Metamorphic rock the foliation is called "gneissosity"), granoblastic (includes granulite, some marbles and quartzite), and hornfelsic (includes hornfels and skarn). 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
Sigma heat extracted by cooling the water vapor below its condensation point does not count towards sigma heat. The reference temperature is usually , although is sometimes used as well. Assuming a reference temperature of 0°F, the following formula may be used under standard temperature ranges and pressure: formula_2 The equivalent metric formula: formula_6 Sigma heat is not the same as the enthalpy of the humid air above the reference temperature. (Enthalpy is sometimes called "total heat" or "true total heat") Unlike sigma heat, enthalpy does include the energy which would be extracted in cooling the condensed water vapor all the way