Geothermal energy is the natural heat of the Earth. Earth's interior heat originated from its fiery consolidation of dust and gas over 4 billion years ago. Heat flow continually regenerated by the decay of radioactive elements that occur in all rocks. From the Earth surface down through the crust, the normal temperature gradient (the increase of temperature with the increase of depth) in the Earth's crust is 17° - 30°C per km of depth. At the mantle the temperature is viscous. For example, partially molten rocks have temperatures between 650° - 1250°C. At the Earth's core, which consists of a liquid outer core and a solid inner core, temperatures vary from 4000°C - 7000 °C.
Major geothermal provinces (fields) are situated in circum-pacific margins, rift zones of East Africa, North Africa, Mediterranean basin of Europe, across Asia to Pacific (See map from http://geothermal.marin.org).