What are two sources of the heat and pressure that cause metamorphism?

There are two sources of heat, one comes from the geothermal gradient - the increase in temperature that occurs with increase in depth in the earth. The average Geothermal Gradient is approximately 1 degree C for every 30 meters (~100 feet) of depth, although under local conditions it may vary from this ideal.

The second source of heat is from intrusive bodies of magma, such as Batholiths. As these bodies cool they release heat to the surrounding country rock, leading to metamorphism.