Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. Earth’s geothermal energy originates from the original formation of the planet (20%) and from radioactive decay of minerals (80%) (Turcotte 2002). The geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface (Figure 12.1). The heat that is used for geothermal energy can be stored deep within the Earth, all the way down to Earth’s core—4,000 miles down.
Figure 12.1 Temperatures in the Earth.