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Weathering is the decomposition of
Rock (geology)s,
soils and their
minerals through direct contact with the
Earth's atmosphere. Weathering occurs
in situ, or "with no 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, ice, wind and gravity.
Two important 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 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 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 Ice age,
eolian or
alluvial sediments) often makes more
fertile soil.
Physical (mechanical) weathering
Mechanical weathering is the cause of the disintegration of rocks. The primary process in mechanical weathering is Abrasion (geology) - 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 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 (geology), insolation weathering or thermal shock, often occurs in areas, like
deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dipping greatly at night. As the rock heats up and expands by day, and cools and contracts by night,
stress (physics) 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.
Freeze thaw weathering
This process can also be called frost shattering. This type of weathering is common in mountain areas where the temperature is around freezing point. 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 heaving upon freezing as a result of 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 liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. Intermolecular forces acting between the mineral surfaces, ice, and water sustain these unfrozen films which transport moisture and generate pressure between mineral surfaces as the lens 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 temperature for extended periods, as one would expect if weathering resulted from the expansion of water as froze. 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.J. B. Murton, R. Peterson, J.-C. Ozouf, Science 314, 1127 (2006). J. G. Dash, A. W. Rempel, J. S. Wettlaufer, Rev. Mod. Phys. 78, 695 (2006).
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 climate 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 where it results in the formation of
tors.
Frost wedging
Formerly believed to be the dominant mode, ice wedging may still be a factor for 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 cracks and joints of rocks freezes and expands. Water can exert pressures up to 21
megapascals (MPa) (2100 kilogram-force/cm²) at −22 °Celsius. This pressure is often higher than the resistance of most rocks and causes the rock to shatter.
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 rock structure.
Pressure release
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, 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"; these processes result in batholiths and granite domes, an example of which is Dartmoor.
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.
Salt-crystal growth (haloclasty)
near Qobustan, Azerbaijan.
Salt crystallization 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 crystallization 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.
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 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
Chemical weathering involves the change in the composition of rocks, often leading to a 'break down' in its form. This type of weathering happens over a period of time.
Dissolution
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 rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0.Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become sulfuric acid within rainwater, which can cause solution weathering to 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 acid 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
Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.
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, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.
was found in Moraine 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, though, 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 hydrolysis 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.
Aluminosilicates when subjected to the hydrolysis 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
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 coloration 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 plants so as to break down aluminium and
iron containing compounds in the soils beneath them. Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils.
Building weathering
Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface. Also statues, monuments and ornamental stonework can be badly damaged by natural weathering processes. This is accelerated in areas severely affected by
acid rain.
See also
References
Weathering is the decomposition of Rock (geology)s, soils and their
minerals through direct contact with the
Earth's atmosphere. Weathering occurs
in situ, or "with no 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, ice, wind and gravity.
Two important 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 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 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
Ice age, eolian or alluvial sediments) often makes more fertile soil.
Physical (mechanical) weathering
Mechanical weathering is the cause of the disintegration of rocks. The primary process in mechanical weathering is
Abrasion (geology) - 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 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 (geology), insolation weathering or
thermal shock, often occurs in areas, like
deserts, where there is a large diurnal temperature range. The temperatures soar high in the day, while dipping greatly at night. As the rock heats up and expands by day, and cools and contracts by night,
stress (physics) 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.
Freeze thaw weathering
This process can also be called frost shattering. This type of weathering is common in mountain areas where the temperature is around freezing point. 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 heaving upon freezing as a result of 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 liquid water from the surrounding pores. The ice crystal growth weakens the rocks which, in time, break up. Intermolecular forces acting between the mineral surfaces, ice, and water sustain these unfrozen films which transport moisture and generate pressure between mineral surfaces as the lens 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 temperature for extended periods, as one would expect if weathering resulted from the expansion of water as froze. 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.J. B. Murton, R. Peterson, J.-C. Ozouf, Science 314, 1127 (2006). J. G. Dash, A. W. Rempel, J. S. Wettlaufer, Rev. Mod. Phys. 78, 695 (2006).
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 climate 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 where it results in the formation of tors.
Frost wedging
Formerly believed to be the dominant mode, ice wedging may still be a factor for 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 cracks and joints of rocks freezes and expands. Water can exert pressures up to 21 megapascals (MPa) (2100
kilogram-force/cm²) at −22 °
Celsius. This pressure is often higher than the resistance of most rocks and causes the rock to shatter.
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 rock structure.
Pressure release
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, 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"; these processes result in batholiths and granite domes, an example of which is Dartmoor.
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.
Salt-crystal growth (haloclasty)
near Qobustan,
Azerbaijan.
Salt crystallization 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 crystallization 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.
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 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
Chemical weathering involves the change in the composition of rocks, often leading to a 'break down' in its form. This type of weathering happens over a period of time.
Dissolution
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 rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0.Sulfur dioxide, SO2, comes from volcanic eruptions or from fossil fuels, can become
sulfuric acid within rainwater, which can cause solution weathering to 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 acid 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
Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.
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, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum.
was found in
Moraine 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, though, 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 hydrolysis 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.
Aluminosilicates when subjected to the hydrolysis 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
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 coloration 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 plants so as to break down aluminium and iron containing compounds in the soils beneath them. Extreme release of chelating compounds can easily affect surrounding rocks and soils, and may lead to
podsolisation of soils.
Building weathering
Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface. Also
statues, monuments and ornamental stonework can be badly damaged by natural weathering processes. This is accelerated in areas severely affected by acid rain.
See also
References
Weathering factors
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weathering - Hutchinson encyclopedia article about weathering
Process by which exposed rocks are broken down on the spot (in situ) by the action of rain, frost, wind, and other elements of the weather. It differs from erosion in that no ...
WUN Weathering Science Consortium - home
The Biological Weathering in Earth Systems intitiative is a UK Natural Environment Research Council (NERC) funded grouping of 3 UK Universities in partnership with The British ...
BBC - Schools - KS3 Bitesize - Science - Chemistry - The Rock Cycle
KS3 revision on The Rock Cycle ... Rocks gradually wear away. This is called weathering. There are three types of weathering:
Weathering
Classic sheeted granite along the Tioga Road, Yosemite National Park. The granite is broken into gently dipping plates by unloading joints. Unloading joints probably form as the ...
Rocks and weathering
Rocks and weathering Rocks. BBC 'Essential Guide to Rocks' page - a brilliant introduction and general reference site, with animations, time lines and ...
