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Geothermal energy is derived from heat within the earth. Earth's internal heat is due to the residual heat that was produced when Earth formed, in addition to heat generated by radioactive decay. Earth's temperature increases with depth below the surface. The inner core has a temperature of about 4,000 degrees C. The increase in temperature with depth is referred to as the geothermal gradient. A normal geothermal gradient is 15 to 30 degrees C/kilometer. The geothermal gradient is much higher, double or triple, in areas of recent volcanic activity. Generally, the higher the geothermal gradient, the higher the heat flow to the surface.

Generally, geothermal energy is tapped either by drilling wells into bedrock and allowing hot water and steam to flows up to turn ...

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