metamorphic rocks

rocks that formed earlier as sedimentary or igneous rocks but have undergone alteration (metamorphism) in the earth’s interior under the influence of abyssal fluids, temperature, and pressure or alteration near the surface under the influence of the heat of intrusive masses.

Metamorphic rocks formed deep within the earth (products of regional metamorphism) are characterized by schistosity that developed as a result of directional pressure; they are called crystalline schists. As the degree of metamorphism increases, clays are converted to phyllites, mica schists, and gneisses (schisty rocks with a large amount of granite material). Marls and basic igneous rocks are converted to chlorite schists and actinolite-chlorite schists (greenschists) and amphibolites. Eclogites—garnet-jadeite rocks—form at very great depths. When sandstones and limestones undergo metamorphism, quartzites and marble form.

Metamorphic rocks resulting from contact with intrusives (contact metamorphism) have a characteristic corneous texture. Argillaceous and other aluminosilicate rocks that undergo metamorphism become such types of hornfelses as pyroxene, biotite, and amphibolite hornfelses; limestones become marbles, and bauxites are converted to corundum rocks (emeries).

Metasomatic rocks are frequently associated with meta-morphic rocks.