In caves just below Earth’s ground surface in continental crust, the temperature tends to be a constant ~15⁰C (~59⁰F). The deepest mine on Earth is currently the Mponeg gold mine just southwest of Johannesburg, South Africa, which reaches just over 4 km below the surface. At the bottom of that mine, the rock temperature reaches 66⁰C (~151⁰F). Using the cave temperature to represent the surface temperature of the crust at ~0 km depth, what is the rate at which temperature changes in Earth between 0 and 4 km – the near-surface geothermal gradient?
