Metamorphic rocks are formed through the tranformation of pre-existing rock into new rock from high pressures and/or temperatures, a process called metamorphism. There are are two types of metamorphism:
Contact Metamorphism: a type of metamorphism where high temperature is the dominant factor. It can occur surrounding a pluton where a body of magma intrudes into relatively cool rock baking the area (~1-100m) around the intrusion. These types of metamorphic rocks are usually non-foliated.
Regional Metamorphism: a type of metamorphism that takes place at great depth (usually >5 km below the surface). Pressure is high and temperatures are variable (~ 300-800°C). Regional metamorphic rocks are almost always foliated.
A metamorphic rock is identified based on it’s texture and mineral composition.
The texture of a metamorphic rocks refers to whether or not a rock exhibits foliation (a planar texture).
The mineral composition of a metamorphic rock is controlled by the composition of the original parent rock.
Non-Foliated metamorphic rocks are named based on their composition. For example, quartzite is made of quartz.
Foliated schists have variable mineral compositions and are named using the two most predominant minerals. This sample of schist is a biotite kyanite schist.