Geothermal Energy The earth formed approximately 4.5 billion years ago. Our planet's geothermal heat originates from the energetic remnants of that coalescence. Over time, the interior of the earth "became differentiated into several distinct compositional zones: a large, iron rich core; a thick surrounding mantle; and at the surface, a thin, low density crust." (Montgomery 10). This fact contributed to a number of important attributes of the earth, including the formation of the atmosphere -- which came into existence thanks to gases escaping from the hot interior.

Despite the fact that the earth has been gradually cooling for the past 4 billion years, "The earth still retains enough internal heat to drive large-scale mountain-building processes, to produce volcanic eruptions, to make continents mobile, and indirectly to trigger earthquakes." (Montgomery 10).

A cross-section of the earth would reveal that it is surrounded by a thin outer crust that is about 10 kilometers deep. Below that is a thick mantle that extends another 2900 kilometers. Even further down is what is known as the outer core; this is approximately 2100 kilometers thick and houses convective liquid iron. Finally, at the center is the inner core. The inner core is believed to be composed of solid iron (Montgomery 10). Additionally, it is estimated that the core lies about 4000 miles beneath the surface of the earth, and can reach temperatures of over 9000 degrees Fahrenheit (Geothermal Education Office).

This rate of change is defined as the geothermal gradient and, averaged over the entire surface, is about 30 degrees Celsius for every kilometer of depth (Montgomery 134). "Local geothermal gradients can be increased by plutonism, emplacement of hot magma into the crust." (Montgomery 134). Generally, this is associated with plate boundaries.
The earth's mantle is primarily comprised of solid and liquid rock. Liquid rock, or magma, is what we see when a volcano erupts. However, magma has other effects that can be felt on the surface of the earth. "Magma rising to the crust from the mantle brings usually hot material nearer the surface. Heat from the cooling magma heats any ground-waters circulating nearby. This is the basis for geothermal power." (Montgomery 466).

The rock of the earth's mantle is motivated to move to the surface largely because of the substantial pressure it is under. Mantle rock, which is made up of mostly iron and magnesium, "is squeezed this way and that, like toothpaste in an uncapped tube. Since the molecules in the rock are under so much pressure, they are forced to move closer together they give off energy in the form of heat." (Gallant 42-43). Some of this magma actually reaches the surface, and either erupts above ground or under water. Much more of the magma, however, contributes to platonic ridges…