View source for Earth:Weathering

{{Short description|Deterioration of rocks and minerals through exposure to the elements}}
[[File:KharazaArch.jpg|thumb|upright=1.35|A [[Earth:Natural arch|natural arch]] produced by erosion of differentially weathered rock in Jebel Kharaz (Jordan)]]
[[File:Rocks weathering and salt formation.jpg|alt=Weathering |thumb|Rocks into salt ]]
'''Weathering'''  is the deterioration of [[Earth:Rock (geology)|rocks]], [[Chemistry:Soil|soil]]s and [[Earth:Mineral|mineral]]s (as well as [[Biology:Wood|wood]] and artificial materials) through contact with water, atmospheric gases, [[Astronomy:Sunlight|sunlight]], and biological organisms. Weathering occurs ''[[Biology:In situ|in situ]]'' (on-site, with little or no movement), and so is distinct from [[Earth:Erosion|erosion]], which involves the transport of rocks and minerals by agents such as [[Chemistry:Water|water]], [[Earth:Ice|ice]], [[Physics:Snow|snow]], [[Earth:Wind|wind]], [[Physics:Wind wave|waves]] and [[Physics:Gravity|gravity]].

Weathering processes are divided into ''physical'' and ''chemical weathering''. Physical weathering involves the breakdown of rocks and soils through the mechanical effects of heat, water, ice, or other agents. Chemical weathering involves the chemical reaction of water, atmospheric gases, and biologically produced chemicals with rocks and soils. Water is the principal agent behind both physical and chemical weathering,<ref name="leeder-4">{{cite book |last1=Leeder |first1=M. R. |title=Sedimentology and sedimentary basins : from turbulence to tectonics |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex, UK |isbn=9781405177832 |edition=2nd |page=4}}</ref> though atmospheric oxygen and carbon dioxide and the activities of biological organisms are also important.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |pages=245–246}}</ref> Chemical weathering by biological action is also known as biological weathering.<ref>{{cite web |last1=Gore |first1=Pamela J. W. |url=http://facstaff.gpc.edu/~pgore/geology/geo101/weather.htm |title=Weathering |archive-url=https://web.archive.org/web/20130510224332/http://facstaff.gpc.edu/~pgore/geology/geo101/weather.htm |archive-date=2013-05-10 |website=Georgia Perimeter College}}</ref>

The materials left over after the rock breaks down combine with organic material to create [[Chemistry:Soil|soil]]. Many of Earth's [[Earth:Landform|landform]]s and landscapes are the result of weathering processes combined with erosion and re-deposition. Weathering is a crucial part of the [[Earth:Rock cycle|rock cycle]], and [[Earth:Sedimentary rock|sedimentary rock]], formed from the weathering products of older rock, covers 66% of the Earth's continents and much of its ocean floor.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |edition=2nd |page=217}}</ref>

==Physical weathering==
'''Physical weathering''', also called '''mechanical weathering''' or ''disaggregation'', is the class of processes that causes the disintegration of rocks without chemical change. Physical weathering involves the breakdown of rocks into smaller fragments through processes such as expansion and contraction, mainly due to temperature changes. Two types of physical breakdown are freeze-thaw weathering and thermal fracturing. Pressure release can also cause weathering without temperature change. It is usually much less important than chemical weathering, but can be significant in subarctic or alpine environments.{{sfn|Blatt|Middleton|Murray|1980|p=247}} Furthermore, chemical and physical weathering often go hand in hand. For example, cracks extended by physical weathering will increase the surface area exposed to chemical action, thus amplifying the rate of disintegration.{{sfn|Leeder|2011|p=3}}

Frost weathering is the most important form of physical weathering. Next in importance is wedging by plant roots, which sometimes enter cracks in rocks and pry them apart. The burrowing of worms or other animals may also help disintegrate rock, as can "plucking" by lichens.{{sfn|Blatt|Middleton|Murray|1980|pp=249-250}}

===Frost weathering===
[[File:Abiskorock.JPG|thumb|A rock in Abisko, Sweden, fractured along existing joints possibly by frost weathering or thermal stress]]
{{Main|Earth:Frost weathering}}

''Frost weathering'' is the collective name for those forms of physical weathering that are caused by the formation of ice within rock outcrops. It was long believed that the most important of these is ''frost wedging'', which results from the expansion of pore water when it freezes. However, a growing body of theoretical and experimental work suggests that ''ice segregation'', in which supercooled water migrates to lenses of ice forming within the rock, is the more important mechanism.<ref name="murton-etal-2006">{{cite journal |last1=Murton |first1=J. B. |last2=Peterson |first2=R. |last3=Ozouf |first3=J.-C. |title=Bedrock Fracture by Ice Segregation in Cold Regions |journal=Science |date=17 November 2006 |volume=314 |issue=5802 |pages=1127–1129 |doi=10.1126/science.1132127|pmid=17110573 |bibcode=2006Sci...314.1127M |s2cid=37639112 }}</ref>{{sfn|Leeder|2011|p=18}}

