Metamorphism Defined

When rocks are subjected to deep burial, tectonic forces such as folding, and high pressures and temperatures, the textures and mineral compositions begin to change. This process, called metamorphism, is the solid‐state transformation (no melting) of a rock mass into a rock of generally the same chemistry but with different textures and minerals.

Usually the metamorphic rock looks quite different from the original rock, called the parent rock or protolith. Metamorphic rocks often show contorted patterns of folding that indicate they were soft enough to bend (plastic deformation). Folding is achieved by the application of great pressure over long periods. The intensity of the metamorphism increases with increasing temperature and/or pressure, and the highest “grade” of metamorphism approaches partial melting of the rock, almost completing the rock cycle.