<P> Earth's internal heat budget is fundamental to the thermal history of the Earth . 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> <P> Based on calculations of Earth's cooling rate, which assumed constant conductivity in the Earth's interior, in 1862 William Thomson (later made Lord Kelvin) estimated the age of the Earth at 98 million years, which contrasts with the age of 4.5 billion years obtained in the 20th century by radiometric dating . As pointed out by John Perry in 1895 a variable conductivity in the Earth's interior could expand the computed age of the Earth to billions of years, as later confirmed by radiometric dating . Contrary to the usual representation of Kelvin's argument, the observed thermal gradient of the Earth's crust would not be explained by the addition of radioactivity as a heat source . More significantly, mantle convection alters how heat is transported within the Earth, invalidating Kelvin's assumption of purely conductive cooling . </P>

The majority of the heat energy on the earth's surface comes from where