What is Geothermal energy?
[[File:NesjavellirPowerPlant edit2.jpg|thumb|280px|Steam rising from the Nesjavellir Geothermal Power Station in [[Iceland]]]] '''Geothermal energy''' is a [[renewable energy]] made by heat inside the [[Earth's crust]].<ref name=holt/> Although the [[Sun]] does heat the surface of the Earth, the heat from inside the Earth is not caused by the Sun. The geothermal energy of the Earth's [[Crust (geology)|crust]] comes 20% from the original formation of the planet, and 80% from the [[radioactive decay]] of minerals.<ref name=ucsusa>[http://www.ucsusa.org/clean_energy/our-energy-choices/renewable-energy/how-geothermal-energy-works.html How Geothermal energy works]. Ucsusa.org. Retrieved on 2013-04-24.</ref><ref name=turcotte>Turcotte D.L. & Schubert G. 2002. ''Geodynamics''. 2nd ed, Cambridge University Press, 136–137. {{ISBN|978-0-521-66624-4}}</ref> The Earth is hottest at its [[Earth's core|core]] and, from the core to the surface, the temperature gets gradually cooler. Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles or kilometers beneath the Earth's surface, and down even deeper to the extremely high temperatures of molten rock called [[magma]]. It has been used for [[bath]]ing from [[Paleolithic]] times, but is now better known for making [[electricity]].<ref name=holt>{{cite book | title = Earth Science| url = https://archive.org/details/holtsciencetechn00na_0| publisher = Holt, Rinehart and Winston| date = 2001| location = United States of America| pages = 211| isbn = 0-03-055667-8}}</ref> All over the world, geothermal energy was used to make about 10 [[gigawatt]]s of electricity in the year 2007, about 0.3% of the electricity used around the world. When used to generate electricity, geothermal power plants typically offer constant output.<ref name=IPCC2011/> Geothermal energy is also used directly for district heating, or in other heating and cooling applications. Buildings in [[Iceland]] are heated in this way from the country's many geothermal