Earth's temperature increases gradually. with depth, at the center reaching more than 4,200C (7600F). Some of this heat is a relic of the globe's fiery formation about 4.5 billion years ago, but most has been generated by the decay of radioactive isotopes. As heat naturally moves from hotter to cooler regions, so earth's heat flows along a geothermal gradient from the center to the surface, where an estimated 42 million thermal megawatts are continually radiated into space.

The bulk of this immense heat supply cannot be practically captured, because it arrives at the surface at too low a temperature. Fortunately, the fundamental geologic process know as plate tectonics (responsible for seismic, mountain building, and volcanism) ensures that some of this heat is concentrated at temperatures and depths favorable for its commercial extraction.

The planet's thin lithosphere its rigid shell of crust and outermost mantel has been broken into 12 large and several smaller moving plates by thermally and gravitationally driven convection of the underlying mantle, at rates measured in millimeters per year. The world's geothermal provinces are conspicuously concentrated at the margins of these jostling slabs. Where plates move apart, along globe encircling mid ocean ridges, basaltic magma rises in the fissures to form vast undersea volcanoes. Where two plates collide, one is commonly thrust (subducted) beneath the other, causing formation of a deep ocean trench and occasionally inducing powerful earthquakes.

At great depth just above the down-going plate, temperatures become high enough to melt rock. The resulting magma bodies are less dense than surrounding rocks, and ascend buoyantly through the upper mantle into the crust, where they sometimes give rise to explosive volcanoes, and are always profound shallow pools of heat. Under the right conditions, these near surface heat anomalies can be harnessed for commercial production of geothermal energy.