When water freezes, its volume increases by 9.2%. This expansion can theoretically generate pressures greater than {{convert|200|MPa|}}, though a more realistic upper limit is {{convert|14|MPa|}}. This is still much greater than the tensile strength of granite, which is about {{convert|4|MPa|}}. This makes frost wedging, in which pore water freezes and its volumetric expansion fractures the enclosing rock, appear to be a plausible mechanism for frost weathering. However, ice will simply expand out of a straight, open fracture before it can generate significant pressure. Thus frost wedging can only take place in small, tortuous fractures.{{sfn|Blatt|Middleton|Murray|1980|p=247}} The rock must also be almost completely saturated with water, or the ice will simply expand into the air spaces in the unsaturated rock without generating much pressure. These conditions are unusual enough that frost wedging is unlikely to be the dominant process of frost weathering.<ref name="matsuoka-murton-2008">{{cite journal |last1=Matsuoka |first1=Norikazu |last2=Murton |first2=Julian |title=Frost weathering: recent advances and future directions |journal=Permafrost and Periglacial Processes |date=April 2008 |volume=19 |issue=2 |pages=195–210 |doi=10.1002/ppp.620|s2cid=131395533 }}</ref> Frost wedging is most effective where there are daily cycles of melting and freezing of water-saturated rock, so it is unlikely to be significant in the tropics, in polar regions or in arid climates.{{sfn|Blatt|Middleton|Murray|1980|p=247}}

Ice segregation is a less well characterized mechanism of physical weathering.<ref name="murton-etal-2006"/> It takes place because ice grains always have a surface layer, often just a few molecules thick, that resembles liquid water more than solid ice, even at temperatures well below the freezing point. This ''premelted liquid layer'' has unusual properties, including a strong tendency to draw in water by [[Physics:Capillary action|capillary action]] from warmer parts of the rock. This results in growth of the ice grain that puts considerable pressure on the surrounding rock,<ref>{{cite journal |last1=Dash |first1=J. G. |last2=Rempel |first2=A. W. |last3=Wettlaufer |first3=J. S. |title=The physics of premelted ice and its geophysical consequences |journal=Reviews of Modern Physics |date=12 July 2006 |volume=78 |issue=3 |pages=695–741 |doi=10.1103/RevModPhys.78.695|bibcode=2006RvMP...78..695D }}</ref> up to ten times greater than is likely with frost wedging. This mechanism is most effective in rock whose temperature averages just below the freezing point, {{convert|-4 to -15|C|}}. Ice segregation results in growth of ice needles and [[Earth:Ice lens|ice lens]]es within fractures in the rock and parallel to the rock surface, which gradually pry the rock apart.{{sfn|Leeder|2011|p=18}}

===Thermal stress===
''Thermal stress weathering'' results from the expansion and contraction of rock due to temperature changes. Thermal stress weathering is most effective when the heated portion of the rock is buttressed by surrounding rock, so that it is free to expand in only one direction.<ref name="hall-1999">{{citation|title=The role of thermal stress fatigue in the breakdown of rock in cold regions|journal=Geomorphology|volume=31|issue=1–4|pages=47–63|doi=10.1016/S0169-555X(99)00072-0|year=1999|last1=Hall|first1=Kevin|bibcode=1999Geomo..31...47H}}</ref>

Thermal stress weathering comprises two main types, [[Physics:Thermal shock|thermal shock]] and thermal fatigue. Thermal shock takes place when the stresses are so great that the rock cracks immediately, but this is uncommon. More typical is thermal fatigue, in which the stresses are not great enough to cause immediate rock failure, but repeated cycles of stress and release gradually weaken the rock.<ref name="hall-1999"/>

{{Anchor|Insolation weathering}}
Thermal stress weathering is an important mechanism in deserts, where there is a large [[Earth:Diurnal temperature variation|diurnal]] temperature range, hot in the day and cold at night.<ref>{{cite book|author=Paradise, T. R.|doi=10.1130/0-8137-2390-6.39|chapter=Petra revisited: An examination of sandstone weathering research in Petra, Jordan|title=Special Paper 390: Stone Decay in the Architectural Environment|date=2005|isbn=0-8137-2390-6|volume=390|pages=39–49}}</ref> As a result, thermal stress weathering is sometimes called '''insolation weathering''', but this is misleading. Thermal stress weathering can be caused by any large change of temperature, and not just intense solar heating. It is likely as important in cold climates as in hot, arid climates.<ref name="hall-1999"/> Wildfires can also be a significant cause of rapid thermal stress weathering.<ref>{{cite journal |last1=Shtober-Zisu |first1=Nurit |last2=Wittenberg |first2=Lea |title=Long-term effects of wildfire on rock weathering and soil stoniness in the Mediterranean landscapes |journal=Science of the Total Environment |date=March 2021 |volume=762 |pages=143125 |doi=10.1016/j.scitotenv.2020.143125 |issn=0048-9697|pmid=33172645 |bibcode=2021ScTEn.762n3125S |s2cid=225117000 }}</ref>

The importance of thermal stress weathering has long been discounted by geologists,{{sfn|Blatt|Middleton|Murray|1980|p=247}}{{sfn|Leeder|2011|p=18}} based on experiments in the early 20th century that seemed to show that its effects were unimportant. These experiments have since been criticized as unrealistic, since the rock samples were small, were polished (which reduces nucleation of fractures), and were not buttressed. These small samples were thus able to expand freely in all directions when heated in experimental ovens, which failed to produce the kinds of stress likely in natural settings. The experiments were also more sensitive to thermal shock than thermal fatigue, but thermal fatigue is likely the more important mechanism in nature. [[Physics:Geomorphology|Geomorphologists]] have begun to reemphasize the importance of thermal stress weathering, particularly in cold climates.<ref name="hall-1999"/>

===Pressure release===
[[File:GeologicalExfoliationOfGraniteRock.jpg|thumb|Exfoliated granite sheets in Texas, possibly caused by pressure release]]

