Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the
Earth's interior. Away from tectonic plate boundaries, it is about
25 °C per km of depth (1 °F per 70 feet of depth) near the surface in most of the world. Strictly speaking,
geo-thermal necessarily refers to the
Earth but the concept may be applied to other planets. A line tracing the gradient through the planetary body is called a
geotherm on Earth and other terrestrial planets. On the Moon it is called a
selenotherm.