Metamorphic rocks form when igenous or sedimentary rocks are subjected to high temperatures or crushed by huge pressures underground. This changes the properties and the appearance of the rocks. For example the sedimentary rock limestone becomes marble.

There are two types of metamorphism. In contact metamorphism molten magma from the Earth's mantle pushes its way up into the rocks of the crust. The rocks that surround the intrufding magma are heated by hot magma. In regional metamorphism, rocks are heated as they are buried and squeezed as they are pushed together in the middle of mountain ranges.