METAMORPHIC ROCK

any of a class of rocks that result from the alteration of preexisting rocks in response to changing environmental conditions, such as variations in temperature, pressure, and mechanical stress, and the addition or subtraction of chemical components. The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks. The relative influence of the above-mentioned physical conditions defines the type of metamorphism involved. There are four basic kinds of metamorphic processes: (1) cataclastic, stress with low pressure; (2) contact, high temperature with low pressure; (3) regional, low to high temperature generally correlated with pressure; (4) metasomatic, wide range of temperatures and pressures in the presence of solutions. The structure and mineralogy of any given metamorphic rock are reflective of the particular type of metamorphism that produced the rock. Its texture is determined by changes in the interrelationship between grains resulting from growth in the solid state, as opposed to liquid crystallization. The shape of the new crystals may be irregular or regular with crystal faces depending on the mineral type and the presence or absence of stress. Granoblastic texture describes a rock with grains having no preferred shape or relative orientation. Preferred orientation is exhibited by some rocks because of the growth of minerals with platy or elongate habit. This particular texture is manifested by micas in a rock called schist; the parallel alignment of most mica imparts a layered or foliated texture. Relic textures refer to those inherited from the preexisting rocks. Common relic textures include a preexisting mineral shielded from reaction by a new stable mineral overgrowth, or mineral layering that represents sedimentary layering. Folds or other deformation structures can be used to ascertain the direction of stress. Metamorphic rocks formed by contact metamorphism are limited in extent and closely associated with an igneous intrusion. Pressure gradients are absent and fluid effects are dependent on the type of intrusion. A typical resultant rock has a hornfels texture with fine-grained, equidimensional grains. In common sedimentary rocks a sequence of minerals forms in response primarily to temperature changes. Such minerals include andalusite, cordierite, anorthite, diopside, grossularite, and wollastonite. Because of the large areas affected, the most common metamorphic rocks are those produced by regional metamorphism. Typical rocks of this kind include schists and gneisses, coarse-grained rocks rich in feldspar with equidimensional grains. The coarse-grain size and regional extent both suggest long-term metamorphism characteristic of geosynclinal subsidence. Sedimentary sequences can frequently be traced for considerable distances and the change of mineralogy in response to metamorphism recognized for any restricted compositional range. For each composition, as represented by a particular type of sedimentary or igneous rock, the changes in mineralogy can be shown on a temperature/pressure graph as areas in which a certain mineral assemblage is stable. An examination of a sequence of metamorphic rocks reveals that there is also a general pattern in the structures or textures of the rocks in addition to mineralogic changes. In the case of common sedimentary shale low-grade metamorphism produces a slate, which is a micaceous, fine-grained rock. In turn, the slate becomes phyllite or a coarser-grained, laminated slate, and then schist. Further metamorphism results in a loss of volatile materials and metasomatic changes that produce a gneiss and subsequently a granulite (laminated rock without micas but with pyroxene-feldspar-garnet mineralogy), and finally migmatite, in which evidence of partial melting is present. Other rock compositions show parallel sequences: quartz sandstones become quartzites; limestones become marbles with a sequence of silicate minerals changing with pressure and temperature; and basic igneous rocks become greenschist, amphibolites, and ultimately eclogites. The appearance of characteristic minerals provides a mappable feature indicative of metamorphic intensity: the first occurrence of biotite, garnet, staurolite, kyanite, or sillimanite is represented by an isograd, which is an indicator of constant metamorphic conditions. any of a class of rocks that result from the alteration of preexisting rocks in response to changing environmental conditions, such as variations in temperature, pressure, and mechanical stress, and the addition or subtraction of chemical components. The preexisting rocks may be igneous, sedimentary, or other metamorphic rocks. 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 of changes 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 grain boundaries 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 where metamorphic 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, but occasionally 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 new high-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 depth within 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 that permits 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 or large-scale folding) into a new depth-temperature regime in, for example, areas of collision between two continents. Regardless of which process occurs, the result is that a collection of minerals that are thermodynamically stable at the initial conditions are placed under a new set of conditions at which they may or may not be stable. If they are no longer in equilibrium with one another under the new conditions, the minerals will react in such a way as to approach a new equilibrium state. This may involve a complete change in mineral assemblage or simply a shift in the compositions of the preexisting mineral phases. The resultant mineral assemblage will reflect the chemical composition of the original rock and the new pressure-temperature conditions to which the rock was subjected. Because protolith compositions and the pressure-temperature conditions under which they may be placed vary widely, the diversity of metamorphic rock types is large. Many of these varieties are repeatedly associated with one another in space and time, however, reflecting a uniformity of geologic processes over hundreds of millions of years. For example, the metamorphic rock associations that developed in the Appalachian Mountains of eastern North America in response to the collision between the North American and African lithospheric plates during the Paleozoic are very similar to those developed in the Alps of south-central Europe during the Mesozoic-Cenozoic collision between the European and African plates. Likewise, the metamorphic rocks exposed in the Alps are grossly similar to metamorphic rocks of the same age in the Himalayas of Asia, which formed during the continental collision between the Indian and Eurasian plates. Metamorphic rocks produced during collisions between oceanic and continental plates from different localities around the world also show striking similarities to each other (see below Regional metamorphism) yet are markedly different from metamorphic rocks produced during continent-continent collisions. Thus, it is often possible to reconstruct tectonic events of the past on the basis of metamorphic rock associations currently exposed at the Earth's surface. Additional reading Standard references and basic textbooks include, in order of increasing complexity, Con Gillen, Metamorphic Geology: An Introduction to Tectonic and Metamorphic Processes (1982); Roger Mason, Petrology of the Metamorphic Rocks, 2nd ed. (1990); B.W.D. Yardley, An Introduction to Metamorphic Geology (1989); Akiho Miyashiro, Metamorphism and Metamorphic Belts (1973); Francis J. Turner, Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects, 2nd ed. (1981); Helmut G.F. Winkler, Petrogenesis of Metamorphic Rocks, 5th ed. (1979; originally published in German, 1965); and Anthony R. Philpotts, Principles of Igneous and Metamorphic Petrology (1990). Metamorphic fabric development is discussed in Alan H. Spry, Metamorphic Textures (1969); and R.H. Vernon, Metamorphic Processes: Reactions and Microstructure Development (1975, reissued 1983). Major techniques for studying metamorphic rocks and the metamorphic histories of many of the world's mountain belts are reviewed in J.S. Daly, R.A. Cliff, and B.W.D. Yardley (eds.), Evolution of Metamorphic Belts (1989). Jane Selverstone