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{{Short description|Rate of temperature increase with depth in Earth's interior}}
[[File:Geothermal gradient.png|thumb|upright=1.35|Temperature profile of inner Earth, schematic view ([[estimation|estimated]]). The red dashed line shows the minimum temperature for the respective mantle rock to melt. The geothermal gradient remains below the melting temperature of the rock, except in the asthenosphere. Sharp rises occur in the uppermost mantle and at the core–mantle boundary.]]

'''Geothermal gradient''' is the rate of change in temperature with respect to increasing depth in [[Earth]]'s [[ internal structure of Earth|interior]]. As a general rule, [[Earth's crust|the crust]] temperature rises with depth due to the heat flow from the much hotter [[Earth's mantle|mantle]]; away from [[tectonic plate]] [[plate boundary|boundaries]], temperature rises in about 25–30&nbsp;°C/km (72–87&nbsp;°F/mi) of depth near the surface in the continental crust.<ref name="IPCC" /> However, in some cases the temperature may drop with increasing depth, especially near the surface, a phenomenon known as {{em|inverse}} or {{em|negative}} geothermal gradient. The effects of weather, the Sun, and season only reach a depth of roughly {{cvt|10–20|m|ft}}.

Strictly speaking, ''geo''-thermal necessarily refers to Earth, but the concept may be applied to other planets. In [[International System of Units|SI units]], the geothermal gradient is expressed as °C/km,<ref name="IPCC" /> K/km,<ref>{{cite journal |last=Jones |first=M. Q. W. |title=Virgin rock temperatures and geothermal gradients in the Bushveld Complex |journal=Journal of the Southern African Institute of Mining and Metallurgy |date=July 2018 |volume=118 |issue=7 |pages=671–680 |doi=10.17159/2411-9717/2018/v118n7a1 |ref=vrt |issn=2225-6253 |doi-access=free }}{{Dead link|date=May 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> or mK/m.<ref>{{cite journal |publisher=Global Heat Flow Compilation Group |title=Component parts of the World Heat Flow Data Collection |url=https://doi.pangaea.de/10.1594/PANGAEA.810104 |journal=Pangaea |date=11 April 2013 |access-date=2021-09-23 |ref=pangea |language=en |doi=10.1594/PANGAEA.810104 |author1=Global Heat Flow Compilation Group }}</ref> These are all equivalent.

[[Earth's internal heat budget|Earth's internal heat]] comes from a combination of residual heat from [[planetary accretion]], heat produced through [[radioactive decay]], latent heat from core crystallization, and possibly heat from other sources. The major heat-producing [[nuclide]]s in Earth are [[potassium|potassium-40]], [[uranium|uranium-238]], [[uranium-235]], and [[thorium|thorium-232]].<ref>{{cite news
 | first=Robert | last=Sanders
 | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=2003-12-10 | url=http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml
 | access-date=2007-02-28 }}</ref> The [[inner core]] is thought to have temperatures in the range of 4000 to 7000&nbsp;K, and the pressure at the centre of the planet is thought to be about 360&nbsp;[[GPa]] (3.6 million atm).<ref>{{cite journal
 |author1=Alfè, D. |author2=Gillan, M. J. |author3=Vocadlo, L. |author4=Brodholt, J. |author5=Price, G. D. | title=The ''ab initio'' simulation of the Earth's core
 | journal= Philosophical Transactions of the Royal Society
 | date=2002 | volume=360 | issue=1795 | pages=1227–44 | url=http://chianti.geol.ucl.ac.uk/~dario/pubblicazioni/PTRSA2002.pdf
 | access-date=2007-02-28 | doi=10.1098/rsta.2002.0992|bibcode = 2002RSPTA.360.1227A | pmid=12804276 |s2cid=21132433 }}</ref> (The exact value depends on the density profile in Earth.) Because much of the heat is provided for by radioactive decay, scientists believe that early in Earth's history, before nuclides with short [[Half-life|half-lives]] had been depleted, Earth's heat production would have been much higher. Heat production was twice that of present-day at approximately 3&nbsp;billion&nbsp;years ago,<ref name="turcotte" /> resulting in larger temperature gradients within Earth, larger rates of [[mantle convection]] and [[plate tectonics]], allowing the production of igneous rocks such as [[komatiites]] that are no longer formed.<ref>{{cite journal
 | last1=Vlaar | first1=N | title=Cooling of the earth in the Archaean: Consequences of pressure-release melting in a hotter mantle | date=1994 |journal=Earth and Planetary Science Letters
 | volume=121
 | issue=1–2 | doi=10.1016/0012-821X(94)90028-0
 | pages=1–18
 | last2=Vankeken
 | first2=P
 | last3=Vandenberg
 | first3=A | bibcode=1994E&PSL.121....1V}}</ref>

