Heat sources and heat flow.

Several significant sources contribute to the internal heat of the earth. Radioactive decay of isotopes of thorium, uranium, and potassium in the crust produces most of the heat observed at the surface of the continents. Terrestrial heat flow is readily observed in deep wells, mines, and tunnels that penetrate below the narrow zone on the surface, which is heated by daily and seasonal radiation changes. Another major source is heat convected outward from the earth's core and through the mantle. This heat produces the majority of the heat flow measured in the oceans, especially at the mid-oceanic ridges.