<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, and recent results indicate their contributions may be roughly equal . </P> <P> The structure of the Earth is a rigid outer crust that is composed of thicker continental crust and thinner oceanic crust, solid but plastically flowing mantle, a liquid outer core, and a solid inner core . The fluidity of a material is proportional to temperature; thus, the solid mantle can still flow on long time scales, as a function of its temperature and therefore as a function of the flow of Earth's internal heat . The mantle convects in response to heat escaping from Earth's interior, with hotter and more buoyant mantle rising and cooler, and therefore denser, mantle sinking . This convective flow of the mantle drives the movement of Earth's lithospheric plates; thus, an additional reservoir of heat in the lower mantle is critical for the operation of plate tectonics and one possible source is an enrichment of radioactive elements in the lower mantle . </P> <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>

Where does the heat in the mantle come from
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