The top of the geothermal gradient is influenced by [[atmospheric temperature]]. The uppermost layers of the solid planet are at the temperature produced by the local weather, decaying to approximately the annual [[mean average|mean-average]] ground temperature (MAGT) at a shallow depth of about 10-20 metres depending on the type of ground, rock etc.;<ref>Kalogirou, Soteris & Florides, Georgios. (2004). Measurements of Ground Temperature at Various Depths, conference paper 3rd International Conference on Sustainable Energy Technologies, Nottingham, UK, https://www.researchgate.net/publication/30500372_Measurements_of_Ground_Temperature_at_Various_Depths
https://ktisis.cut.ac.cy/bitstream/10488/870/3/C55-PRT020-SET3.pdf {{Webarchive|url=https://web.archive.org/web/20221005194149/https://ktisis.cut.ac.cy/bitstream/10488/870/3/C55-PRT020-SET3.pdf |date=2022-10-05 }}</ref><ref>Williams G. and Gold L. Canadian Building
Digest 180m 1976. National Research Council of Canada, Institute for Research in Construction. https://nrc-publications.canada.ca/eng/view/ft/?id=386ddf88-fe8d-45dd-aabb-0a55be826f3f, </ref>
<ref>{{cite web |title=Groundwater temperature's measurement and significance - National Groundwater Association |url=http://www.ngwa.org/Fundamentals/studying/Pages/Groundwater-temperature's-measurement-and-significance.aspx |archive-url=https://web.archive.org/web/20150823030910/http://www.ngwa.org/Fundamentals/studying/Pages/Groundwater-temperature's-measurement-and-significance.aspx |url-status=dead |archive-date=23 August 2015 |publisher=National Groundwater Association |date=23 August 2015}}</ref><ref>{{cite web |title=Mean Annual Air Temperature - MATT |url=http://www.icax.co.uk/Mean_Annual_Air_Temperature.html |website=www.icax.co.uk}}</ref><ref>{{cite web |title=Ground Temperatures as a Function of Location, Season, and Depth |url=https://builditsolar.com/Projects/Cooling/EarthTemperatures.htm |website=builditsolar.com}}</ref> it is this depth which is used for many [[ground-source heat pumps]].<ref name="survival">{{Cite news | last = Rafferty | first = Kevin |date = April 1997| title =An Information Survival Kit for the Prospective Residential Geothermal Heat Pump Owner | periodical =Geo-Heat Centre Quarterly Bulletin | location =Klmath Falls, Oregon | publisher =Oregon Institute of Technology | volume =18 | issue =2 | pages = 1–11 | url =http://geoheat.oit.edu/bulletin/bull18-2/art1.pdf | issn =0276-1084 | access-date =2009-03-21 |archive-url=https://web.archive.org/web/20120217184539/http://geoheat.oit.edu/bulletin/bull18-2/art1.pdf |archive-date=17 February 2012}} The author issued an [http://geoheat.oit.edu/ghp/survival.pdf updated version] {{Webarchive|url=https://web.archive.org/web/20130217043915/http://geoheat.oit.edu/ghp/survival.pdf |date=2013-02-17 }} of this article in February 2001.</ref> The top hundreds of meters reflect past climate change;<ref name="Huang">Huang, S., H. N. Pollack, and P. Y. Shen (2000), Temperature trends over the past five centuries reconstructed from borehole temperatures, Nature, 403, 756–758.</ref> descending further, warmth increases steadily as interior heat sources begin to dominate.

