Metamorphic Rocks

Rocks formed by the transformation of an existing igneous, sedimentary or metamorphic rock under heat, pressure or chemically active fluid, without full melting, following the regional, contact and dynamic divisions of standard metamorphic petrology. Includes regionally metamorphosed rocks formed during mountain building at a range of grades (greenschist and blueschist at low grade, schist and amphibolite at medium to high grade, gneiss, migmatite and eclogite at the highest grades, plus slate and soapstone as low-grade products of a shale or ultramafic parent), contact metamorphic rocks formed by nearby igneous heat (hornfels, skarn), a dynamically metamorphosed rock formed by ductile shearing along fault zones (mylonite), a rock formed by the hydration of ultramafic material at depth (serpentinite), and rocks named for a specific sedimentary or igneous parent transformed by heat and pressure (marble from limestone, quartzite from sandstone). Belongs here: any material whose defining origin is the alteration of a pre-existing rock texture or mineralogy under heat or pressure without melting. Does not belong here: the original unaltered igneous or sedimentary rock it formed from, or a rock formed directly from magma with no earlier solid parent (Igneous Rocks).