<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 minimally relevant 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> <P> Estimates of the total heat flow from Earth's interior to surface span a range of 43 to 49 terawatts (TW) (a terawatt is 10 watts). One recent estimate is 47 TW, equivalent to an average heat flux of 91.6 mW / m, and is based on more than 38,000 measurements . The respective mean heat flows of continental and oceanic crust are 70.9 and 105.4 mW / m . </P> <P> While the total internal Earth heat flow to the surface is well constrained, the relative contribution of the two main sources of Earth's heat, radiogenic and primordial heat, are highly uncertain because their direct measurement is difficult . Chemical and physical models give estimated ranges of 15--41 TW and 12--30 TW for radiogenic heat and primordial heat, respectively . </P>

Which of the following best describes the source of earth's internal heat