Metamorphic rocks are, as noted above, those rocks that have recrystallized in response to changes in pressure, temperature, and the composition of fluids percolating through them. The word metamorphism is taken from the Greek for "change of form"; metamorphic rocks are derived from igneous or sedimentary rocks that have altered their form ( recrystallized) as a result ofchanges in their physical environment. Metamorphism comprises changes both in mineralogy and in the fabric of the original rock. In general, these alterations are brought about either by the intrusion of hot magma into cooler surrounding rocks (contact metamorphism) or by large-scale tectonic movements of the Earth's lithospheric plates that alter the pressure-temperature conditions of the rocks(regional metamorphism; see also PLATE TECTONICS ). Minerals within the original rock, or protolith, respond to the changing conditions by reacting with one another to produce a new mineral assemblage that is thermodynamically stable under the new pressure-temperature conditions. These reactions occur in the solid state but may be facilitated by the presence of a fluid phase lining the grainboundaries of the minerals. In contrast to the formation of igneous rocks, metamorphic rocks do not crystallize from a silicate melt, although high-temperature metamorphism can lead to partial melting of the host rock.
Because metamorphism represents a response to changing physical conditions, those regions of the Earth's surface where dynamic processes are most active will also be regions wheremetamorphic processes are most intense and easily observed. The vast region of the Pacific margin, for example, with its seismic and volcanic activity, is also an area in which materials are being buried and metamorphosed intensely. In general, the margins of continents and regions of mountain building are the regions where metamorphic processes proceed with intensity. But in relatively quiet places,where sediments accumulate at slow rates, less spectacular changes also occur in response to changes in pressure and temperature conditions. Metamorphic rocks are therefore distributed throughout the geologic column.
Because most of the Earth's mantle is solid, metamorphic processes may also occur there. Mantle rocks are seldom observed at the surface because they are too dense to rise, butoccasionally a glimpse is presented by their inclusion in volcanic materials. Such rocks may represent samples from a depth of a few hundred kilometres, where pressures of about 100 kilobars (3,000,000 inches of mercury) may be operative. Experiments at high pressure have shown that few of the common minerals that occur at the surface will survive at depth within the mantle without changing to newhigh-density phases in which atoms are packed more closely together. Thus, the common form of SiO2, quartz, with a density of 2.65 grams per cubic centimetre, transforms to a new phase, stishovite, with a density of 4.29 grams per cubic centimetre. Such changes are of critical significance in the geophysical interpretation of the Earth's interior.
In general, temperatures increase with depthwithin the Earth along curves referred to as geotherms. The specific shape of the geotherm beneath any location on Earth is a function of its corresponding local tectonic regime. Metamorphism can occur either when a rock moves from one position to another along a single geotherm or when the geotherm itself changes form. The former can take place when a rock is buried or uplifted at a rate thatpermits it to maintain thermal equilibrium with its surroundings; this type of metamorphism occurs beneath slowly subsiding sedimentary basins and also in the descending oceanic plate in some subduction zones. The latter process occurs either when hot magma intrudes and alters the thermal state of a stationary rock, or when the rock is rapidly transported by tectonic processes (e.g., thrust faulting...