(adj.) Geology. One of three main classes of rock (igneous, metamorphic and sedimentary). Metamorphic rocks form from the alteration of preexisting rocks by changes in ambient temperature, pressure, volatile content, or all of these. Such changes can occur through the activity of fluids in the Earth and movement of igneous bodies or regional tectonic activity. The texture of metamorphic rocks can vary from almost homogeneous, or nonfoliated, to foliated rocks with a strong planar fabric or foliation produced by alignment of minerals during recrystallization or by reorientation. Common foliated metamorphic rocks include gneiss, schist and slate. Marble, or metamorphosed limestone, can be foliated or non-foliated. Hornfels is a nonfoliated metamorphic rock. Graphite, chlorite, talc, mica, garnet and staurolite are distinctive metamorphic minerals. See: Basement, Chlorite, Feldspar, Hard Rock, Igneous, Massif, Metamorphism, Mica, Nonconformity, Plate Tectonics, Reservoir, Rock, Sedimentary, Soft Rock, Unconformity