==Heat from the earth==
The geothermal energy of the Earth's crust originates from the original formation of the planet (20%) and from radioactive decay of materials (80%). The geothermal gradient—the difference in temperature between the core and the surface of the planet—drives a continuous conduction of thermal energy in the form of heat from the core to the surface.
Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core–mantle boundary can exceed 4000 °C (7,200 °F). The high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of mantle, which is lighter than the surrounding rock, to convect upward. In the crust, rock and water is heated, sometimes up to 370 °C (700 °F).
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==Noteworthy papers in OnePetro==
Anderson, D.N. 1972. Geothermal Development in California. Presented at the SPE California Regional Meeting, 8-10 November, Bakersfield, California, USA.SPE-4180-MS. http://dx.doi.org/10.2118/4180-MS.