Geothermal Geology
. The distance from Earth’s surface to its center is about 6,500 km (about 4,000 mi). From Earth’s surface down through the crust, the normal temperature gradient (the increase of temperature with increase of depth) is 10° to 30° C per km (29° to 87°F per mi). Underlying the crust is the mantle, which is made of partially molten rock. Temperatures in the mantle may reach 3700° C (6700° F). The convective (circulating) motion of this mantle rock drives plate tectonics —the "drift" of Earth's crustal plates that occurs at a rate of 1 to 5 cm (0.4 to 2 in) per year. Where plates spread apart, molten rock (magma) rises up into the rift (opening), solidifying to form new crust. Where plates collide, one plate is generally forced (subducted) beneath the other. As the subducted plate slides slowly downward into the mantle’s ever-increasing heat, it melts, forming new magma. Plumes of this magma can rise and intrude into the crust, bringing vast quantities of heat relatively clos...