Editing Weathering (section)

==Physical==
'''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===
[[File:Abiskorock.JPG|thumb|A rock in [[Abisko]], Sweden, fractured along existing [[joint (geology)|joints]] possibly by frost weathering or thermal stress]]
{{Main|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 is the widening of cracks or joints in rocks resulting from the expansion of porewater when it freezes. A growing body of theoretical and experimental work suggests that ice segregation, whereby 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}}<ref name=dkp>{{cite book|title=The Ultimate Family Visual Dictionary|chapter=Geology, Geography, and Meteorology|publisher=[[DK (publisher)|D.K. Pub.]]|language=en|year=2012|page=282|isbn=978-0-1434-1954-9|location=New Delhi}}</ref>

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. 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|bibcode=2008PPPr...19..195M |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 [[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 [[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, [[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 rocks. Block disintegration, when rock joints weaken from temperature fluctuations and the rock splits into rectangular blocks, can be attributed to thermal fatigue.<ref name="hall-1999"/><ref name=dkp/>

{{Anchor|Insolation weathering}}
Thermal stress weathering is an important mechanism in [[deserts]], where there is a large [[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.76243125S |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. [[Geomorphology|Geomorphologists]] have begun to reemphasize the importance of thermal stress weathering, particularly in cold climates.<ref name="hall-1999"/>

===Pressure release===
{{See also|Erosion and tectonics}}
[[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 [[Exhumation (geology)|exhumed]]. Intrusive igneous rocks, such as [[granite]], are formed deep beneath the Earth's surface. They are under tremendous [[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 (geology)|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 toward 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|[[Tafoni]] at [[Salt Point State Park]], Sonoma County, California]]
{{main|Haloclasty}}
{{distinguish-redirect|Salt wedging|Salt wedge (hydrology)}}

''Salt crystallization'' (also known as '''salt weathering''', '''salt wedging''' or [[haloclasty]]) causes disintegration of rocks when [[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 [[pyrite]] in sedimentary rock is chemically weathered to [[iron(II) sulfate]] and [[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 [[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 [[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>

===Biomechanical relationship===
Living organisms may contribute to mechanical weathering, as well as chemical weathering (see [[#Biological weathering|§ Biological weathering]] below). [[Lichen]]s and [[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}}