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
2012 North American heat wave the west, causing the core of the hot weather to the build in the Mountain States and the southwest U.S. shifting eastwards again by mid-July. By early August, the core of the heat remained over the Southern Plains. The heat wave formed when high pressure aloft over the Baja of Mexico strengthened and moved over the southern plains around June 20–23 and then spread east and northward, remaining fixed over the center of North America through July 2012. The direct cause of the heat is that, under high pressure, the air subsides (sinks) toward the surface. This sinking air acts