Earth's Underground Furnace: Power From Volcanoes

In Iceland, almost every building is heated by water that was never touched by a furnace, boiler, or power plant burning fuel. Instead, the heat comes straight from rocks deep underground that are naturally as hot as an oven, warmed by the same forces that create volcanoes. About 90% of Icelandic homes are heated this way, using something called geothermal energy.

- Where the heat inside the Earth actually comes from - How geothermal power plants turn underground heat into electricity - Why volcanic regions are the best places to find geothermal energy - The difference between geothermal heating and geothermal electricity

About 4 miles below your feet in some places, and much closer near volcanoes, rock temperatures can exceed 700 degrees Fahrenheit. This heat comes from two sources: leftover heat from when the Earth formed 4.5 billion years ago, and heat still being released today by radioactive elements like uranium and potassium decaying deep in the Earth's core and mantle. Near volcanoes and the edges of tectonic plates, this heat rises much closer to the surface than usual, because magma (molten rock) is closer to the crust. That is exactly why volcanic countries like Iceland, which sits directly on the Mid-Atlantic Ridge where two tectonic plates are pulling apart, have such easy access to geothermal energy.

A geothermal power plant drills a well, sometimes over a mile deep, down to naturally hot water or steam. That steam shoots up the well and spins a turbine, a large fan-like wheel connected to a generator, the same basic idea used in coal or gas power plants, except no fuel is burned at all. After the steam spins the turbine, it cools back into water and is pumped back underground to be reheated and reused, creating a nearly endless cycle. The Hellisheiði plant in Iceland, one of the largest geothermal plants in the world, produces enough electricity for hundreds of thousands of homes this way.

Geothermal energy is clean and constant (unlike solar or wind, it works day and night, rain or shine), but it is not available everywhere. Drilling several miles into solid rock is expensive, and it only makes economic sense where heat is close enough to the surface to reach, which is mainly near volcanic activity or tectonic plate boundaries, like Iceland, parts of the western United States, and New Zealand. Scientists are working on new drilling technology to reach deeper, hotter rock in more places, which could someday make geothermal energy available even in regions without volcanoes nearby.