''Pressure release'' or ''unloading'' is a form of physical weathering seen when deeply buried rock is [[Earth:Exhumation (geology)|exhumed]]. Intrusive igneous rocks, such as [[Chemistry:Granite|granite]], are formed deep beneath the Earth's surface. They are under tremendous [[Earth:Overburden pressure|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, a process known as exfoliation.  Exfoliation due to pressure release is also known as ''sheeting''.{{sfn|Blatt|Middleton|Murray|1980|p=249}}

As with thermal weathering, pressure release is most effective in buttressed rock. Here the differential stress directed towards the unbuttressed surface can be as high as {{convert|35|MPa||}}, easily enough to shatter rock. This mechanism is also responsible for spalling in mines and quarries, and for the formation of joints in rock outcrops.{{sfn|Leeder|2011|p=19}}

Retreat of an overlying glacier can also lead to exfoliation due to pressure release. This can be enhanced by other physical wearing mechanisms.<ref>{{cite journal |last1=Harland |first1=W. B. |title=Exfoliation Joints and Ice Action |journal=Journal of Glaciology |date=1957 |volume=3 |issue=21 |pages=8–10 |doi=10.3189/S002214300002462X|doi-access=free }}</ref>

===Salt-crystal growth {{anchor|Salt weathering}}===
[[File:Tafoni 03.jpg|thumb|[[Earth:Tafoni|Tafoni]] at Salt Point State Park, Sonoma County, California]]

''Salt crystallization'' (also known as '''salt weathering''', '''salt wedging''' or haloclasty) causes disintegration of rocks when [[Earth:Salinity|saline]] solutions seep into cracks and joints in the rocks and evaporate, leaving salt crystals behind. As with ice segregation, the surfaces of the salt grains draw in additional dissolved salts through capillary action, causing the growth of salt lenses that exert high pressure on the surrounding rock. Sodium and magnesium salts are the most effective at producing salt weathering. Salt weathering can also take place when [[Chemistry:Pyrite|pyrite]] in sedimentary rock is chemically weathered to [[Chemistry:Iron(II) sulfate|iron(II) sulfate]] and [[Chemistry:Gypsum|gypsum]], which then crystallize as salt lenses.{{sfn|Leeder|2011|p=18}}

Salt crystallization can take place wherever salts are concentrated by evaporation. It is thus most common in [[Earth:Arid|arid]] climates where strong heating causes strong evaporation and along coasts.{{sfn|Leeder|2011|p=18}} Salt weathering is likely important in the formation of [[Earth:Tafoni|tafoni]], a class of cavernous rock weathering structures.<ref>{{cite journal |last1=Turkington |first1=Alice V. |last2=Paradise |first2=Thomas R. |title=Sandstone weathering: a century of research and innovation |journal=Geomorphology |date=April 2005 |volume=67 |issue=1–2 |pages=229–253 |doi=10.1016/j.geomorph.2004.09.028|bibcode=2005Geomo..67..229T }}</ref>

===Biological effects on mechanical weathering===
Living organisms may contribute to mechanical weathering, as well as chemical weathering (see § Biological weathering below). [[Biology:Lichen|Lichen]]s and [[Biology:Moss|moss]]es 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. Lichens have been observed to pry mineral grains loose from bare shale with their hyphae (rootlike attachment structures), a process described as ''plucking'', {{sfn|Blatt|Middleton|Murray|1980|p=249}} and to pull the fragments into their body, where the fragments then undergo a process of chemical weathering not unlike digestion.<ref>{{cite journal |last1=Fry |first1=E. Jennie |title=The Mechanical Action of Crustaceous Lichens on Substrata of Shale, Schist, Gneiss, Limestone, and Obsidian |journal=Annals of Botany |date=July 1927 |volume=os-41 |issue=3 |pages=437–460 |doi=10.1093/oxfordjournals.aob.a090084}}</ref> 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 infiltration.{{sfn|Blatt|Middleton|Murray|1980|pp=249-250}}

=={{anchor|Chemical weathering}}Chemical weathering==
[[File:Weathering Limestone State College PA.jpg|thumb|Comparison of unweathered (left) and weathered (right) limestone]]
Most rock forms at elevated temperature and pressure, and the minerals making up the rock are often chemically unstable in the relatively cool, wet, and oxidizing conditions typical of the Earth's surface. Chemical weathering takes place when water, oxygen, carbon dioxide, and other chemical substances react with rock to change its composition. These reactions convert some of the original ''primary'' minerals in the rock to ''secondary'' minerals, remove other substances as solutes, and leave the most stable minerals as a chemically unchanged ''resistate''. In effect, chemical weathering changes the original set of minerals in the rock into a new set of minerals that is in closer equilibrium with surface conditions. However, true equilibrium is rarely reached, because weathering is a slow process, and leaching carries away solutes produced by weathering reactions before they can accumulate to equilibrium levels. This is particularly true in tropical environments.{{sfn|Blatt|Middleton|Murray|1980|pp=245-246}}

Water is the principal agent of chemical weathering, converting many primary minerals to clay minerals or hydrated oxides via reactions collectively described as [[Chemistry:Hydrolysis|hydrolysis]]. Oxygen is also important, acting to [[Chemistry:Redox|oxidize]] many minerals, as is carbon dioxide, whose weathering reactions are described as [[Chemistry:Carbonation|carbonation]].{{sfn|Blatt|Middleton|Murray|1980|pp=246}}

The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca<sup>2+</sup> and other ions into surface waters.<ref>Hogan, C. Michael (2010) [http://www.eoearth.org/article/Calcium?topic=49557 "Calcium"], in A. Jorgenson and C. Cleveland (eds.) ''[[Encyclopedia of Earth]]'', National Council for Science and the Environment, Washington DC</ref>

===Dissolution===

[[File:Weathered limestone cores.jpg|thumb|[[Earth:Limestone|Limestone]] core samples at different stages of chemical weathering, from very high at shallow depths (bottom) to very low at greater depths (top). Slightly weathered limestone shows brownish stains, while highly weathered limestone loses much of its carbonate mineral content, leaving behind clay. Limestone drill core taken from the carbonate West Congolian deposit in Kimpese, Democratic Republic of Congo.]]

