Geothermal Energy and Earth’s Interior

The word geothermal has Greek origins = geo (earth) + therme (heat).

The term, “geothermal energy,” refers to using subsurface heat reservoirs within a rock for:

• Generating electricity
• Cooling and heating buildings
• Direct use

Heat at the Earth’s Core is Similar to the Sun

Temperature increases with depth in the earth. Geothermal energy is a renewable resource that harnesses heat flowing from the Earth’s interior, where the temperature is similar to the sun. The driving force behind heat flow to the surface is this temperature difference (differential) between the earth’s surface and its interior.¹U.S. Dept. of Energy. (2019, May). GeoVision: Harnessing the Heat Beneath Our Feet. https://www.energy.gov/eere/geothermal/articles/geovision-full-report-0.

Earth’s Temperature Differential

At the surface, the crust forms the earth’s outer shell of rock and varies in thickness depending on if the crust is oceanic or continental. As we go deeper into the earth, the earth’s interior structure comprises the mantle, outer core, and inner core. Geothermal energy is the thermal energy in the earth’s crust that originates from the earth’s core.

Let’s examine the four major parts, or layers, of the earth in more detail. Earth is comprised of:²U.S. Energy Information Administration (EIA). (2022, December 27). Geothermal explained. EIA. https://www.eia.gov/energyexplained/geothermal/

• A crust of solid rock that forms the continents and ocean floors that is 15 miles to 35 miles thick under the continents and 3 miles to 5 miles thick under the oceans
• A mantle of magma and rock surrounding the outer core that is about 1,800 miles thick
• An outer core of hot molten rock called magma that is about 1,500 miles thick.
• An inner core of solid iron that is about 1,500 miles in diameter

The gravitational pull and friction that formed Earth produce a small amount of the core’s heat. But most of the core’s heat results from radioactive isotopes. Some people think of geothermal energy as a form of renewable nuclear energy without the problems of waste disposal. To fully understand the origin of geothermal energy available to us as a resource, we must examine the role of isotopes in Earth’s core.