The geothermal ___ is about 25C for every 1 km of depth below Earth's surface.

Gradient

Explanation:

There is an increase in the internal temperature of the earth with an increase in the depth from the surface. The average geothermal gradient near the surface is about 25 degrees centigrade for every one kilometer of depth. In some areas there is a much higher heat flows because of the presence of deep fault zones, rifting, magmatic intrusions, or active tectonic forces. Conditions in the deep mines can be quite uncomfortable and hot enough to explode rocks or bend steel due to the presence of geothermal gradient.

The geothermal gradient is 25c for every 1k of depth below Earth's surface