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Earth’s Interior Heat

Geothermal energy is thermal energy that is generated and stored within the Earth. The word geothermal has Greek origins = geo (earth) + therme (heat).

Heat at 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.

Earth’s various layers are composed of different materials. These layers transfer heat from the interior of the planet to the surface.

Let’s examine the four major parts, or layers, of the earth in more detail. Earth is comprised of:

  • 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 diameter2U.S. Energy Information Administration (EIA). (2022, December 27). Geothermal explained. EIA. https://www.eia.gov/energyexplained/geothermal/

To fully understand how geothermal energy is available to us as a resource, we must examine the sources of Earth’s heat and how that heat moves from the interior of the Earth to the surface.

Images: “Steamboat Geyser in Yellowstone” by Brocken Inaglory ; “Geothermal equals Earth plus Heat” by EERE, Geothermal Technologies Office; “Earth’s Temperature Differential” by DOE, GeoVision; “Earth’s Interior Structure” by EIA, Geothermal explained