Temperatures beneath the Earth's surface increase with depth at a rate of about 25 degrees Celsius per kilometer. Scientists estimate that the temperature at the base of the crust is about 800 to 1200 degrees Celsius. This heat is generated from the decay of radioactive materials in the mantle and the core . Magma can sometimes migrate up through the crust forming an igneous intrusion . This is especially true along continental boundaries, like the western side of North America, where subduction is taking place.
Metamorphism takes place in the rock surrounding the magma body because of heat dissipation.
Because of the nature of the dissipation process, the level of metamorphic alteration in the influenced rock decreases with distance from the igneous intrusion. Pressure and Metamorphism Rocks that buried are subjected to pressure because of the weight of overlying materials. Pressure can also be exerted on rocks due the forces involved in a variety of tectonic processes. The most obvious effect of pressure on rocks is the reorientation of mineral crystals. Under extreme levels of pressure rocks become plastic creating flow structures in their crystalline structure.