Metamorphic rocks are rocks that have changed from their original form due to heat, pressure, and fluids. There are two main types of metamorphism - contact metamorphism which occurs in small local areas, and regional metamorphism which affects large areas. Metamorphic rocks form through heat from the mantle, pressure from overlying rock layers, and fluids which can change a rock's chemistry and minerals. Metamorphic rocks are classified as either foliated (layered) such as gneiss and slate, or nonfoliated (nonlayered) such as marble and quartzite.