Metamorphic rocks are formed under conditions of heat and pressure. The rock does not melt, but its structure is altered. Metamorphic rocks can be formed from igneous or sedimentary rocks, and typically have a layered structure as that seen in slate or marble.
During metamorphism the size of the particles that a rock is composed of undergoes a transformation. This process is known as recrystallization. For example, fine crystals in limestone and chalk are metamorphosed into larger crystals during metamorphism resulting in marble. Metamorphism can result from contact with magma, or as a result of pressure and heat at great depths beneath the earth’s crust.