Dissolution (also called ''simple solution'' or ''congruent dissolution'') is the process in which a mineral dissolves completely without producing any new solid substance.<ref>{{cite book |last1=Birkeland |first1=Peter W. |title=Soils and geomorphology |date=1999 |publisher=Oxford University Press |location=New York |isbn=978-0195078862 |page=59 |edition=3rd}}</ref> Rainwater easily dissolves soluble minerals, such as [[Chemistry:Halite|halite]] or [[Chemistry:Gypsum|gypsum]], but can also dissolve highly resistant minerals such as [[Chemistry:Quartz|quartz]], given sufficient time.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=7}}</ref> Water breaks the bonds between atoms in the crystal:<ref name="Environmental Studies in Sedimentar">{{cite journal|jstor=43418626|title=Environmental Studies in Sedimentary Geochemistry|last1=Nicholls|first1=G. D.|journal=Science Progress (1933- )|year=1963|volume=51|issue=201|pages=12–31}}</ref>

[[File:SiO H2O.jpg|Hydrolysis of a silica mineral]]

The overall reaction for dissolution of quartz is
:{{chem2|SiO2 + 2 H2O -> H4SiO4}}

The dissolved quartz takes the form of [[Chemistry:Silicic acid|silicic acid]].

A particularly important form of dissolution is carbonate dissolution, in which atmospheric [[Chemistry:Carbon dioxide|carbon dioxide]] enhances solution weathering. Carbonate dissolution affects rocks containing [[Chemistry:Calcium carbonate|calcium carbonate]], such as [[Earth:Limestone|limestone]] and [[Earth:Chalk|chalk]]. It takes place when rainwater combines with carbon dioxide to form [[Chemistry:Carbonic acid|carbonic acid]], a [[Chemistry:Weak acid|weak acid]], which dissolves calcium carbonate (limestone) and forms soluble [[Chemistry:Calcium bicarbonate|calcium bicarbonate]]. Despite a slower reaction kinetics, this process is thermodynamically favored at low temperature, because colder water holds more dissolved carbon dioxide gas (due to the retrograde [[Chemistry:Solubility|solubility]] of gases). Carbonate dissolution is therefore an important feature of glacial weathering.<ref>{{cite journal |last1=Plan |first1=Lukas |title=Factors controlling carbonate dissolution rates quantified in a field test in the Austrian alps |journal=Geomorphology |date=June 2005 |volume=68 |issue=3–4 |pages=201–212 |doi=10.1016/j.geomorph.2004.11.014|bibcode=2005Geomo..68..201P }}</ref>

Carbonate dissolution involves the following steps:

:CO<sub>2</sub> + H<sub>2</sub>O → H<sub>2</sub>CO<sub>3</sub>
:carbon dioxide + water → carbonic acid

:H<sub>2</sub>CO<sub>3</sub> + CaCO<sub>3</sub> → Ca(HCO<sub>3</sub>)<sub>2</sub>
:carbonic acid + calcium carbonate → calcium bicarbonate

Carbonate dissolution on the surface of well-jointed limestone produces a dissected [[Earth:Limestone pavement|limestone pavement]]. This process is most effective along the joints, widening and deepening them.<ref name="Geology and geomorphology">{{cite web|url=http://www.limestone-pavements.org.uk/geology.html|title=Geology and geomorphology|last=Anon|work=Limestone Pavement Conservation|publisher=UK and Ireland Biodiversity Action Plan Steering Group|accessdate=30 May 2011|archive-url=https://web.archive.org/web/20110807234809/http://www.limestone-pavements.org.uk/geology.html|archive-date=7 August 2011|url-status=dead}}</ref>

In unpolluted environments, the [[Chemistry:PH|pH]] of rainwater due to dissolved carbon dioxide is around 5.6.  [[Earth:Acid rain|Acid rain]] occurs when gases such as sulfur 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. [[Chemistry:Sulfur dioxide|Sulfur dioxide]], SO<sub>2</sub>, comes from volcanic eruptions or from fossil fuels, and can become [[Chemistry:Sulfuric acid|sulfuric acid]] within rainwater, which can cause solution weathering to the rocks on which it falls.<ref>{{cite journal |last1=Charlson |first1=R. J. |last2=Rodhe |first2=H. |title=Factors controlling the acidity of natural rainwater |journal=Nature |date=February 1982 |volume=295 |issue=5851 |pages=683–685 |doi=10.1038/295683a0|bibcode=1982Natur.295..683C |s2cid=4368102 }}</ref>

===Hydrolysis and carbonation===
[[File:Iddingsite.JPG|thumb|right|[[Chemistry:Olivine|Olivine]] weathering to iddingsite within a mantle xenolith]]
[[Chemistry:Hydrolysis|Hydrolysis]] (also called ''incongruent dissolution'') is a form of chemical weathering in which only part of a mineral is taken into solution. The rest of the mineral is transformed into a new solid material, such as a [[Chemistry:Clay mineral|clay mineral]].{{sfn|Boggs|2006|pp=7-8}} For example, [[Chemistry:Forsterite|forsterite]] (magnesium [[Chemistry:Olivine|olivine]]) is hydrolyzed into solid [[Chemistry:Brucite|brucite]] and dissolved silicic acid:

