Geothermal Energy the Earth Formed Approximately 4.5

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).

Because of our planet's innate thermal energy and the manner by which it formed, temperature increases with increasing depth. 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 -- effectively moving the continents and reshaping the face of the earth.

Aside from volcanic eruptions and plate tectonics, there is another way to witness and to use the thermal energy generated in the interior of our planet: geysers and hot springs. As aforementioned, magma rising towards the crust can heat natural water reservoirs. "Some of this hot geothermal water travels back up through faults and cracks and reaches the earth's surface as hot springs or geysers, but most of it stays deep underground, trapped in cracks and porous rock. This natural collection of hot water is called a geothermal reservoir." (Geothermal Education Office).

Humans have used this natural form of energy since before the dawn of civilization. The Romans used geothermal water for medical purposes, and for heating homes in some cities. American Indians, for perhaps as long as ten thousand years, have also made use of hot springs for medicine as well as for cooking (Geothermal Education Office).

Today, much more advanced and efficient techniques are employed to take advantage of this free form of energy. Geologists work to locate possible geothermal reservoirs, and then drill into them to release their steam. This rapid escape of steam can be used to turn turbine generators and transform what was once thermal energy into forms that can be much more useful and mobile -- like electricity. Presently, humans employ three different types of geothermal power plants: dry steam power plants, flash power plants, and binary power plants (Geothermal Education Office). The general difference between the three being the temperature of the water, and subsequently, the amount of steam that is generated.