Weathering is the breakdown and alteration of rocks at Earth’s surface through physical and chemical reactions with the atmosphere and the hydrosphere. Weathered rock or minerals will break down into smaller units and eventually disintegrate into even smaller particles. This takes place in two major ways: physical (or mechanical) and chemical weathering; to a lesser extent, biological weathering is also a factor. The following defines these weathering processes:
PHYSICAL WEATHERING
Physical weathering is the mechanical fragmentation of rocks from stress acting on them. Ice wedging may be the most important type. The agents responsible for physical weathering include gravity, wind, tumbling rocks, and moving water, all of which mechanically affect rock. They are responsible for the development of fractures in rocks, generating fragments of rocks, and creating sediment. The development of rock fragments and sediment is most often caused by abrasion, which is the grinding of rock by friction and impact as fragments are transported in rivers and streams, under and around glaciers, and the movement of sand by winds.
CHEMICAL WEATHERING
Chemical weathering involves chemical reactions with minerals that progressively decompose the solid rock. The major types of chemical weathering are dissolution, acid hydrolysis, and oxidation. Chemical weathering takes place when rocks react with chemicals in solution, essentially decomposing the rocks and soil by a chemical reaction. This usually occurs in water that is rich in carbon dioxide, which is, in turn, produced mainly by the decomposition of plants. For example, limestone caves are weathered in this way.
Dissolution is a process of dissolving a mineral or rock completely into solution, as when an acidic solution dissolves limestone. Halite (salt) is perhaps the best-known example. It is extremely soluble, surviving at Earth’s surface only in the most arid regions. Gypsum is less soluble than halite but is also easily dissolved by surface water. In karst, refers to the process of dissolving rock to produce landforms, in contrast to solution, the chemical product of dissolution.
Hydrolysis is a chemical reaction wherein reaction between silicate minerals and either pure water or aqueous solution. In such reactions, H+ ions or OH- ions are consumed, thus changing the H+/OH- ratio. Hydrolysis can occur in pure water, but in the natural world it usually accompanies reactions with acids; thus, this kind of reaction between a mineral and an acid is usually called acid hydrolysis.
Oxidation is the chemical combination of oxygen, in the atmosphere or dissolved in water, with one mineral to form a completely different mineral in which at least one of the elements has a higher oxidation state. Among the elements that have variable charges, iron is the most important in weathering reactions on Earth. In most silicates, iron is present as Fe2+, but in the presence of Earth’s modern oxygen-rich atmosphere, Fe3+ is the favoured oxidation state. Therefore, oxidation is especially important in the weathering of minerals that have high iron content, such as olivine, pyroxene, and amphibole.
BIOLOGICAL WEATHERING
Biological weathering is caused by organisms and other vegetation that break down rock either physically or chemically. It includes a large range of organisms-from bacteria to plants to animals. For example, lichens play an important part in weathering because they are rich in chelating agents, which trap the metallic elements of the decomposing rock. Some lichens live on rock surfaces (epilithic), some actively bore into the rock’s surface (endolithic), and still others live in the hollows and cracks.
PRODUCTS OF WEATHERING
The major products of weathering are (1) rock bodies modified into spherical shapes; (2) a blanket of loose, decayed rock debris, known as regolith, of which soil is an important part; and (3) ions in solution.