<P> The flow of heat from Earth's interior to the surface is estimated at 47 terawatts (TW) and comes from two main sources in roughly equal amounts: the radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and the primordial heat left over from the formation of the Earth . </P> <P> Earth's internal heat powers most geological processes and drives plate tectonics . Despite its geological significance, this heat energy coming from Earth's interior is actually only 0.03% of Earth's total energy budget at the surface, which is dominated by 173,000 TW of incoming solar radiation . The insolation that eventually, after reflection, reaches the surface penetrates only several tens of centimeters on the daily cycle and only several tens of meters on the annual cycle . This renders solar radiation irrelevant for internal processes . </P>

What drives the earth's internal heat engine