:Mg<sub>2</sub>SiO<sub>4</sub> + 4 H<sub>2</sub>O ⇌ 2 Mg(OH)<sub>2</sub> + H<sub>4</sub>SiO<sub>4</sub>
:forsterite + water ⇌ brucite + silicic acid

Most hydrolysis during weathering of minerals is ''acid hydrolysis'', in which protons (hydrogen ions), which are present in acidic water, attack chemical bonds in mineral crystals.{{sfn|Leeder|2011|p=4}} The bonds between different cations and oxygen ions in minerals differ in strength, and the weakest will be attacked first. The result is that minerals in igneous rock weather in roughly the same order in which they were originally formed (Bowen's Reaction Series).{{sfn|Blatt|Middleton|Murray|1980|p=252}} Relative bond strength is shown in the following table:<ref name="Environmental Studies in Sedimentar"/> 
{| class="wikitable" style="text-align: left;"
|+<!--Bond strengths between oxygen and common cations-->
! Bond
! Relative strength
|-
| Si&ndash;O
| 2.4
|-
| Ti&ndash;O
| 1.8
|-
| Al&ndash;O
| 1.65
|- 
| Fe<sup>+3</sup>&ndash;O
| 1.4
|- 
| Mg&ndash;O
| 0.9
|- 
| Fe<sup>+2</sup>&ndash;O
| 0.85
|- 
| Mn&ndash;O
| 0.8
|- 
| Ca&ndash;O
| 0.7
|- 
| Na&ndash;O
| 0.35
|-
| K&ndash;O
| 0.25
|}

This table is only a rough guide to order of weathering. Some minerals, such as [[Chemistry:Illite|illite]], are unusually stable, while silica is unusually unstable given the strength of the [[Chemistry:Silicon–oxygen bond|silicon–oxygen bond]].{{sfn|Blatt|Middleton|Murray|1980|p=258}}

Carbon dioxide that dissolves in water to form carbonic acid is the most important source of protons, but organic acids are also important natural sources of acidity.{{sfn|Blatt|Middleton|Murray|1980|p=250}} Acid hydrolysis from dissolved carbon dioxide is sometimes described as ''carbonation'', and can result in weathering of the primary minerals to secondary carbonate minerals.<ref name="thornbury-1969">{{cite book |last1=Thornbury |first1=William D. |title=Principles of geomorphology |date=1969 |publisher=Wiley |location=New York |isbn=0471861979 |pages=303–344 |edition=2d}}</ref> For example, weathering of forsterite can produce [[Chemistry:Magnesite|magnesite]] instead of brucite via the reaction:

:Mg<sub>2</sub>SiO<sub>4</sub> + 2 CO<sub>2</sub> + 2 H<sub>2</sub>O ⇌ 2 MgCO<sub>3</sub> + H<sub>4</sub>SiO<sub>4</sub> 
:forsterite + carbon dioxide + water ⇌ magnesite + silicic acid in solution

[[Chemistry:Carbonic acid|Carbonic acid]] is consumed by [[Chemistry:Silicate|silicate]] weathering, resulting in more [[Chemistry:Alkaline|alkaline]] solutions because of the [[Chemistry:Bicarbonate|bicarbonate]]. This is an important reaction in controlling the amount of CO<sub>2</sub> in the atmosphere and can affect climate.<ref>{{cite journal |last1=Berner |first1=Robert A. |editor1-first=Arthur F |editor1-last=White |editor2-first=Susan L |editor2-last=Brantley |title=Chapter 13. CHEMICAL WEATHERING AND ITS EFFECT ON ATMOSPHERIC CO2 AND CLIMATE |journal=Chemical Weathering Rates of Silicate Minerals |date=31 December 1995 |pages=565–584 |doi=10.1515/9781501509650-015|isbn=9781501509650 }}</ref>

[[Chemistry:Aluminosilicate|Aluminosilicate]]s containing highly soluble cations, such as sodium or potassium ions, will release the cations as dissolved bicarbonates during acid hydrolysis:

:2 KAlSi<sub>3</sub>O<sub>8</sub> + 2 H<sub>2</sub>CO<sub>3</sub> + 9 H<sub>2</sub>O ⇌ Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> + 4 H<sub>4</sub>SiO<sub>4</sub> + 2 K<sup>+</sup> + 2 HCO<sub>3</sub><sup>−</sup>
:[[Chemistry:Orthoclase|orthoclase]] (aluminosilicate feldspar) + carbonic acid + water ⇌ [[Chemistry:Kaolinite|kaolinite]] (a clay mineral) + silicic acid in solution + potassium and bicarbonate ions in solution

===Oxidation===
[[File:GoldinPyriteDrainage acide.JPG|thumb|A pyrite cube has dissolved away from host rock, leaving gold particles behind.]]
[[File:PyOx.JPG|thumb|Oxidized pyrite cubes]]
Within the weathering environment, chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe<sup>2+</sup> ([[Chemistry:Iron|iron]]) by oxygen and water to form Fe<sup>3+</sup> oxides and hydroxides such as [[Chemistry:Goethite|goethite]], [[Chemistry:Limonite|limonite]], and [[Chemistry:Hematite|hematite]]. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, as does sulfur during the weathering of [[Chemistry:Sulfide mineral|sulfide mineral]]s such as [[Chemistry:Chalcopyrite|chalcopyrite]]s or CuFeS<sub>2</sub> oxidizing to copper hydroxide and [[Chemistry:Iron oxide|iron oxide]]s.{{sfn|Boggs|2006|p=9}}

