Magma is formed when rocks melt deep underground. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet. Temperature increases can be caused by layers of sediments being buried deeper and deeper under the surface of the Earth. As we descend into the earth the temperature increases about 25 degrees Celsius for every kilometer that we descend. The deeper the layers are buried the hotter the temperatures become.