How do thermorocks occur?

pivoxa15

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.

billiards

NO!
1)'Thermoenergy' as you call it is not generated in any meaningful sense by passing water through the rocks, although it is fair to say that the water advects the heat, that is it transports it.
2)Heating may occur by adiabatic means, basically the more you pressurize something the hotter it gets, pressure increases with depth. Heat is also sourced by radioactive decay, the chemical release of latent heat at phase transitions, and the gravitational release of heat (which is basically a conversion of potential energy into heat - not by the mechanism that you propose though).

Heat does increase with depth, the temperature of the earth is well constrained at the surface and at the core mantle boundary, the gradient of the temperature with depth (the geotherm) is not know precisely, we just know two points.

matthyaouw

Rock does get hotter with depth pretty much anywhere, but geothermal energy can only be readily utilised in areas where heat is closer to the surface, for example where magma penetrates into the crust in the sort of areas where you are already likely to find hot springs and the like.

pivoxa15

Are you suggesting the fact that the deeper you go, the hotter it is, is due to the high pressure the deeper you go? What about this idea:

"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." in my OP?

How is geothermal power generated if what I stated (controlled pass of water through tunnels what contain hot dry rocks thereby heating the water and turning it to steam to drive the turbines) was incorrect?

billiards

I believe that to be partially true. I think Lord Kelvin did a calculation based on that idea and concluded that the Earth was something like 30 million years old (or something far less than the actual 4.5 billion years that we now believe the Earth to be), so you see, the remnant heat from initial formation will have been more or less lost. Furthermore the fact that we have a magnetic field is strong evidence to suggest that there is something down there that drives it, that is, heat! Other surface features that require an ongoing source of heat include the plate movement which is associated with mountain building, earthquakes and volcanism.

As I mentioned earlier, there are sources of heat: radiogenic, chemical and gravitational, which power the Earth as a dynamic engine. How the Earth gets that heat to the surface is a hot topic at the moment (no pun intended). Obviously convection plays a large role, or does it?? That depends on how well we can constrain the geotherm (geothermal gradient), at the moment various lines of inquiry (linking mainly seismics to mineral physics experiments) have shown that the geotherm is adiabtic at the top of the asthenosphere suggesting that convection occurs there (which is kind of the definiton of the asthenosphere: the depth at which convection occurs - above that we have the lithosphere with no convection and a super-adiabatic geotherm).

The water in geothermal heat is only a middle man so-to-speak. It is not the passing of the water than creates the heat, it is the water that makes the heat accessible. The heat is already down there, water just helps us convert it to usable energy.

pivoxa15

I never said the passing of the water creates the heat. The hot dry rock is at a higher temperture than the water and when water is passed down, it is heated up by the rock and liquid water is converted to steam which drifts upwards and does work such as driving turbines. Isn't this basically correct?

Looking back in my OP, I said in the very first sentence "Thermoenergy is generated from passing water through rocks at high temperture a few kilometers beneth the earth. " This could be a mistake as thermoenergy is in the rock. The heat in the rock is given off to the water and it turns to steam and drives the turbines.

billiards

I think the word you're looking for is is.

pivoxa15

I take your point. At least I know that when I typed that sentence, I didn't mean it and what I meant was stated in the first paragraph of my previous post. It was a case of what I thought didn't match what I meant, which doesn't occur often, fortunately.

billiards

Hey no worries, this is merely an internet forum after all. The only reason I didn't let it go was because:
1) Some kid could be reading this and potentially get confused
2) Every young scientist should learn to be as clear and honest as possible, if you turn out to be wrong just accept it and learn, I've made countless mistakes and have found this approach to be the most honest and best for all concerned. To your credit that is exactly what you did, it doesn't even matter whether you knew what you meant or not, you're pretty much unaninimous on a site like this so it won't affect your reputation amongst your colleagues and peers.

pivoxa15

Good point. I agree that the need to be clear is extremely important.