===Hydration===
[[Earth:Mineral hydration|Mineral hydration]] is a form of chemical weathering that involves the rigid attachment of water molecules or H+ and OH- ions to the atoms and molecules of a mineral. No significant dissolution takes place. For example, [[Chemistry:Iron oxide|iron oxide]]s are converted to iron hydroxides and the hydration of [[Chemistry:Anhydrite|anhydrite]] forms [[Chemistry:Gypsum|gypsum]].{{sfn|Boggs|1996|p=8}}

Bulk hydration of minerals is secondary in importance to dissolution, hydrolysis, and oxidation,{{sfn|Boggs|2006|p=9}} but hydration of the crystal surface is the crucial first step in hydrolysis. A fresh surface of a mineral crystal exposes ions whose electrical charge attracts water molecules. Some of these molecules break into H+ that bonds to exposed anions (usually oxygen) and OH- that bonds to exposed cations. This further disrupts the surface, making it susceptible to various hydrolysis reactions. Additional protons replace cations exposed on the surface, freeing the cations as solutes. As cations are removed, silicon-oxygen and silicon-aluminium bonds become more susceptible to hydrolysis, freeing silicic acid and aluminium hydroxides to be leached away or to form clay minerals.{{sfn|Blatt|Middleton|Murray|1980|p=258}}{{sfn|Leeder|2011|pp=653-655}} Laboratory experiments show that weathering of feldspar crystals begins at dislocations or other defects on the surface of the crystal, and that the weathering layer is only a few atoms thick. Diffusion within the mineral grain does not appear to be significant.<ref>{{cite journal |last1=Berner |first1=Robert A. |last2=Holdren |first2=George R. |title=Mechanism of feldspar weathering: Some observational evidence |journal=Geology |date=1 June 1977 |volume=5 |issue=6 |pages=369–372 |doi=10.1130/0091-7613(1977)5<369:MOFWSO>2.0.CO;2|bibcode=1977Geo.....5..369B }}</ref>

[[File:Weathering 9039.jpg|thumb|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.]]

===Biological weathering===
Mineral weathering can also be initiated or accelerated by soil microorganisms. Soil organisms make up about 10&nbsp;mg/cm<sup>3</sup> of typical soils, and laboratory experiments have demonstrated that [[Chemistry:Albite|albite]] and [[Chemistry:Muscovite|muscovite]] weather twice as fast in live versus sterile soil. Lichens on rocks are among the most effective biological agents of chemical weathering.{{sfn|Blatt|Middleton|Murray|1980|p=250}} For example, an experimental study on hornblende granite in New Jersey, US, demonstrated a 3x – 4x increase in weathering rate under lichen covered surfaces compared to recently exposed bare rock surfaces.<ref>{{cite journal|doi=10.1016/j.chemgeo.2011.10.009|title=Effect of lichen colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux|date=2012|last1=Zambell|first1=C.B.|last2=Adams|first2=J.M.|last3=Gorring|first3=M.L.|last4=Schwartzman|first4=D.W.|journal=Chemical Geology|volume=291|pages=166–174|bibcode=2012ChGeo.291..166Z}}</ref>

[[File:lava z14.jpg|thumb|right|Biological weathering of [[Earth:Basalt|basalt]] by lichen, La Palma]]
The most common forms of biological weathering result from the release of chelating compounds (such as certain organic acids and [[Chemistry:Siderophore|siderophore]]s) and of carbon dioxide and organic acids by plants. Roots can build up the carbon dioxide level to 30% of all soil gases, aided by adsorption of {{CO2}} on clay minerals and the very slow diffusion rate of {{CO2}} out of the soil.<ref>{{cite journal |last1=Fripiat |first1=J. J. |title=Interlamellar Adsorption of Carbon Dioxide by Smectites |journal=Clays and Clay Minerals |date=1974 |volume=22 |issue=1 |pages=23–30 |doi=10.1346/CCMN.1974.0220105|bibcode=1974CCM....22...23F |s2cid=53610319 |doi-access=free }}</ref> The {{CO2}} and organic acids help break down [[Chemistry:Aluminium|aluminium]]- and [[Chemistry:Iron|iron]]-containing compounds in the soils beneath them. Roots have a negative electrical charge balanced by protons in the soil next to the roots, and these can be exchanged for essential nutrient cations such as potassium.{{sfn|Blatt|Middleton|Murray|1980|pp=251}} Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.<ref>{{cite book|last=Chapin III|first=F. Stuart|author2=Pamela A. Matson |author3=Harold A. Mooney |title=Principles of terrestrial ecosystem ecology|date=2002|publisher=Springer|location=New York|isbn=9780387954431|pages=54–55|edition=[Nachdr.]}}</ref> Chelating compounds, mostly low molecular weight organic acids, are capable of removing metal ions from bare rock surfaces, with aluminium and silicon being particularly susceptible.{{sfn|Blatt|Tracy|1996|p=233}} The ability to break down bare rock allows lichens to be among the first colonizers of dry land.{{sfn|Blatt|Middleton|Murray|1980|pp=250-251}} The accumulation of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils.<ref>{{Cite journal|last1=Lundström|first1=U. S.|last2=van Breemen|first2=N.|last3=Bain|first3=D. C.|last4=van Hees|first4=P. A. W.|last5=Giesler|first5=R.|last6=Gustafsson|first6=J. P.|last7=Ilvesniemi|first7=H.|last8=Karltun|first8=E.|last9=Melkerud|first9=P. -A.|last10=Olsson|first10=M.|last11=Riise|first11=G.|date=2000-02-01|title=Advances in understanding the podzolization process resulting from a multidisciplinary study of three coniferous forest soils in the Nordic Countries|url=http://www.sciencedirect.com/science/article/pii/S0016706199000774|journal=Geoderma|language=en|volume=94|issue=2|pages=335–353|doi=10.1016/S0016-7061(99)00077-4|bibcode=2000Geode..94..335L|issn=0016-7061}}</ref><ref>{{cite book|last=Waugh|first=David|title=Geography : an integrated approach|date=2000|publisher=Nelson Thornes|location=Gloucester, U.K.|isbn=9780174447061|page=272|edition=3rd}}</ref>