== Heat sources ==
[[Image:Earth-crust-cutaway-english.svg|thumb|275px|right|Earth cutaway from core to exosphere]]
[[File:Geothermaldrilling.jpg|275px|thumb|Geothermal drill machine in Wisconsin, USA]]
Temperature within Earth increases with depth. Highly viscous or partially molten rock at temperatures between {{convert|650|and|1200|C|F|sigfig=2}}  are found at the margins of tectonic plates, increasing the geothermal gradient in the vicinity, but only the outer core is postulated to exist in a molten or fluid state, and the temperature at Earth's inner core/outer core boundary, around {{convert|3500|km|mi}} deep, is estimated to be 5650 ± 600 [[Kelvin]].<ref name='Alfe2003'>{{cite journal |title=Thermodynamics from first principles: temperature and composition of Earth's core |url=http://www.es.ucl.ac.uk/people/d-price/papers/153.pdf |access-date=2007-03-01 |date=2003-02-01 |first=D. |last=Alfe |author2=M. J. Gillan |author3=G. D. Price |volume=67 |issue=1 |pages=113–123 |doi=10.1180/0026461026610089 |journal=Mineralogical Magazine |url-status=dead |archive-url=https://web.archive.org/web/20070316082958/http://www.es.ucl.ac.uk/people/d-price/papers/153.pdf |archive-date=2007-03-16 |bibcode=2003MinM...67..113A |s2cid=98605003 }}</ref><ref name='Gerd2001'>{{cite news | first=Gerd | last=Steinle-Neumann |author2=Lars Stixrude |author3=Ronald Cohen  | title=New Understanding of Earth's Inner Core | date=2001-09-05 | publisher=[[Carnegie Institution of Washington]] | url =http://www.carnegieinstitution.org/news_010905.html | access-date = 2007-03-01 |archive-url = https://web.archive.org/web/20061214151031/http://www.carnegieinstitution.org/news_010905.html |archive-date = 2006-12-14}}</ref> The heat content of Earth is [[1 E31 J|10<sup>31</sup> joules]].<ref name="IPCC">{{cite journal | first1=Ingvar B. | last1=Fridleifsson | first2=Ruggero | last2=Bertani | first3=Ernst | last3=Huenges | first4=John W. | last4=Lund | first5=Arni | last5=Ragnarsson | first6=Ladislaus | last6=Rybach | date=2008-02-11 | title=The possible role and contribution of geothermal energy to the mitigation of climate change|journal=IPCC Scoping Meeting on Renewable Energy Sources, Proceedings| editor=O. Hohmeyer and T. Trittin | location=Luebeck, Germany | pages=59–80 | citeseerx=10.1.1.362.1202 | url=http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.362.1202&rep=rep1&type=pdf#page=75 | access-date=2013-11-03 | archive-url=https://web.archive.org/web/20130312182133/http://ipcc.ch/pdf/supporting-material/proc-renewables-lubeck.pdf | archive-date=2013-03-12 | url-status=dead }}</ref>
* Much of the heat is created by [[radioactive decay|decay]] of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from Earth originates from radioactive decay of elements, mainly located in the mantle.<ref name="turcotte">{{cite book
 | last=Turcotte | first=DL |author2=Schubert, G
 | title=Geodynamics | publisher=Cambridge University Press
 | location=Cambridge, England, UK| date=2002 | edition=2nd
 | pages=136–7 | chapter=4 | isbn=978-0-521-66624-4 }}</ref><ref name='Anuta2006'>{{cite news | first=Joe | last=Anuta | title=Probing Question: What heats the earth's core? | date=2006-03-30 | publisher=physorg.com | url=http://www.physorg.com/news62952904.html | access-date = 2007-09-19 }}</ref><ref name=physicsworld>{{cite web|last=Johnston|first=Hamish|title=Radioactive decay accounts for half of Earth's heat|url=http://physicsworld.com/cws/article/news/2011/jul/19/radioactive-decay-accounts-for-half-of-earths-heat|work=PhysicsWorld.com|publisher=Institute of Physics|access-date=18 June 2013|date=19 July 2011}}</ref> 
* [[Gravitational energy|Gravitational potential energy]], which can be further divided into:
** Release during the [[Accretion (astrophysics)|accretion]] of Earth.
** Heat released during [[Planetary differentiation|differentiation]], as abundant [[heavy metals]] ([[iron]], [[nickel]], [[copper]]) descended to Earth's core.
* Latent heat released as the liquid [[outer core]] [[crystallization|crystallizes]] at the [[inner core]] boundary. 
* Heat may be generated by [[tidal force]]s on Earth as it rotates (conservation of angular momentum). The resulting [[earth tide]]s dissipate energy in Earth's interior as heat.
[[File:Evolution of Earth's radiogenic heat.svg|thumb|275px|The [[radiogenic heat]] from the decay of <sup>238</sup>U and <sup>232</sup>Th are now the major contributors to [[Earth's internal heat budget]].]]

