Metamorphic rocks are rocks that have been modified by heat, pressure, and chemical processes. When rocks are buried deep in the Earth at high temperatures and pressures, they form new minerals and textures all without melting. If melting occurs, magma is formed, starting the rock cycle all over again.
Geologists can learn the following about the Earth from the study of metamorphic rocks:
the temperature and pressure conditions (metamorphic environment) in which the rock was formed
the composition of the parent, or original unmetamorphosed, rock
aids in the interpretation of the platetectonic setting in which the metamorphism took place
aids in the reconstruction of the geological history of an area.
The term "metamorphic" means "to change form." Changes in the temperature and pressure conditions cause the minerals in the rock to become unstable so they either reorient themselves into layers (foliation) or recrystallize into larger crystals, all without undergoing melting.
Below are some examples of metamorphic rocks below.