The symbiotic [[Biology:Mycorrhiza|mycorrhizal fungi]] associated with tree root systems can release inorganic nutrients from minerals such as apatite or biotite and transfer these nutrients to the trees, thus contributing to tree nutrition.<ref>{{cite journal|author=Landeweert, R. |author2=Hoffland, E. |author3=Finlay, R.D. |author4=Kuyper, T.W. |author5=van Breemen, N. |pmid=11301154|date=2001|title=Linking plants to rocks: Ectomycorrhizal fungi mobilize nutrients from minerals|volume=16|issue=5|pages=248–254|journal=Trends in Ecology & Evolution|doi=10.1016/S0169-5347(01)02122-X}}</ref>  It was also recently evidenced that bacterial communities can impact mineral stability leading to the release of inorganic nutrients.<ref>{{cite journal|author=Calvaruso, C.|author2=Turpault, M.-P.|author3=Frey-Klett, P.|doi=10.1128/AEM.72.2.1258-1266.2006|title=Root-Associated Bacteria Contribute to Mineral Weathering and to Mineral Nutrition in Trees: A Budgeting Analysis|date=2006|journal=Applied and Environmental Microbiology|volume=72|issue=2|pages=1258–66|pmid=16461674|pmc=1392890 |bibcode=2006ApEnM..72.1258C}}</ref> A large range of bacterial strains or communities from diverse genera have been reported to be able to colonize mineral surfaces or to weather minerals, and for some of them a plant growth promoting effect has been demonstrated.<ref>{{cite journal|author=Uroz, S.|author2=Calvaruso, C.|author3=Turpault, M.-P.|author4=Frey-Klett, P.|title=Mineral weathering by bacteria: ecology, actors and mechanisms|journal=Trends Microbiol|date= 2009|volume=17|issue=8|pages=378–87|doi=10.1016/j.tim.2009.05.004|pmid=19660952}}</ref>  The demonstrated or hypothesised mechanisms used by bacteria to weather minerals include several oxidoreduction and dissolution reactions as well as the production of weathering agents, such as protons, organic acids and chelating molecules.

==Weathering on the ocean floor==
Weathering of basaltic oceanic crust differs in important respects from weathering in the atmosphere. Weathering is relatively slow, with basalt becoming less dense, at a rate of about 15% per 100 million years. The basalt becomes hydrated, and is enriched in total and ferric iron, magnesium, and sodium at the expense of silica, titanium, aluminum, ferrous iron, and calcium.{{sfn|Blatt|Middleton|Murray|1960|p=256}}

==Building weathering==
[[File:-47 Concrete weathering.jpg|thumb|upright|Concrete damaged by acid rain]]
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 [[Earth:Acid rain|acid rain]].<ref>{{cite book |last1=Schaffer |first1=R. J. |title=Weathering of Natural Building Stones. |date=2016 |publisher=Taylor and Francis |isbn=9781317742524}}</ref>

Accelerated building weathering may be a threat to the environment and occupant safety. Design strategies can moderate the impact of environmental effects, such as using of pressure-moderated rain screening, ensuring that the HVAC system is able to effectively control humidity accumulation and selecting concrete mixes with reduced water content to minimize the impact of freeze-thaw cycles. <ref> {{cite web |url=https://www.bchousing.org/publications/MBAR-Chronic-Stressors.pdf |title= Design Discussion Primer - Chronic Stressors |publisher=BC Housing |access-date=13 July 2021}}</ref>

==Properties of well-weathered soils==
Granitic rock, which is the most abundant crystalline rock exposed at the Earth's surface, begins weathering with destruction of [[Chemistry:Hornblende|hornblende]]. [[Chemistry:Biotite|Biotite]] then weathers to [[Chemistry:Vermiculite|vermiculite]], and finally [[Chemistry:Oligoclase|oligoclase]] and [[Chemistry:Microcline|microcline]] are destroyed. All are converted into a mixture of clay minerals and iron oxides.{{sfn|Blatt|Middleton|Murray|1980|p=252}} The resulting soil is depleted in calcium, sodium, and ferrous iron compared with the bedrock, and magnesium is reduced by 40% and silicon by 15%. At the same time, the soil is enriched in aluminium and potassium, by at least 50%; by titanium, whose abundance triples; and by ferric iron, whose abundance increases by an order of magnitude compared with the bedrock.{{sfn|Blatt|Middleton|Murray|p=253}}

