Weathering

File:YehliuTaiwan-HoneycombWeathering.jpg

The surface pattern on this pedestal rock is honeycomb weathering, caused by salt crystallization. This example is at Yehliu, Taiwan.

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.

The breakdown products following chemical weathering of rock and sediment minerals, and the leaching out of the more soluble parts, can be combined with decaying organic material to constitute soil. The mineral content of the soil is determined by the parent material from which the minerals are derived from. 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 more fertile soil.

Physical (mechanical) weathering

Mechanical weathering is a cause of the disintegration of rocks or wood. Most of the time it produces smaller angular fragments (like scree), as compared to chemical weathering [which produces...]. 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, exfoliation or thermal shock, 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 minus 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 is enhanced by the presence of moisture.

Frost induced weathering

A rock in southern Iceland fragmented by freeze-thaw action

Frost induced weathering, although often attributed to the expansion of freezing water captured in cracks, is generally independent of the water-to-ice expansion. It has long been known that moist soils expand, or frost heave, upon freezing as a result of the growth of [ice lenses - water migrating along from unfrozen areas via thin films to collect at growing ice lenses.] This same phenomena occurs within pore spaces of rocks. They grow larger as they attract water that has not frozen from the surrounding pores. The development of the ice crystal weakens the rock which, in time, breaks up. Intermolecular forces between the mineral surfaces, ice, and water sustain these unfrozen films which transport moisture and generate pressure between mineral surfaces as the lens(es?) aggregates. Experiments show that chalk, sandstone and limestone do not fracture at the nominal freezing temperature of water [of slightly below 0°C], even when cycled or held at low temperatures for extended periods, [as one would expect if weathering resulted from the expansion of water upon freezing]. For the more porous types of rocks, the temperature range critical for rapid, ice-lens-induced fracture is -3 to -6°C, significantly below freezing temperatures.

Freeze induced weathering 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. An example of rocks susceptible to frost action is chalk, which has many pore spaces for the growth of ice crystals. This process can be seen in Dartmoor, a southwest region of England, where it results in the formation of exposed granite hilltops, or tors <Dartmoor on wiki>.

Frost wedging

Formerly believed to be the dominant mode, ice wedging may still be a factor for the weathering of nonporous rock, although recent research has demonstrated it less important than previously thought. Frost action, sometimes known as ice crystal growth, ice wedging, frost wedging or freeze-thaw occurs when water in the cracks and joints of rocks freezes and expands. In the expansion, it was argued that expanding water can exert pressures up to 21 megapascals (MPa) (2100 kgf/cm²) at −22 °C, and this pressure is often higher than the resistance of most rocks, causing the rock to shatter.

When water that has entered the joints freezes, the expanding ice strains the walls of the joints and causes the joints to deepen and widen. This is because the volume of water expands by about 10% when it freezes. <"Water expansion after frozen." Ask a Scientist: Environmental Science Archive. Newton. Dec 2004. http://www.newton.dep.anl.gov/askasci/eng99/eng99358.htm>

When the ice thaws, water can flow further into the rock. Once the temperature drops below freezing 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.

Pressure release

Pressure release of granite.

In pressure release (also known as unloading), 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; as in a moving glacier, for example. 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. As a response to the decrease in pressure, the underlying rocks then expand upward. 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"; these processes result in batholiths and granite domes, an example of which is found in Dartmoor. <McConnell, David. "Physical Weathering." Weathering and Soils. Sept. 09, 2001. http://www.mhhe.com/earthsci/geology/mcconnell/wss/pw.htm (would like to use the figure 3 on this website, if possible)>

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 (haloclasty)

Salt weathering of building stone on the island of Gozo, Malta

Salt crystallisation or otherwise known as Haloclasty 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) 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.[where to get this info]

It is normally associated with arid climates where strong heating causes strong evaporation and therefore salt crystallisation. It is also common along coasts. An example of salt weathering can be seen in the honeycombed stones in sea walls.

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 crevice and plant roots exert physical pressure as well as provide a pathway for water and chemical inlfitration. Burrowing animals and insects disturb the soil layer adjacent to the bedrock surface, 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.

Chemical weathering

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

Solution

Rainfall is naturally slightly acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when gases such as sulphur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rainwater to produce stronger acids and can lower the pH to 4.5 or even 4.0. Sulfur dioxide, SO₂, from volcanic eruptions or from fossil fuels, can become sulfuric acid when exposed to rainwater, which can cause solution weathering of the rocks on which it falls.

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 (such as 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 [exfoliation] E.g. the hydration of anhydrite forms gypsum

A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of sandstone was found in glacial drift near Angelica, New York

Hydrolysis

Hydrolysis is a chemical weathering process affecting Silicate minerals. In such reactions, pure water ionizes slightly and reacts with silicate minerals. An example reaction:

Mg₂SiO₄ + 4H⁺ + 4OH^- —> 2Mg²⁺ + 4OH^- + H₄SiO₄

olivine (forsterite) + four ionized water molecules —> ions in solution + silicic acid in solution

This reaction results in complete dissolution of the original mineral, assuming enough water is available to drive the reaction. However, the above reaction is to a degree deceptive because pure water rarely acts as a H⁺ donor. Carbon dioxide, however, dissolves readily in water forming a weak acid and H⁺ donor.

Mg₂SiO₄ + 4CO₂ + 4H₂O —> 2Mg²⁺ + 4HCO₃^- + 4H₄SiO₄

olivine (forsterite) + carbon dioxide + water —> Magnesium and bicarbonate ions in solution + silicic acid in solution

This hydrolosis reaction is much more common. Carbonic acid is consumed by silicate weathering, resulting in more alkaline solutions because of the bicarbonate. This is an important reaction in controlling the amount of CO₂ in the atmosphere and can affect climate [Where do you get this info from?] Aluminosilicates, when subjected to the hydrolosis reaction, produce a secondary mineral rather than simply releasing cations.

2KAlSi₃O₈ + 2H₂CO₃ + 9H₂O —> Al₂Si₂O₅(OH)₄ + 4H₄SiO₄ + 2K⁺ + 2HCO₃^-

Orthoclase - aluminosilicate feldspar + carbonic acid + water —> Kaolinite - a clay mineral + silicic acid in solution + potassium and bicarbonate ions in solution

Oxidation

Chemical oxidation of a variety of metals occurs within the weathering process. The most commonly observed is the oxidation of Fe²⁺ (iron) in 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'.

Sulfation

Sulfur dioxide can react directly with limestone producing gypsum (calcium sulfate) which is more soluble than calcium carbonate and which is easily dissolved and washed away by subsequent rain. On areas of a building which are sheltered from rain, a gypsum crust may accumulate and trap soot particles derived from fossil fuel combustion.

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, secreted by trees in order to break down elements such as Aluminium and Iron in the surrounding soil. These elements can be toxic and disruptive in plant growth if they are left alone. Once broken down, such elements are more easily washed away by rainwater, and extreme release of chelating compounds can easily affect surrounding rocks and soils by the leaching out of these elements from the soil, known as Podsolisation <podsolisation from wiki>.

Building weathering

Buildings made of limestone are particularly susceptible to weathering. Weeds grow almost anywhere without many problems. They can sometimes germinate in the gutters of buildings where they have been transported to by the wind. As they proceed to grow they plant their roots down into the rock that the building is made up of, forcing their way further down. This causes the rock to exfoliate over a long time, small fragments crumbling away now and then. Statues and ornamental features can be badly damaged by weathering, especially in areas severely affected by acid rain which is caused by polutants put into the air.

See also

• Erosion
• Pedogenesis
• Soil production function
• Space weathering
• Spheroidal weathering

References

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