Mechanical weathering occurs when physical forces break rock materials into smaller pieces that retain the chemical composition of the parent material. Granite, for example, may be mechanically weathered to yield smaller pieces of granite, or disintegration may liberate individual mineral grains from it. The physical processes responsible for mechanical weathering include frost action, pressure release, thermal expansion and contraction, salt crystal growth, and the activities of organisms.
Frost action involves the repeated freezing and thawing of water in cracks and freezes, it expands by about 9% and exerts great force on the walls of the crack, thereby widening and extending it by frost wedging. As a consequence of repeated freezing and thawing, pieces of rock are eventually detached from parent material. Frost wedging is particularly effective if the crack is convoluted. If the crack is a simple wedge-shaped opening, much of the force of expansion is released upward toward the surface.
Frost action is most effective in areas where temperatures commonly fluctuate above and below freezing (Alabama). In the high mountains of the western United States and Canada, frost action is very effective even during summer months. In the tropics and in areas where water is permanently frozen, frost action is of little or no importance.
The debris produced by frost wedging in mountains commonly accumulates as large cones of talus lying at the bases of slopes. The materials that form the talus are simply angular pieces of rock from a larger body that has been mechanically weathered. Most rocks have a system of fractures called joints along which frost action is particularly effective. Water seeps along the joint surfaces and eventually wedges pieces of rock loose; these then fall downslope to accumulate with other loosened rocks.
In the phenomenon known as frost heaving, a mass of sediment or soil undergoes freezing, expansion, and actual lifting, followed by thawing, contraction, and lowering of the mass. Frost heaving is particularly evident where water freezes beneath roadways and sidewalks.
Activities of Organisms
Animals, plants, and bacteria all participate in the mechanical and chemical alteration of rocks. Burrowing animals, such as worms, reptiles, rodents, and many others, constantly mix soil and sediment particles and bring materials from depth to the surface where further weathering may occur. Even materials ingested by worms are further reduced in size, and animals burrows allow gases and water to have easier access to greater depths. The root of plants, especially large bushes and trees, wedge themselves into cracks in rocks and further widen them. Tree roots that grow under or through sidewalks and foundations may do considerable damage.
In most places the land surface is covered by a large layer of unconsolidated rock and mineral fragments called regolith. Regolith may consist of volcanic ash, sediments deposited by wind, streams, or glaciers, or weathered rock material formed in place as a residue. Some regolith that consist of weathered material, water, air, and organic matter and can support plant growth is recognized as soil. Soil is an essential link between the parent material below and the life above. Most land-dwelling organisms are dependent on soil for their existence. Plants derive their nutrients and most of their water from soils, and land-dwelling animals depend directly or indirectly on plants for nutrients.
About 45% of a good, fertile soil for gardening or farming is weathered rock material including sand, silt, and clay, but an essential constituent of such soils is humus. Humus, which gives many soils their dark color, is derived by bacterial decay of organic matter. It contains more carbon and less nitrogen than the original material and is nearly resistant to further bacterial decay. Although a fertile soil may contain only a small amount of humus, it is an essential source of plant nutrients and enhances moisture retention.
Some weathered materials in soils are simply sand and silt-sized mineral grains, especially quartz, but other weathered materials may be present as well. Such solid particles are important because they hold soil particles apart, allowing oxygen and water to circulate more freely. Clay minerals are also important constituents of soil and aid in the retention of water as well as supplying nutrients to plants. Soils with excess clay minerals, however, drain poorly and are sticky when wet and hard when dry.
Soils are commonly characterized as residual or transported. If a body of rock weathers and the weathering residue accumulates over it, the soil so formed is residual, meaning that it formed in place. In contrast, transported soil develops on weathered material that has been eroded and transported form the weathering site and deposited elsewhere, such as on a stream's floodplain. Many fertile transported soils of the Mississippi River valley and the Pacific Northwest developed on deposits of windblown dust called loess.