Basaltic rock is more easily weathered than granitic rock, due to its formation at higher temperatures and drier conditions. The fine grain size and presence of volcanic glass also hasten weathering. In tropical settings, it rapidly weathers to clay minerals, aluminium hydroxides, and titanium-enriched iron oxides. Because most basalt is relatively poor in potassium, the basalt weathers directly to potassium-poor [[Chemistry:Montmorillonite|montmorillonite]], then to [[Chemistry:Kaolinite|kaolinite]]. Where leaching is continuous and intense, as in rain forests, the final weathering product is [[Earth:Bauxite|bauxite]], the principal ore of aluminium. Where rainfall is intense but seasonal, as in monsoon climates, the final weathering product is iron- and titanium-rich [[Earth:Laterite|laterite]]. {{sfn|Blatt|Middleton|Murray|p=254}} Conversion of kaolinite to bauxite occurs only with intense leaching, as ordinary river water is in equilibrium with kaolinite.{{sfn|Blatt|Middleton|Murray|p=262}}

[[Earth:Soil formation|Soil formation]] requires between 100 and 1,000 years, a very brief interval in geologic time. As a result, some formations show numerous [[Earth:Paleosol|paleosol]] (fossil soil) beds. For example, the [[Earth:Willwood Formation|Willwood Formation]] of Wyoming contains over 1,000 paleosol layers in a {{convert|770|m||sp=us}} section representing 3.5 million years of geologic time. Paleosols have been identified in formations as old as [[Earth:Archean|Archean]] (over 2.5 billion years in age). However, paleosols are difficult to recognize in the geologic record.{{sfn|Blatt|Middleton|Murray|p=233}} Indications that a sedimentary bed is a paleosol include a gradational lower boundary and sharp upper boundary, the presence of much clay, poor sorting with few sedimentary structures, rip-up clasts in overlying beds, and desiccation cracks containing material from higher beds.{{sfn|Blatt|Tracy|1996|p=236}}

The degree of weathering of a soil can be expressed as the ''chemical index of alteration'', defined as {{chem2|100 Al2O3/(Al2O3 + CaO + Na2O + K2O)}}. This varies from 47 for unweathered upper crust rock to 100 for fully weathered material.{{sfn|Leeder|2011|11}}

==Weathering of non-geological materials==
Wood can be physically and chemically weathered by hydrolysis and other processes relevant to minerals, but in addition, wood is highly susceptible to weathering induced by [[Physics:Ultraviolet|ultraviolet]] radiation from sunlight. This induces photochemical reactions that degrade the wood surface.<ref>{{cite book |last1=Williams |first1=R.S. |editor1-last=Rowell |editor1-first=Roger M. |title=Handbook of wood chemistry and wood composites |date=2005 |publisher=Taylor & Francis |location=Boca Raton |isbn=9780203492437 |pages=139–185 |chapter=7}}</ref> Photochemical reactions are also significant in the weathering of paint<ref>{{cite journal |last1=Nichols |first1=M.E. |last2=Gerlock |first2=J.L. |last3=Smith |first3=C.A. |last4=Darr |first4=C.A. |title=The effects of weathering on the mechanical performance of automotive paint systems |journal=Progress in Organic Coatings |date=August 1999 |volume=35 |issue=1–4 |pages=153–159 |doi=10.1016/S0300-9440(98)00060-5}}</ref> and plastics.<ref>{{cite book |title=Weathering of plastics : testing to mirror real life performance |date=1999 |publisher=Society of Plastics Engineers |location=[Brookfield, Conn.] |isbn=9781884207754}}</ref>

==Gallery==
<gallery widths="180" heights="180">
File:Salt weathering in gozo.jpg|Salt weathering of building stone on the island of Gozo, Malta
File:Qobustan-salt.jpg|Salt weathering of [[Earth:Sandstone|sandstone]] near Qobustan, [[Place:Azerbaijan|Azerbaijan]]
File:Weathered sandstone, Sedona.jpg|[[Earth:Permian|Permian]] sandstone wall near Sedona, Arizona, United States, weathered into a small [[Earth:Alcove (landform)|alcove]]
File:Weathered sandstone DSC01497.jpg|Weathering on a sandstone pillar in Bayreuth
File:Pollution - Damaged by acid rain.jpg|Weathering effect of [[Earth:Acid rain|acid rain]] on statues
File:Skulptur aus Sandstein, Dresden 2012-09-06-0555.jpg|Weathering effect on a sandstone statue in Dresden, Germany
File:Physical weathering.jpg| Physical weathering of the pavements of Azad University Science and Research Branch, which is located in the heights of [[Place:Tehran|Tehran]], the capital of [[Religion:Iran|Iran]] 
</gallery>

==See also==
{{Div col}}
* {{annotated link|Earth:Aeolian processes}}
* {{annotated link|Earth:Biorhexistasy}}
* {{annotated link|Earth:Case hardening of rocks}}
* {{annotated link|Medicine:Decomposition}}
* {{annotated link|Earth:Eluvium}}
* {{annotated link|Earth:Exfoliating granite}}
* {{annotated link|Chemistry:Factors of polymer weathering}}
* {{annotated link|Earth:Metasomatism}}
* {{annotated link|Earth:Meteorite weathering}}
* {{annotated link|Earth:Pedogenesis}}
* {{annotated link|Earth:Reverse weathering}}
* {{annotated link|Soil production function}}
* {{annotated link|Astronomy:Space weathering}}
* {{annotated link|Earth:Spheroidal weathering}}
* {{annotated link|Chemistry:Weather testing of polymers}}
{{Div col end}}

==References==
{{reflist|30em}}

==Other links==

{{Weathering}}
{{Geologic Principles}}
{{Earth}}

[[Category:Geological processes]]
[[Category:Climatology]]
[[Category:Pedology]]
[[Category:Geomorphology]]
[[Category:Earth sciences]]

{{Sourceattribution|Weathering}}