Are deep mines (2000 meters or so) hot because of the weight of the rock above (compression), nearness to the mantle, local “hot spot” condition or for another reason altogether?
Basically, nearness to the mantle.
Granted, you still have a hell of a long way to go to reach anything approximating lava or what not, but you’re getting, “warm.”
No pun intended.
Virtually all of the Earth is made of really hot material. The “solid” crust is like a paper thin layer on a volleyball. The rest is either molten or would be molten if the pressure wasn’t so high (<- people still working on if there there is solid, hot!, stuff in the core). So the lower you go, the closer you get to the heat.
The drawings you see in grade school texts of crust, mantle, outer and inner core and horribly not to scale.
Now, the cause of the heat is another matter. I have always heard that radioactive decay is the bulk of it, but lately a lot of Dopers have been making serious posts suggesting that it’s due to gravitational collapse. I’ll stand aside on that.
As a former mining engineering student and a person who has worked in a mine somewhere around the depth of 400 feet, I can tell you that the heat is caused by nearness to the mantle. A person really doesn’t notice the heat as much due to the fresh air intakes (they carry away all the dust and what little bit of methane is emitted and supply the mine with fresh air for the workers). I have heard stories in classes where there are mines around Pakistan that are insanely hot due to their depth.
You mean it’s not the proximity to Hell?
Gravitational collapse wouldn’t produce nearly as much heat as radioactive decay in a terrestrial planet such as earth. Without the steady heat produced by decay, earth wouldn’t still be warm enough to have active plate tetonics.
On a gas giant like Jupiter however, most of the heat is from the still-ongoing process of gravitational collapse. As the helium slowly rains out of the moslty hydrogen atmosphere, it converts gravitaional energy into heat. The core of the planet might have enough radioactive elements to produce significant heating, but it’s kinda hard to say from here, and the far greater bulk and effect of the overlying atmosphere means we’ll likely never know.