Weathering
From Free net encyclopedia
Weathering is the process of disintegration of rocks, soils and their minerals through natural, chemical, and biological processes. It is not to be confused with erosion, which is the movement of rocks and/or weathering products by water, wind, ice or gravity. In the field of miniature modeling, including model railroading, weathering refers to the process of making a model look as though it weathered by adding simulated dirt, rust, etc.
The breakdown products, after chemical weathering of rock and sediment minerals and the leaching out of the more soluble parts, when combined with decaying organic material, is called soil. The mineral content of the soil is determined by the parent material, thus a soil derived from a single rock type can often be deficient in one or more minerals for good fertility, while a soil weathered from a mix of rock types (as in glacial, eolian or alluvial sediments) often makes a richer soil.
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Mechanical (Physical) Weathering
Mechanical weathering is the cause of the disintegration of rocks or wood. Most of the times it produces smaller angular fragments (like scree), as compared to chemical weathering. However, chemical and physical weathering often go hand in hand. For example, cracks exploited by mechanical weathering will increase the surface area exposed to chemical action. Furthermore, the chemical action at minerals in cracks can aid the disintegration process.
Thermal Expansion
Thermal Expansion, also known as onion-skin weathering, often occurs in hot areas, like deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dipping to a few degrees at night. As the rock heats up and expands by day, and cools and contracts by night, stress is often exerted on the outer layers. The stress causes the peeling off of the outer layers of rocks in thin sheets. Though this is caused mainly by temperature changes, thermal expansion cannot take place without the presence of moisture.
Freeze-thaw
Freeze-thaw action, sometimes known as ice crystal growth or frost wedging, occurs when water in cracks and joints of rocks freeze and expand. In the expansion, it can exert pressures up to 21 megapascals (MPa) (2100 kgf/cm2) at −22 °C. This pressure is often higher than the resistance of most rocks and causes the rock to shatter.
Freeze-thaw action occurs mainly in environments where there is a lot of moisture, and temperatures frequently fluctuate above and below freezing point—that is, mainly alpine and periglacial areas.
When water that has entered the joints freezes, the ice formed strains the walls of the joints and causes the joints to deepen and widen. This is because the volume of water expands by 9% when it freezes.
When the ice thaws, water can flow further into the rock. When the temperature drops below freezing point and the water freezes again, the ice enlarges the joints further.
Repeated freeze-thaw action weakens the rocks which, over time, break up along the joints into angular pieces. The angular rock fragments gather at the foot of the slope to form a talus slope (or scree slope). The splitting of rocks along the joints into blocks is called block disintegration. The blocks of rocks that are detached are of various shapes depending on their rock structure.
Ice crystals can also form in the pore spaces of rocks. They grow larger as they attract water that has not frozen from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals.
Laboratory tests show that frequent daily freeze-thaw cycles are more conducive than seasonal freeze-thaw cycles to frost shattering.
Pressure release
In pressure release, overlying materials (not necessarily rocks) are removed (by erosion, or other processes), which causes underlying rocks to expand and fracture parallel to the surface. Often the overlying material is heavy, and the underlying rocks experience high pressure under them, for example, a moving glacier. Pressure release may also cause exfoliation to occur.
Intrusive igneous rocks (e.g. granite) are formed deep beneath the earth's surface. They are under tremendous pressure because of the overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures. Pressure release is also known as "exfoliation" or "sheeting".
Salt-crystal growth
Image:YehliuTaiwan-HoneycombWeathering.jpg Salt crystallisation causes disintegration of rocks when saline (see salinity) solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up exerting pressure on the confining rock.
Salt crystallisation may also take place when solutions decompose rocks (for example, limestone and chalk) to form salt solutions of sodium sulfate or sodium carbonate, of which the moisture evaporates to form their respective salt crystals.
The salts which have proved most effective in disintegrating rocks are sodium sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up to three times or even more.
Organic Activity
Living organisms contribute to mechanical as well as chemical weathering. Lichens and mosses grow on essentially bare rock surfaces and create a more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. On a larger scale seedlings sprouting in a crevace and plant roots exert physical pressure as well as providing a pathway for water and chemical inlfitration. Burrowing animals and insects disturb the soil layer adjacent to the bedrock surface thus further increasing water and acid infiltration and exposure to oxidation processes.
Chemical Weathering
Carbonation-solution
Carbonation occurs on rocks which contain calcium carbonate such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate and forms calcium bicarbonate.
The reactions as follows:
- CO2 + H2O ⇌ H2CO3
- carbon dioxide + water ⇌ carbonic acid
- H2CO3 + CaCO3 ⇌ Ca(HCO3)2
- carbonic acid + calcium carbonate ⇌ calcium bicarbonate
Hydration
Hydration (not to be confused with hydrolysis) is the process whereby minerals in the rock absorb water and expand, and sometimes change. One example is how anhydrite changes to gypsum by absorbing water.
- CaSO4 + 2H2O → CaSO4·2H2O
- anhydrite + water → gypsum
Though chemical, this process may contribute to mechanical weathering as well, as some materials expand upon absorption of water. These materials may expand up to sixteen times their size, especially mudstones containing montmorillonite clays or bentonite.
Hydrolysis
Hydrolysis involves the action of acidic water on rock forming minerals like pyroxenes, amphiboles, and feldspars. For example feldspar reacts with acidic water to form kaolin clay and quartz. The soluble ions are removed in solution.
Oxidation
Within the weathering environment chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ and combination with oxygen and water to form Fe3+ hydroxides and oxides such as goethite, limonite, and hematite. This gives the affected rocks a reddish-brown colouration on the surface.
Acids
Acids are formed as sulfur and nitrogen compounds dissolve in rainwater forming acid rain. Carbonic and organic acids are formed in the soil by plants and micro-organisms. These acids contribute to the chemical weathering of rocks.
See also
bg:Изветряне de:Verwitterung et:Murenemine es:Meteorización eo:Efloresko he:בליה lt:Dūlėjimas nl:Verwering ja:風化 pl:Wietrzenie pt:Meteorização fi:Rapautuminen sv:Vittring