Metamorphic rock Metamorphic rock Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure ( or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath
Pressure melting point Pressure melting point The pressure melting point is the temperature at which ice melts at a given pressure. The pressure melting point is nearly a constant 0 °C at pressures above the triple point at 611.7 Pa, where water can exist in only the solid or liquid phases, through atmospheric pressure (100 kPa) until about 10 MPa. With increasing pressure above 10 MPa, the pressure melting point decreases to a minimum of −21.9 °C at 209.9 MPa. Thereafter, the pressure melting point rises rapidly with pressure, passing back through 0 °C at 632.4 MPa. Glaciers are subject to geothermal heat