ABSTRACT
The temperature gradient in the crust of the Earth is 17-30°C per kilometer of depth. For example, deep mines are hot, and most need cooling for the miners. Plumes of magma ascend by buoyancy and force themselves into the crust, generally along the edges of tectonic plates (Figure 11.1), which results in volcanoes. There are huge regions of subsurface hot rocks with cracks and faults that allow water to seep into the reservoir, which then results in hot springs, geysers, mud pots, and fumaroles. Two famous examples are Yellowstone Park and Iceland, which is an exposed section of the Mid-Atlantic Ridge.
