Everywhere on Earth, heat comes up to the surface from below.Temperatures in mine workings get higher as the workings are deeper. It would be impossible for people to withstand the temperatures some 3 kilometres down in South Africa’s deepest diamond mines without very extensive air-cooling machinery.
The oil which comes up from deeper drillings may be at 180 degrees centigrade, much hotter than boiling water. The heat is generated by processes deep with in the planet, such as nuclear conversion processes, the most familiar of which is radioactive decay of atoms.
In nuclear conversion, actual mass is converted into energy, according to the famous Einstein equation E=mc2 (Energy equals mass times the velocity of light squared). So the heat coming up from below the surface (the geothermal flux, in technical terms) reflects the fact that our planet is losing weight in generating it.
This heat is very small compared to that received by radiation from the Sun, but is still quite significant — for example it is more than twice the energy that mankind generates from all sources, including fossil fuels, capture of solar energy, and hydroelectric and wind power.
A good source on the heat balance of the Earth is at Temperatures of the Earth — a Globe in Space.
Now the amount of mass the Earth loses each year has been calculated in a task at the Zombal.com scientific outsourcing site. It turns out that our planet must be getting lighter by over 14 tonnes per year if its internal heat all comes from nuclear conversion.
The actual calculations are available in the Zombal Vault. They were done for just $25.