Rocks adjust to new environments by changing their minerals and textures. Metamorphism is the mineralogical and structural alteration of rock in the solid state under elevated temperature, pressure, or changing chemical conditions. Melting is not involved; that would produce igneous rock.

Contact metamorphism occurs adjacent to intrusions where heat is the dominant factor. Regional metamorphism accompanies mountain building and affects large volumes of crust. Burial metamorphism and hydrothermal metamorphism are additional settings. Index minerals help define metamorphic zones of increasing grade.

Metamorphic reactions release or consume fluids and can significantly alter rock strength and density. The process is a major agent of crustal differentiation and ore formation. By studying metamorphic rocks, geologists recover the pressure-temperature paths that rocks followed during orogeny and subsequent exhumation.

• Solid-state alteration of rock by heat and pressure
• Produces new minerals and textures
• Occurs in contact, regional, and other settings
• Records crustal burial and heating histories