Metamorphism is the change of
minerals or
geologic texture (distinct arrangement of minerals) in pre-existing
rocks (
protoliths), without the protolith melting into liquid
magma (a
solid-state change). The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The
chemical components and
crystal structures of the minerals making up the rock may change even though the rock remains a
solid. Changes at or just beneath Earth's surface due to
weathering and/or
diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis 200°C, and
melting 850°C.