Editing Weathering (section)

===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}}