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±2

terawatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named in honor ...

s (TW) and comes from two main sources in roughly equal amounts: the ''radiogenic heat'' produced by the

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

of isotopes in the mantle and crust, and the ''primordial heat'' left over from the formation of Earth. Earth's internal heat travels along

geothermal gradient Geothermal gradient is the rate of change in temperature with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plat ...

s and powers most geological processes. It drives

mantle convection Mantle convection is the very slow creep of Earth's solid silicate mantle as convection currents carry heat from the interior to the planet's surface. Mantle convection causes tectonic plates to move around the Earth's surface. The Earth's l ...

plate tectonics Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

mountain building Mountain formation occurs due to a variety of geological processes associated with large-scale movements of the Earth's crust (List of tectonic plates, tectonic plates). Fold (geology), Folding, Fault (geology), faulting, Volcano, volcanic acti ...

, rock metamorphism, and

volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

. Convective heat transfer within the planet's high-temperature metallic core is also theorized to sustain a

geodynamo In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field. The dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid can ...

which generates

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...

. Despite its geological significance, Earth's interior heat contributes only 0.03% of Earth's total energy budget at the surface, which is dominated by 173,000 TW of incoming

solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...

. This external energy source powers most of the planet's atmospheric, oceanic, and biologic processes. Nevertheless on

land Land, also known as dry land, ground, or earth, is the solid terrestrial surface of Earth not submerged by the ocean or another body of water. It makes up 29.2% of Earth's surface and includes all continents and islands. Earth's land sur ...

and at the

ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

, the

sensible heat Sensible heat is heat exchanged by a body or thermodynamic system in which the exchange of heat changes the temperature of the body or system, and some macroscopic variables of the body or system, but leaves unchanged certain other macroscopic vari ...

absorbed from non-reflected

insolation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...

flows inward only by means of

thermal conduction Thermal conduction is the diffusion of thermal energy (heat) within one material or between materials in contact. The higher temperature object has molecules with more kinetic energy; collisions between molecules distributes this kinetic energy ...

, and thus penetrates only a few dozen centimeters on the daily cycle and only a few dozen meters on the annual cycle. This renders solar radiation minimally relevant for processes internal to

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

. Global data on heat-flow density are collected and compiled by the International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior.

Heat and early estimate of Earth's age

Based on calculations of Earth's cooling rate, which assumed constant conductivity in the Earth's interior, in 1862 William Thomson, later

Lord Kelvin William Thomson, 1st Baron Kelvin (26 June 182417 December 1907), was a British mathematician, Mathematical physics, mathematical physicist and engineer. Born in Belfast, he was the Professor of Natural Philosophy (Glasgow), professor of Natur ...

, estimated the

age of the Earth The age of Earth is estimated to be 4.54 ± 0.05 billion years. This age may represent the age of Earth's accretion (astrophysics), accretion, or Internal structure of Earth, core formation, or of the material from which Earth formed. This dating ...

at 98 million years, which contrasts with the age of 4.5 billion years obtained in the 20th century by

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...

. 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 Thomson'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,

alters how heat is transported within the Earth, invalidating Thomson's assumption of purely conductive cooling.

Global internal heat flow

Estimates of the total heat flow from Earth's interior to surface span a range of 43 to 49 terawatts (TW) (a terawatt is 10¹²

watt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...

s).Dye, S. T. (2012). Geoneutrinos and the radioactive power of the Earth. Reviews of Geophysics, 50(3). doi:10.1029/2012RG000400 One recent estimate is 47 TW, equivalent to an average heat flux 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. 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. The structure of Earth is a rigid outer crust that is composed of thicker

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

and thinner

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

, solid but plastically flowing mantle, a liquid

outer core Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid Earth's inner core, inner core and below its Earth's mantle, mantle. The outer core begins approximately beneath Earth's surface ...

, and a solid

inner core Earth's inner core is the innermost internal structure of Earth, geologic layer of the planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. T ...

. 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.Arevalo Jr, R., McDonough, W. F., & Luong, M. (2009). The K/U ratio of the silicate Earth: Insights into mantle composition, structure and thermal evolution. Earth and Planetary Science Letters, 278(3), 361–369. Earth heat transport occurs by conduction,

, hydrothermal convection, and volcanic

advection In the fields of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is a ...

