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Geology Center - Weathering


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دوشنبه دوازدهم تیر 1391 :: نویسنده : مجید

What is weathering?

The process of weathering alters rocks at the earth's surface and breaks them into fine-grained particles of sediment and soil. Weathering is the result of the interactions of air, water, and temperature on rocks and prepares the rock for erosion. Erosion is the movement of the particles by ice, wind, or water. The particles are then transported by that agent until they are deposited to form sedimentary deposits, which can be later eroded again or transformed into sedimentary rocks .

Types of weathering

There are two kinds of weathering: mechanical and chemical.

a) Mechanical weathering

Mechanical weathering is the process by which rocks are broken down into smaller pieces by external conditions, such as the freezing of water in cracks in the rock. Five major processes cause mechanical weathering:

1. Pressure-release fracturing: If a large intrusion is brought to the surface through tectonic uplift and the erosion of overlying rocks, the confining pressure above the intrusion has been released, but the pressure underneath is still being exerted, forcing the rock to expand. This process is called unloading and gives rise to pressure-release fracturing. Because the outer layers expand the most, cracks, or sheet joints, develop that parallel the curved outer surface of the rock. Sheet joints become surfaces along which curved pieces of rock break loose, exposing a new surface. This process is called exfoliation.

2. Frost wedging: Water expands when it freezes. If water accumulates in a crack and then freezes, its expansion pushes the rock apart in a process called frost wedging.

3. Abrasion: Many rocks along a stream or beach are rounded and smooth. The mechanical wearing and grinding of rock surfaces by friction and impact is called abrasion.

4. Organic activity: Less important agents of mechanical weathering include the burrowing of animals, plant roots that grow in surface cracks, and the digestion of certain minerals, such as metal sulfides, by bacteria.

5. Thermal expansion and contraction: Daily temperature changes, especially in those regions where temperatures can vary by 30 degrees centigrade, result in the expansion and contraction of minerals, which weaken rocks.

b) Chemical weathering

Chemical weathering occurs when air and water chemically react with rock to alter its composition and mineral content. For example, feldspar crystals in a volcanic tuff commonly weather to form new clay minerals. Other minerals, such as calcite, dissolve. Quartz, on the other hand, is very resistant to weathering. The most important processes of chemical weathering are dissolution, hydrolysis, and oxidation. Water, carbon dioxide, acids and bases, and oxygen are common substances that cause these processes to decompose rocks.

1. Dissolution: A few minerals dissolve readily in water. Acids and bases often markedly increase the solubility of minerals.

2. Hydrolysis: During dissolution, a mineral dissolves but does not react chemically with the solution. However, during hydrolysis, water reacts with a mineral to form a new mineral with the water incorporated into its crystal structure. Many common minerals weather by hydrolysis. For example, feldspar, the most abundant mineral in the Earths crust, weathers by hydrolysis to form clay.

3. Oxidation: Many elements react with atmospheric oxygen, O2. Iron rusts when it reacts with water and oxygen. Rusting is one example of a more general process called oxidation.

Soils

The layers of weathered particles of earth material that contain organic matter and can support vegetation are defined as soil. Soil can be all or just part of the sedimentary material that covers the bedrock. Soil commonly consists of sand, silt, clay, and organic material. The most fertile soils contain a mixture of sand, clay, and silt as well as abundant amounts of organic matter. Such a mixture is called loam.

Soil profiles

A typical mature soil consists of several layers called soil horizons. The uppermost layer is called the O horizon, named for its Organic component. This layer consists mostly of litter and humus with a small proportion of minerals .





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