weathering BY 11013 Weathering Weathering is the breaking down of Earth’s rocks, soil and minerals through direct contact with the planet’s atmosphere. Weathering occurs in situ or “with no movement” and thus should not be confused with erosion, which involves the movement of rock and minerals by agents such as water, ice, wind and gravity. In addition, weathering is the effect of atmospheric exposure to man-made structures and materials.

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Two important classification of weathering processes exist – physical and chemical weathering. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric condition, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biological produced chemicals in the breakdown of rocks, soils and minerals. The materials left over after the rock breaks down combined with organic material creates soil.

The mineral content of the soil is determined by the parent materials, thus a soil derived from a single rock type can ften be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial, Aeolian or alluvial sediments )often makes more fertile soil. Physical Weathering: Physical weathering is the class of processes that causes the disintegration of rocks without chemical change. The primary process in physical weathering is abrasion (the process by which clasts and other particles are reduced in size).

However, chemical and physical weathering often go hand in hand. For example, cracks xploited by physical chemical action at mineral in cracks can aid the disintegration process. Another type of exfoliation occurs where boulders are spheroidally weathered. These boulders are rounded by concentric shells of rock spalling off, similar to the way shells may be removed from an onion. The outer shells are formed by chemical weathering of certain minerals to a product with a greater volume than the original material.

For example, feldspar in granite is converted to clay which occupies a larger volume. Igneous rocks are very susceptible to Physical weathering. Biological Weathering: Living organisms may contribute to mechanical weathering (as well as chemical weathering, see ‘biological’ weathering below). Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of those organisms to the rock surface enhances physical as well as chemical breakdown of the surface micro layer of the rock.

On a large scale seeding sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. Burrowing animals and insects disturb he soil layer adjacent to the bedrock surface thus further increasing water and acid infiltration and exposure to oxidation processes. Biological weathering involves processes that can be either chemical or physical in character. Some of the more particles by animals. Particles can also fracture because of animal burrowing or by the pressure put forth by growing roots. 2. Movement and mixing of materials.

Many large soil organisms cause the movement of soil practices. This movement can introduce the materials to different weathering processes found at district location n the soil profile. 3. Simple chemical processes like solution can be enhanced by the carbon-di-oxide produce by respiration. Carbon-di- oxide mixing with water forms carbonic acid. 4. The complex chemical effect that occur as a result of chelation. Chelation is a biological process where organisms produce organic substances, known as chelates, that have the ability to compose minerals and rocks by the removal of metallic cations. 5.

Organisms can influence the moisture regime in soils and therefore enhance weathering. Shade from aerial leaves and stems, the resence of roots masses and humus all act to increase the availability of water in the soil profile. Water is a necessary component in several physical and chemical weathering processes. 6. Organisms can influence the pH of the soil solution. Respiration from plant roots releases carbon-di-oxide. If the carbon-di-oxide mixed the water, carbonic acid is formed which lowers soil PH. Cation exchange reactions by which plants absorb nutrition from the soil can also cause pH changes.

The absorption processes often involves the exchange of basic captions for hydrogen ons. Generally, the higher the concentration of hydrogen ions the more acidic a soil becomes. Chemical weathering: Chemical weathering changes the composition of rocks, often transforming them when water interacts with minerals to create various chemical reactions. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts toe the near surface environment. New or secondary minerals develop from the original mineral of the rock. In this the processes of oxidation and hydrolysis are most important.

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