.Jaupart, C., & Mareschal, J. C. (2007). Heat flow and thermal structure of the lithosphere. Treatise on Geophysics, 6, 217–251. 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 A mid-ocean ridge (MOR) is a undersea mountain range, seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading ...

spreading centers (zones of mantle upwelling), as observed in the global map of Earth heat flow.

Sources of heat

Radiogenic heat

The

of elements in the Earth's mantle and crust results in production of daughter

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

s and release of geoneutrinos and heat energy, or radiogenic heat. About 50% of the Earth's internal heat originates from radioactive decay. Four radioactive isotopes are responsible for the majority of radiogenic heat because of their enrichment relative to other radioactive isotopes:

uranium-238 Uranium-238 ( or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it i ...

(²³⁸U),

uranium-235 Uranium-235 ( or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exists in nat ...

(²³⁵U),

thorium-232 Thorium-232 () is the main naturally occurring isotope of thorium, with a relative abundance of 99.98%. It has a half life of 14.05 billion years, which makes it the longest-lived isotope of thorium. It decays by alpha decay to radium-228; its de ...

(²³²Th), and

potassium-40 Potassium-40 (K) is a long lived and the main naturally occurring radioactive isotope of potassium. Its half-life is 1.25 billion years. It makes up about 0.012% (120 parts-per notation, ppm) of natural potassium. Potassium-40 undergoes four dif ...

(⁴⁰K).Korenaga, J. (2011). Earth's heat budget: Clairvoyant geoneutrinos. Nature Geoscience, 4(9), 581–582. Due to a lack of rock samples from below 200 km depth, it is difficult to determine precisely the radiogenic heat throughout the whole mantle, although some estimates are available. For the Earth's core,

geochemical Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...

studies indicate that it is unlikely to be a significant source of radiogenic heat due to an expected low concentration of radioactive elements partitioning into iron. Radiogenic heat production in the mantle is linked to the structure of

, 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 and ⁴⁰K are not thus detectable. Regardless, ⁴⁰K is estimated to contribute 4 TW of heating.Gando, A., Dwyer, D. A., McKeown, R. D., & Zhang, C. (2011). Partial radiogenic heat model for Earth revealed by geoneutrino measurements. Nature Geoscience, 4(9), 647–651. However, due to the short

half-lives Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * '' Half Life: A Parable for t ...

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

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.Korenaga, J. (2008). Urey ratio and the structure and evolution of Earth's mantle. Reviews of Geophysics, 46(2).

Primordial heat

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.Lay, T., Hernlund, J., & Buffett, B. A. (2008). Core–mantle boundary heat flow. Nature Geoscience, 1(1), 25–32. Estimates of mantle primordial heat loss range between 7 and 15 TW, which is calculated as the remainder of heat after removal of core heat flow and bulk-Earth radiogenic heat production from the observed surface heat flow. 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

; therefore primordial heat from the core enabled Earth's atmosphere and thus helped retain Earth's liquid water. Primordial heat energy comes from the

potential energy In physics, potential energy is the energy of an object or system due to the body's position relative to other objects, or the configuration of its particles. The energy is equal to the work done against any restoring forces, such as gravity ...

released by collapsing a large amount of matter into a

gravity well A sphere of influence (SOI) in astrodynamics and astronomy is the oblate spheroid-shaped region where a particular celestial body exerts the main gravitational influence on an orbiting object. This is usually used to describe the areas in the ...

, and the

kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

of accreted matter.

Heat flow and tectonic plates

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 the processes of

were not active in the Earth before 3.2 billion years ago, and that early Earth's internal heat loss could have been dominated by

via heat-pipe

.Moore, W. B., & Webb, A. A. G. (2013). Heat-pipe Earth. Nature, 501(7468), 501–505. Terrestrial bodies with lower heat flows, such as the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

and

Mars Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...

, 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.

External links

References

{{reflist, 30em Earth Geodynamics Plate tectonics Heat transfer Geothermal energy