distinguish between frost action and pressure release
Answers
Frost action involves the weathering processes caused by repeated cycles of freezing and thawing (the “multigelation” of some European writers). Frost action is thus differentiated from glacial action, which involves the processes related to moving ice. Frost action is limited to climates in which the temperature both drops below, and rises above, 32°F (0°C) and can be described as occurring near the boundary of the cryosphere . The cryosphere is thefrom the Greek kryos, meaning cold; see articles on Cryopedology, etc.). The cryosphere is largely at various elevations in the atmosphere, but it is partly in contact with the lithosphere, especially at high altitudes and latitudes. It is only in the latter situation, and at points through which the boundary of the cryosphere fluctuates, that frost action can take place.
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.
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, a process known as exfoliation. Exfoliation due to pressure release is also known as "sheeting".
Retreat of an overlying glacier can also lead to exfoliation due to pressure release.
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