Heat and pressure can transform a rock without melting it. Metamorphic rock is the product of that solid-state alteration. Original sedimentary or igneous textures and minerals are replaced by new assemblages stable under the changed conditions.

Foliated metamorphic rocks such as slate, schist, and gneiss develop planar fabrics under directed stress. Non-foliated rocks such as marble and quartzite recrystallize without preferred orientation. Grade of metamorphism ranges from low, barely altering the protolith, to high, approaching the onset of melting.

Metamorphic rocks record the temperatures, pressures, and fluid compositions of the deep crust. They are common in mountain belts where burial and deformation are intense. Field relations and mineral assemblages allow geologists to reconstruct the burial and heating history of entire regions.

• Rock changed by heat and pressure without melting
• May be foliated or non-foliated
• Records conditions of the deep crust
• Common in mountain belts