Exfoliation is the term used to describe the peeling away of sheets of rock millimeters to meters in thickness from a rock's surface due a range of physical and chemical processes during exhumation and weathering . Exfoliation can occur due to several processes.
Unloading or release of stress in a rock that produces expansion joints can cause exfoliation. A reduction in stress occurs when rocks previously buried deeply are exposed due to erosion of overlying rocks, or when ice sheets that bury rocks melt. During a combination of physical and chemical weathering, exfoliation may occur parallel to a rock's outer surface due to a combination of chemical breakdown of minerals , especially in the presence of water . Such 'onion-skin' style weathering occurs especially in igneous rocks (e.g., granite ) as micas, amphiboles and pyroxenes, common minerals in many igneous rocks, break down to clay . Clays swell in the presence of water, so alternating wetting and drying of a rock may lead to consecutive expansion and shrinking that can result in disintegration and exfoliation.
Stresses induced in a rock due to the expansion of water trapped between grains or in fractures in a rock during freezing may result in fracturing. Shattering of rock into small fragments by the expansion of water during the formation of ice is common in arctic environments (causing a problem for field geologists looking for rock relationships and structures). Likewise, changes in temperature of a rock may cause exfoliation. Stresses due to variability in the rates and amounts of expansion of different minerals in a rock, or due to alternating expansion and shrinkage from day to night in desert areas, may result in exfoliation. Rapid temperature changes may also occur due to lightning strikes followed by cooling in the ensuing rain. Although generally a naturally occurring process, exfoliation was also induced by man to obtain rock sheets several centimeters in thickness to thin, sharp shards of some fine-grained rocks for use as scrapers and knives by heating the rock with fire, then pouring water on the rock's surface.
See also Weathering and weathering series