Weathering
Weathering, in geology, the natural decay and disintegration of rocks at or near the surface of the earth. Weathering eventually transforms the rocks into sand, clay, and dissolved salts. All soil is derived from weathered materials.
Weathering processes are classified as either chemical or physical. Chemical weathering involves chemical reactions that decompose the minerals that make up a rock; physical weathering involves the fragmentation (breaking up) of rocks.
Chemical weathering is caused primarily by exposure to water and air. Rainfall tends to be slightly acid because carbon dioxide in the atmosphere dissolves in it, producing a weak carbonic acid. (Acid rain is the result of man-made air pollution and its acidity is much greater than the natural acidity of rain. ) Chemical weathering affects different minerals in different ways. Such minerals as feldspar partially dissolve over time, leaving only a clay residue. Limestone and certain other minerals dissolve completely and vare carried away by flowing water. Quartz is relatively resistant to chemical weathering; grains of quartz from the weathered rock generally remain intact and are deposited as particles of sand.
One of the main causes of physical weathering is the freezing of water in small cracks in rocks. As the water freezes, it expands, causing the rock to fracture. When rock is broken into pieces, the amount of exposed rock increases, which in turn accelerates chemical weathering.
The rate at which weathering occurs depends a great deal on the climate—exposed boulders in a desert typically show little chemical weathering, whereas exposed boulders in a rain forest are often so weathered that they will crumble under a single blow.
Plants and other living things contribute to both chemical and physical weathering. For example, lichens and other plants growing on rocks remove certain elements as nutrients, and plant roots growing into a rock crevice can split the rock apart.