What is the ultimate source of heat within the earth? Is it residual heat remaining from the formation of the earth? Or is it the result of ongoing processes such as gravitational pressure, friction between moving internal layers, or radioactive decay?
My expectation is that all of these could be contributors. However,I would think that most of the heat of formation would have dissipated by now, and that there isn’t enough radioactive matter for its decay to be a large contributor. That would leave presure and friction as the main sources.
Does anyone have any numerical data as to the relative contributions of these sources and possibly others to the earth’s heat?
Surprisingly it is listed as a non-renewable resource and to me that answers a lot of your questions. The crust of the earth and atmosphere must be a very good insulator, since under natural conditions we get 500 times more heat from the sun than from geothermal heating.
Radioactively generated heat and that remaining from the initial differentiation of the internal sub-lithosphere drive much of the internal heat engine of the earth, which works in concert with the solar-powered external heat engine to produce the environment in which we can live.
Movements of tectonic plates, vulcanism and mountain uplifting are all evidence of the mechanics driven by subterreanean heat, and we know as well from diggings (such as mines and boreholes) and hot springs that there’s heat within.
The story is that planetary accretion and adiabatic (change in volume or pressure without gaining heat) compression initially warmed the earth’s interior. Thermal increases of the interior from radioactivity soon began and earth’s internal temperature began to rise. When internal temperatures achieved the melting point of iron, the core-mantle separation began as molten iron gravitated towards the core.
Surface, or near surface, granites are radioactively “hot” and the continental crust is largely granitic. So that’s one source of internal heat. But the majority of internally supplied heat energy appears to come from below, i.e., the mantle.
So radioactivily supplied heat of the sources within the mantle is a major source of internal terrestial heat energy, and that will die with time. Conversion of gravitational energy to heat (i.e., pressure with depth) accounts for some, as well