The internal temperature of the earth increases with depth from the surface. Near the surface, the average
geothermal gradient is about 25 degrees centigrade (77 degrees Fahrenheit) for every kilometer of depth. Some areas have much higher heat flows because of deep fault zones, rifting, magmatic intrusions, or active tectonic forces. The geothermal gradient can make conditions in deep mines quite uncomfortable and hot enough to explode rocks or bend steel.
The geothermal gradient of 25 degrees centigrade/kilometer is thought to be restricted to the upper part of the crust. If it continued at this rate uniformly from the surface, the internal temperature of the earth would be greater than 2,000 degrees centigrade within the lithosphere—a temperature that far exceeds the melting temperatures for all rocks at that depth. Since the crust and upper mantle are solid and brittle, this gradient cannot extend to these depths, where it is more likely about 1 degree centigrade/kilometer. Recent laboratory studies have suggested the temperature is about 4,800 degrees centigrade at the base of the lower mantle and about 7,000 degrees centigrade in the inner core.
The heat flow is the amount of heat from the earth's interior that is lost at the surface. The heat is probably generated by a still‐cooling core or by the radioactive decay of elements such as uranium and thorium. Areas of higher heat flow are generally related to magmatic activity or tectonic forces that bring wedges of hot mantle rock (mantle plumes) into thin or faulted crustal areas.