Earth's internal heat budget
The flow of heat from Earth's interior to the surface is estimated at 47 terawatts and comes from two main sources, 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.
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.
Heat and early estimate of Earth's age
Based on calculations of Earth's cooling rate, in 1862 Lord Kelvin (William Thomson) estimated the age of the Earth at 98 million years, in contrast to the age of 4.5 billion years old obtained by radiometric dating. A critical, and as yet undiscovered, source of internal heat that would have sustained an older and hotter Earth was not accounted for in Lord Kelvin's calculations, and this was radioactivity. However, it is recognized that Kelvin's correlation of temperature and age does hold up for surface ages over large areas of the Earth crust.
Global internal heat flow
Estimates of the total heat flow from Earth’s interior to surface span a range of 43 to 49 TW (TW = terawatt = 1012 watt). The closest estimate is 47 TW, an average crust heat flow of 91.6 mW/m2, 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/m2. Based on this mean continental crust heat flow value, the ground beneath our feet is releasing Earth's internal heat at a rate equivalent to 709 100-watt lightbulbs per square kilometer.
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.
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.
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, ~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.
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 (238U), uranium-235 (235U), thorium-232 (232Th), and potassium-40 (40K). Due to a lack of rock samples from below 200 km depth it is not possible to do a simple radiogenic heat estimate off of known radioactive isotope concentrations in rock throughout the whole mantle. For the Earth's core, geochemical studies indicate that it would not 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.
Geoneutrino detectors can detect the decay of 238U and 232Th, and thus allow estimation of their contribution to the present radiogenic heat budget, while 235U is unobserved and 40K is presently not able to be detected but is known to contribute 4 TW of heating. However, the decay of 235U and 40K 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.
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 7–15 TW.
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.
Heat flow and plate tectonics
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 plate tectonics was 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.
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- Geothermal gradient
- Geothermal energy
- Plate tectonics
- Earth's energy budget
- Radioactive decay
- Mantle convection
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