<P> Earth heat transport occurs by conduction, mantle convection, hydrothermal convection, and volcanic advection . Earth's internal heat flow to the surface is thought to be 80% due to mantle convection, with the remaining heat mostly originating in the Earth's crust, with about 1% due to volcanic activity, earthquakes, and mountain building . Thus, about 99% of Earth's internal heat loss at the surface is by conduction through the crust, and mantle convection is the dominant control on heat transport from deep within the Earth . Most of the heat flow from the thicker continental crust is attributed to internal radiogenic sources, in contrast the thinner oceanic crust has only 2% internal radiogenic heat . The remaining heat flow at the surface would be due to basal heating of the crust from mantle convection . Heat fluxes are negatively correlated with rock age, with the highest heat fluxes from the youngest rock at mid-ocean ridge spreading centers (zones of mantle upwelling), as observed in the global map of Earth heat flow . </P> <P> The radioactive decay of elements in the Earth's mantle and crust results in production of daughter isotopes and release of particles and heat energy, or radiogenic heat . Four radioactive isotopes are responsible for the majority of radiogenic heat, uranium - 238 (U), uranium - 235 (U), thorium - 232 (Th), and potassium - 40 (K). Due to a lack of rock samples from below 200 km depth, it is not possible to do a simple radiogenic heat estimate of known radioactive isotope concentrations in rock throughout the whole mantle . For the Earth's core, geochemical studies indicate that it is unlikely to be a significant source of radiogenic heat due to an expected low concentration of radioactive elements . Radiogenic heat production in the mantle is linked to the structure of mantle convection, a topic of much debate, and it is thought that the mantle may either have a layered structure with a higher concentration of radioactive heat - producing elements in the lower mantle, or small reservoirs enriched in radioactive elements dispersed throughout the whole mantle . </P> <P> Geoneutrino detectors can detect the decay of U and Th and thus allow estimation of their contribution to the present radiogenic heat budget, while U is unobserved, and K is not detectable but is known to contribute 4 TW of heating . However, the decay of U and K contributed a large fraction of radiogenic heat flux to the early Earth, which was also much hotter than at present . Initial results from measuring the geoneutrino products of radioactive decay from within the Earth, a proxy for radiogenic heat, yielded a new estimate of half of the total Earth internal heat source being radiogenic, and this is consistent with previous estimates . </P> <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>

What are the three sources of earth's internal heat