In brief, we can extract energy not from heat, but from temperature differences. It's very easy to extract energy from a conventional power plant, because the h

In brief, we can extract energy not from heat, but from temperature differences. It's very easy to extract energy from a conventional power plant, because the hot stuff is right next to the cool stuff - the environment here. The molten rock has a huge amount of energy, but we can't extract it unless it's close enough to something cool to build a heat engine between the hot and the cold. Thus, geothermal is only viable at a select number of sites where the lava or other carriers of heat naturally flow near the surface. To make it viable in general, you'll have to figure out a way to send a working fluid down deep enough to pick up enough heat and bring it back up again without losing all of that heat on the way back up, and do it with huge flowrates to extract enough energy, and in such a way that it can run pretty much continuously for years.