Answer:
The parent rock is exposed to varying degrees of pressure and/or heat either from depth of burial, exposure to a plutonic body of intense heat, or from pressures resulting from the collision of tectonic plates. The heat and/or pressure can transform the minerals inside the parent rock to new minerals, cause recrystallization of existing minerals, or reorganize the existing minerals into bands and layers. Exchanges of elements can also occur from hot fluids associated with plutonic intrusions. All of these processes take place without any melting of the parent rock.