<P> 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 -- 30 ° C / km ( 28 -- 34 ° F / mi ) 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 . </P>
in a deep mine temperatures increase with depth at the rate of
about 25 -- 30 ° C / km ( 28 -- 34 ° F / mi )