Typically fluids are involved in the reaction. Foliation Sets of flat or wavy parallel planes Occur at an angle to bedding layers but can coincide. Represent the preferred orientation of the minerals in the rock under a deforming pressure. The main cause of foliation is the presence of platy minerals which are easily elongated. Mid Ocean Ridges, Arc Orogens, Spreading Ridges.
Temperature increases quickly with depth, due to rising hot magma beneath.
At great depth the rocks start to metamorphose and eventually melt, forming magma. The temperature gradient is then deflected back upwards due to the rising hot magma. Continental areas away from tectonically active zones have average geothermal gradients. The corresponding indicator minerals are chlorite, biotite and garnet. The transition from chlorite grade to biotite grade is the first appearance of biotite.