Weathering

Weathering is the process of disintegration of rocks, soils and their minerals through direct, or indirect contact with the atmosphere. Weathering occurs 'in situ', or 'without movement', and thus should not to be confused with erosion, which involves the movement and disintegration of rocks and minerals by processes such as water, wind, ice or gravity.

Two main classifications of weathering processes exist. 'Mechanical' or 'Physical' weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure. The second classification, 'Chemical' weathering, involves the direct effect of atmospheric chemicals, or biologically produced chemicals (also known as 'Biological' weathering), in the breakdown of rocks, soils and minerals.

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.

Mechanical (physical) weathering

Mechanical weathering is a 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, ice wedging, frost wedging, frost action 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".

Hydraulic action

This is when water (generally from powerful waves) rushes into cracks in the rockface rapidly. This traps a layer of air at the bottom of the crack, compressing it and weakening the rock. When the wave retreats, the trapped air is suddenly released with explosive force. The explosive release of highly pressurised air cracks away fragments at the rockface and widens the crack itself, worsening the process so more air is trapped on the next wave. This progressive system of positive feedback can damage cliffs greatly and cause rapid weathering.

Salt-crystal growth

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.

Biotic 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 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.

Another well known example of animal-caused biotic weathering is by the bivalve mollusc known as a Piddock. These animals, found 'boring' into carboniferous rocks, such as the limestone cliffs of Flamborough Head, bore themselves further into the cliff-face. An example of the characteristic holes caused by such weathering can be found here.

Chemical weathering

Chemical weathering involves the change in the composition of rock, often leading to a 'break down' in its form.

Solution

Three common gasses in the atmosphere may cause weathering of rocks.

Sulphur Dioxide, So₂, caused either through volcanic eruptions or from fossil fuels, can become Sulphuric acid within rainwater, which can cause solution weathering to the rocks on which it falls.

Atmospheric Nitrogen can also lead to solution weathering, if nitrous acids are formed.

However, one of the most well-known solution weathering processes is Carbonation, the process in which atmospheric carbon dioxide leads to solution weathering.

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 (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature and therefore is a large feature of glacial weathering.

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 is a form of Chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral.

When rock minerals take up water, it increases in volume, thus setting up physical stresses within the rock.Iron oxides are converted to Iron hydroxides Evidence: Surface flaking ^* E.g. the hydration of anhydrite forms gypsum

Hydrolysis

Hydrolysis involves the action of acidic water on rock forming minerals like pyroxenes, amphiboles, and feldspars. For example feldspar, found in rocks such as granite, 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⁺ (iron) 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 which crumbles easily and weakens the rock. This process is better known as 'rusting'.

Biological

A number of plants and animals may create chemical weathering through release of acidic compounds.

The most common form of biological weathering is the release of 'chelating compounds', i.e. acids, by trees so as to break down elements such as Aluminium and Iron in the soils beneath them. Once broken down, such elements are more easily washed away by rainwater. This process exists as metals such as iron can be toxic and hinder the a tree's growth. Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to Podsolisation of soils.

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