Metamorphic Rock

Definition

A metamorphic rock forms when igneous, sedimentary, or older metamorphic rocks are subjected to heat, pressure, and strain that result in changes of their texture and mineral assemblage. The new texture, commonly characterized by aligned mineral grains and the new mineral assemblage reveal the metamorphic origin.

Overview

Characteristic metamorphic minerals are mica, garnet, aluminosilicates, chlorite, and serpentine. To give the proper name of a metamorphic rock, the petrologist refers to the nature of protolith, i.e., the rock prior to metamorphism. Most metamorphism is isochemical, except for the gain or loss of water and other volatile components, that is, the metamorphic rock usually retains the chemical composition of its precursor. When shale, a sediment rich in Aluminum, is metamorphosed, clay minerals are converted to aluminous minerals such as micas to produce a schist or metapelite. As metamorphic conditions increase, a part of the micas converts to feldspar and...