Volcanic heat from compression or just friction?

One must also consider tidal forces between the moon and earth. These gravitational forces also contribute to the heat inside the earth.

As another effect, albeit negligible, one must consider the decay of unstable isotopes, such as thos of uranium, radium, barium and many other trace elements.

 

I am not sure as to the mechanics of how compression heats up rock or any other object, for that matter. I only know that it does, sorry for a thin answer. I'd suggest you post your compression question in classical physics, it will be answered easily there.

 

I'm not certain, but I seem to remember hearing that radioactive decay is not negligable, but an important factor. As to compression, you must remember that most all models of planetary formation involve a cloud of gas and dust and small debris falling into a lump. This initial formation causes a great deal of heat and that heat is then trapped as new layers are added.

 

It is thought that original heat of compression has dissipated out a few billion years ago. http://www.me.ucsb.edu/dept_site/vanyo.htm [Broken] may contribute significantly to the heat of the Earth core.

However, "volcanic heat" is not necesarily related to the heat in the Earth core but is mostly thought to be compression heat from subducting tectonic plates in "the ring of fire".