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 cooking stage, the heat source is reduced to the lowest possible heat that still maintains pressure, as extra heat wastes energy and increases liquid loss. Before the pressure cooker lid is sealed airtight, the internal air has to be mostly replaced by steam. Steam has a much higher specific heat than air, and the presence of steam rather than air inside the pressure cooker is how it is able to transfer sufficient heat into the parts of the food that are not submerged in liquid, such as a pot roast. If the lid is sealed before enough air has been removed,