metamorphosed

Metamorphosed is a term used in geology to describe rocks or minerals that have undergone metamorphism, the solid-state transformation of pre-existing rocks under elevated temperatures and pressures, often with chemically active fluids. Metamorphism alters mineralogy, texture, and sometimes chemical composition, without melting the rock. It occurs under conditions distinct from those of the rock’s original formation and is driven by tectonic and thermal processes.

Metamorphism occurs mainly through regional metamorphism during mountain-building, contact metamorphism adjacent to intruding magma, and dynamic metamorphism along shear zones. The result is new minerals stable at higher temperature and pressure, and textures such as foliation or lineation produced by the alignment and recrystallization of minerals. Metamorphosed rocks include slate, phyllite, schist, and gneiss, as well as marble and quartzite, with names often reflecting their protoliths that have been metamorphosed.

The term is commonly appended to the protolith to indicate its metamorphic origin, for example metamorphosed limestone or metamorphosed shale. In describing metamorphic rocks, geologists use grade and facies to indicate the pressure–temperature conditions, with index minerals such as chlorite, biotite, garnet, kyanite, and sillimanite helping to constrain those conditions. Metamorphism may involve partial melting in higher-grade cases, producing migmatites with mixed igneous textures.