To help us understand how this energy from inside the earth can be harnessed for electricity, a brief geology explanation is in order. Inside the earth, at its core, is a layer of hot and molten rock, called magma, which constantly produces heat from decayed uranium and potassium. It is this magma that we see in volcanic eruptions. Above the molten core at the center of the earth is the earth’s mantle: the mantle is that portion of the earth, about 1,800 miles thick, between the earth’s core and its crust.
The earth’s crust can be compared to a thick blanket of insulation, but it is a blanket that heat and magma can pierce.
The earth’s crust and mantle are called the lithosphere. The lithosphere is broken up into massive tectonic plates that cover the entire surface of earth – to include the continents and the oceans. These plates move in very slow motion against each other constantly, and produce earthquakes, volcanic eruptions, mountain building and oceanic trench formation at their plate boundaries, as a result of their motion.
At the plate boundaries is where the highest underground temperatures occur, and these “hot spots” is where we see active volcanoes, seismic activity, and hot water springs. Magma movement travels upward through fluid conduits in the earth’s mantle and up through the crust. In volcanic eruptions the magma lava is blown out of the center of the volcano with tremendous force; in places where hot spots are constant, heat is released in the form of hot water and steam. The temperature of a geyser’s water can be more than 430°F (200°C). These less powerful eruptions we call geysers, and the hot water and steam they emit are a form of untapped geothermal energy. That energy can be captured and used to power turbines to produce electricity. By drilling wells into the underground hot water reservoirs that feed geysers, we can bring up hot water or steam to the surface and use it to make electricity.