What is geothermal energy?

The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is heat from within the earth. This heat can be recovered as steam or as hot water, and it can be used to heat buildings or to generate electricity.

Geothermal energy is a renewable energy source because the heat is continuously produced inside the earth.

Geothermal energy is generated deep inside the earth

Image of the earth's interior, from the outside to the inside, with the crust, the mantle of magma and rock, the outer core of magma, and the innermost core of iron.

Source: Adapted from a National Energy Education Development Project graphic (public domain)

Geothermal energy is generated in the earth's core. Temperatures hotter than the sun's surface are continuously produced inside the earth caused by the slow decay of radioactive particles, a process that happens in all rocks. The earth has a number of different layers:

  • The core has a solid iron core and an outer core made of hot melted rock called magma.
  • The mantle surrounds the core and is about 1,800 miles thick. The mantle is made up of magma and rock.
  • The crust is the outermost layer of the earth. The crust forms the continents and ocean floors. The crust can be 3 to 5 miles thick under the oceans and 15 to 35 miles thick on the continents.

The earth's crust is broken into pieces called plates. Magma comes close to the earth's surface near the edges of these plates. This is where volcanoes occur. The lava that erupts from volcanoes is partly magma. The rocks and water absorb the heat from this magma deep underground. The rocks and water found deeper underground have higher temperatures.

People around the world use geothermal energy to heat their homes and to produce electricity by drilling deep wells and pumping the hot underground water or steam to the surface. People can also make use of the stable temperatures near the surface of the earth to heat and cool buildings.