, however, limestone and dolomite resist weathering much better, although they may show some evidence of carbonation at the surface

Biological weathering is the breakdown of rock caused by the actions of living organisms. This kind of weathering plays an important role in the formation of soils. It is through the breakdown of rocks and the accumulation of a layer of minerals that plants can grow–plants whose roots and other parts, in turn, contribute to the weathering processes. But it is likely that the role of plant roots in weathering is somewhat overestimated. The roots follow paths of least resistance and adapt to every small irregularity in the rock. Roots certainly keep cracks open once they have been formed. More importantly, however, are as of roots tend to collect decaying organic material that is involved in chemical-weathering- processes.

One of the most important aspects of biological weathering is the mixing of soil by burrowing animals and worms. Another interesting aspect is the action of lichens, a combination of algae and fungi that live on bare rock. Lichens draw minerals from the rock through an absorption process. The swelling and contraction of lichens as they alternately get wet and dry may also cause small particles of rock to fall off.?

1.The minerals that rocks are made of are subject to alteration by chemical weathering. Some minerals, such as?quartz, resist this alteration quite successfully, but others, such as the calcium carbonate of?limestone, dissolve easily. In any rock made up of a combination of minerals, the chemical breakdown of one set of mineral grains leads to the disintegration of the whole mass. In granite(made chiefly of the minerals quartz and feldspar), the quartz resists chemical decay much more effectively than does the feldspar, which is chemically more reactive and weathers to become clay Often granite surfaces are heavily pitted (marked with many small holes or depressions). In such cases, the feldspar grains are likely to have been weathered to clay and blown or washed away. The quartz grains still remain, but they may eventually be loosened too. So even rock as hard as granite cannot withstand the weathering process forever.?