Metamorphism occurs below the surface zones of weathering and cementation.
Conditions of metamorphism differ from the conditions under which the rocks in question were originally formed. Metamorphism produced as a result of the progressive increase in temperature and pressure, i.e. by burial of a rock within the earth, is termed prograde metamorphism and in general terms is characterized by dehydration reactions, which release water. These gradients cause dehydration reactions to be driven to the right, where water vapour is a producrt of the reaction.