An area of the Earth characterized by a relatively high heat flow. The anomolously high rate of heat flow may be due to present, or fairly recent, orogenic or magmatic activity, or to the radioactive decay of isotopes of K, Th, and U where these occur at very high concentrations in crustal granites (hot dry rocks). In sedimentary basins, low thermal-conductivity values for the rocks are balanced by high thermal gradients, thus maintaining a constant heat flow. The high thermal gradients raise the temperature of deep, permeating water. Extraction of the water up deep boreholes provides surface water at temperatures useful for space heating. Hot springs and fumaroles can be important surface manifestations of a geothermal field.