What is geothermal energy?

The word geothermal comes from the Greek words geo (earth) and therme (heat). Geothermal energy is heat within the earth. People can use this heat as steam or as hot water to heat buildings or to generate electricity.

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

Geothermal energy comes from 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)

The slow decay of radioactive particles in the earth's core, a process that happens in all rocks, produces geothermal energy. The earth's core is hotter than the sun's surface.

The earth has a number of different layers:

  • The inner core is solid iron and is surrounded by an outer core of hot molten 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 tectonic plates. Magma comes close to the earth's surface near the edges of these plates, which is where many volcanoes occur. The lava that erupts from volcanoes is partly magma. Rocks and water absorb heat from magma deep underground. The rocks and water found deeper underground have the highest 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 use the stable temperatures near the surface of the earth to heat and cool buildings.