But metamorphic rocks are rocks that have been changed within the Earth. Because rocks are not easily changed, metamorphic rocks must develop in environments where heat and pressure are intense and extreme. These extreme conditions are only present deep inside the Earth. There are several types of metamorphism, the process by which rocks change into new rocks. The two most general types are contact metamorphism and regional metamorphism.
Contact metamorphism occurs at the boundary between magma and older rock.
The intense heat of the magma alters the rock, often causing its minerals to recrystallize. Thus, the new rock has new or larger mineral crystals than the older rock. Sometimes, the hot magma will even introduce new minerals and modify the entire chemical composition of the original rock. The area of rock affected by contact metamorphism is appropriately known as the baked zone. Compression along one axis causes rock to foliate.