Metamorphism refers to the physical and chemical changes that rocks undergo when exposed to conditions of high temperature, high pressure, or some combination thereof. Rocks that have undergone metamorphism exhibit chemical and structural changes that result from the partial or complete recrystallization of minerals within them. These transformations occur while the rock is in the solid state, i.e., no melting occurs during metamorphism. The conditions of high temperature and pressure under which metamorphism occurs are typically the result of processes such as mountain building, plate convergence, volcanism, and sedimentation.

Any type of rock may be metamorphosed and several agents can be involved in altering a parent rock into its metamorphic product. The composition of the parent rock limits the mineral composition of the product, although subsurface gases and fluids may contribute new elements. Thermal energy at depth, either from the geothermal gradient or from plutonic activity, may...