Geothermal

Overview
In geology, geothermal refers to heat sources within the planet. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets.

Geothermal is technically an adjective (e.g., geothermal energy) but in U.S. English the word has attained frequent use as a noun (otherwise expressed as g. heat, g. source, or geotherm).

The planet's internal heat was originally generated during its accretion, due to gravitational binding energy, and since then additional heat has continued to be generated by the radioactive decay of elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun.

Heat flow
Heat flows constantly from its sources within the Earth to the surface. Global terrestrial heat flow is about 45 TW (1 TW = 1012 watts).

Hot spots
Geothermal heat at the surface is highly concentrated where magma is close to the surface. This primarily occurs in volcanic and hotspot areas and at spreading ridge areas.