Editing Weathering (section)

===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>