<P> Primordial heat is the heat lost by the Earth as it continues to cool from its original formation, and this is in contrast to its still actively - produced radiogenic heat . The Earth core's heat flow--heat leaving the core and flowing into the overlying mantle--is thought to be due to primordial heat, and is estimated at 5--15 TW . Estimates of mantle primordial heat loss range between 7 and 15 TW . </P> <P> The early formation of the Earth's dense core could have caused superheating and rapid heat loss, and the heat loss rate would slow once the mantle solidified . Heat flow from the core is necessary for maintaining the convecting outer core and the geodynamo and Earth's magnetic field, therefore primordial heat from the core enabled Earth's atmosphere and thus helped retain Earth's liquid water . </P> <P> Controversy over the exact nature of mantle convection makes the linked evolution of Earth's heat budget and the dynamics and structure of the mantle difficult to unravel . There is evidence that the processes of plate tectonics were not active in the Earth before 3.2 billion years ago, and that early Earth's internal heat loss could have been dominated by advection via heat - pipe volcanism . Terrestrial bodies with lower heat flows, such as the Moon and Mars, conduct their internal heat through a single lithospheric plate, and higher heat flows, such as on Jupiter's moon Io, result in advective heat transport via enhanced volcanism, while the active plate tectonics of Earth occur with an intermediate heat flow and a convecting mantle . </P>

Where does most of the earth's heat come from