Factors that Control Metamorphism Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature. When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism. Temperature Temperature increases with depth in the Earth along the Geothermal Gradient. Thus higher temperature can occur by burial of rock.
Temperature can also increase due to igneous intrusion.
Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth. Pressure is defined as a force acting equally from all directions. It is a type of stress , called hydrostatic stress , or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock.