Foliated metamorphic rocks are rocks that have gone through extreme temperature and pressure changes, resulting in a different chemical composition. They are distinct for the appearance of a layered pattern throughout the rock.
Unlike non-foliated metamorphic rocks, which have a marbled appearance and form under uniform pressure, foliated metamorphic rocks form from uneven amounts of pressure. The areas of greatest pressure force minerals to line up, forming the layered bands throughout the rock.
Metamorphic rocks form beneath Earth's surface and move up to the surface through tectonic movement. The temperature range for metamorphism is between 200 and 800 degrees Celsius. The amount of heat and pressure that a metamorphic rock undergoes determines its grade. For example, slate, phyllite, schist and gneiss are grades of foliated metamorphic rocks. Slate forms under the lowest temperature and pressure changes, whereas gneiss experiences the highest temperature and pressure.
Slate, used as a material in roofing and pool tables, breaks into flat pieces. The layered bands in slate are less visible due to its fine grain texture. Phyllite contains more crystals than slate. Its layered bands often appear wrinkled or wavy. Schist, the grade between phyllite and gneiss, easily breaks into thin sections. Gneiss, pronounced “nice,” often contains feldspar or quartz. Its grain texture is coarse, making the layered bands of alternating minerals highly visible.