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. It is continually regenerated by the decay of radioactive elements, that occur in all rocks.
From the 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 °C -- 30 °C per kilometer of depth (50 °F -- 87 °F per mile).
Below the crust is the mantle, made of highly viscous, partially molten rocks with temperatures between 650 °C -- 1250 °C (1200 °F -- 2280 °F). At the Earth's core, which consists of a liquid outer core and a solid inner core, temperatures vary from 4000 °C -- 7000 °C (7200 °F-- 12600 °F).
Major geothermal fields are situated in circum-pacific margins, rift zones of East Africa, North Africa, Mediterranean basin of Europe, across Asia to Pacific