Thermoenergy is generated from passing water through rocks at high temperture a few kilometers beneth the earth. Why are these rocks so hot? Are they specific to a particular region or are rocks usually hot beneth the ground anywhere on earth? I have seen diagrams where they show the earth gets hotter the more inward we look. Does all that thermal energy come from low gravitational potential energy since total gravitational energy = gravitational potential + gravitational kinetic. Assume we model the earth as a point mass, When potential is low, kinetic is high by following the conservation of energy principle. But they cannot move anywhere since the rocks are all squshed together so all that kinetic energy is converted to heat. That is why the further to the centre of the earth, the hotter it is? However, my physics textbook did say that for a certain object in the earth's crust, the force on an object reaches a maximum at a certain depth and then decreases as the particle descends further. So my arugment above could only work for objects away from the earth's surface? For objects in the earth a different argument is needed? The evidence is that the further into the centre of the earth, the hotter it is so there must be a good reason for this. What is this reason? Could it simply be the case that it is very well insulated and chemical reactions are usually exothermic and so a lot of heat is trapped inside? Or is it the case that the material such as iron that formed the core in the first place was extremely hot (they all came from the sun) and other layers quickly formed so there wasn't much time for it to cool. This reason plus the fact that the insulation is extremely tight (trapped under thousands of kilometers of rock) and that is why even today, it is still extremely hot. But the insulation decreases the further away from the centre. Plus the rocks further away had more time to cool at the earth's formation so today they are cooler. Hence there is a rough negative temperture gradient as we go from the centre to the surface.