There are three principle categories of geothermal resource. The simplest to exploit is a source of hot underground water (the geothermal reservoir) that either reaches the surface naturally or can be tapped by drilling boreholes.
This is the geothermal source upon which all existing commercial geothermal power plants are based.
Where there are no underground water sources, anomalies in the crust can create regions where the rock close to the surface is much hotter than usual. This hot rock can be accessed by drilling and pumping a heat-transfer fluid into the rock, then bringing it back to the surface. The process has been tested but never exploited commercially.
The third, and potentially the richest source of geothermal energy, is the magma itself. The magma contains by far the greatest amount of heat energy,
but because of the temperatures and pressures found within it, this is also the most difficult geothermal energy source to exploit.
Estimating the amount of energy in Earth’s crust that could be exploited for power generation is not easy. It has been suggested that there is between 10 and 100 times as much heat energy available for power generation as there is energy recoverable from uranium and thorium in nuclear reactors. Certainly, the resource is enormous, if difficult to access.