When rocks are subjected to high enough pressures and temperatures, their characteristics begin to change and they become metamorphic rocks.
Sample of gneiss. Note the banded layering of minerals (foliation), which is a result of pressure. Model by Nate Siddle and the University of Queensland, School of Earth and Environmental Sciences (Sketchfab).
Regional metamorphism describes a metamorphic rock that has been altered due to deep burial and great pressure, for example at subduction zones. This type of metamorphic rock tends to occur in long belts. Different types of metamorphic rock are created depending on the gradients of heat and pressure applied.
High temperatures lead to recrystallization, causing individual grains to reform as interlocking crystals. This makes the resulting metamorphic rock much harder than the original parent rock.
Contact metamorphism describes a metamorphic rock that has been altered by direct contact with magma. Changes that occur due to contact metamorphism are greatest at the point of contact. The farther away the rock is from the point of contact, the less pronounced the change.
Contact metamorphism occurs when a hot igneous intrusion heats and changes pre-existing bedrock (for example, sedimentary rock).