Mineralogic and structural changes in solid rocks caused by physical conditions different from those under which the rocks originally formed.
Changes produced by surface conditions such as compaction are usually excluded. The most important agents of metamorphism are temperature (from 300k–2,200kF, or 150k–1200kC), pressure (from 10 to several hundred kilobars, or 150,000 to several million lbs. per sq in.), and stress. Dynamic metamorphism results from mechanical deformation with little long-term temperature change. Contact metamorphism results from increases in temperature with minor differential stress, is highly localized, and may occur relatively quickly. Regional metamorphism results from the general increase, usually correlated, of temperature and pressure over a large area and a long period of time, as in mountain-building processes. See also metamorphic rock .