In Earth's continental crust, the decay of natural radioactive nuclides makes a significant contribution to geothermal heat production. The continental crust is abundant in lower density minerals but also contains significant concentrations of heavier [[Goldschmidt classification#Lithophile elements|lithophilic elements]] such as uranium. Because of this, it holds the most concentrated global reservoir of radioactive elements found in Earth.<ref name="Geothermal">William, G. E. (2010). ''Geothermal Energy: Renewable Energy and the Environment'' (pp. 1-176). Boca Raton, FL: CRC Press.</ref> Naturally occurring radioactive elements are enriched in the granite and basaltic rocks, especially in layers closer to Earth's surface.<ref>Wengenmayr, R., & Buhrke, T. (Eds.). (2008). ''Renewable Energy: Sustainable Energy Concepts for the future'' (pp. 54-60). Weinheim, Germany: WILEY-VCH Verlag GmbH & Co. KGaA.</ref> These high levels of radioactive elements are largely excluded from Earth's mantle due to their inability to substitute in mantle minerals and consequent enrichment in melts during mantle melting processes. The mantle is mostly made up of high density minerals with higher concentrations of elements that have relatively small atomic radii, such as magnesium (Mg), titanium (Ti), and calcium (Ca).<ref name="Geothermal" />

{| class="wikitable" style="text-align: center;"
|+ Present-day major heat-producing nuclides<ref name="T&S 137">{{cite book|last=Turcotte|first=D. L.|author2=Schubert, G.|title=Geodynamics|publisher=Cambridge University Press|location=Cambridge, England, UK|date=2002|edition=2nd|page=137|chapter=4|isbn=978-0-521-66624-4}}</ref>
|-
! Nuclide
! Heat release
[W/kg nuclide]
! Half-life
[years]
! Mean mantle concentration
[kg nuclide/kg mantle]
! Heat release
[W/kg mantle]
|-
| <sup>238</sup>U
| {{nowrap|9.46 × 10<sup>−5</sup>}}
| {{nowrap|4.47 × 10<sup>9</sup>}}
| {{nowrap|30.8 × 10<sup>−9</sup>}}
| {{nowrap|2.91 × 10<sup>−12</sup>}}
|-
| <sup>235</sup>U
| {{nowrap|56.9 × 10<sup>−5</sup>}}
| {{nowrap|0.704 × 10<sup>9</sup>}}
| {{nowrap|0.22 × 10<sup>−9</sup>}}
| {{nowrap|0.125 × 10<sup>−12</sup>}}
|-
| <sup>232</sup>Th
| {{nowrap|2.64 × 10<sup>−5</sup>}}
| {{nowrap|14.0 × 10<sup>9</sup>}}
| {{nowrap|124 × 10<sup>−9</sup>}}
| {{nowrap|3.27 × 10<sup>−12</sup>}}
|-
| <sup>40</sup>K
| {{nowrap|2.92 × 10<sup>−5</sup>}}
| {{nowrap|1.25 × 10<sup>9</sup>}}
| {{nowrap|36.9 × 10<sup>−9</sup>}}
| {{nowrap|1.08 × 10<sup>−12</sup>}}
|}

The geothermal gradient is steeper in the [[lithosphere]] than in the mantle because the mantle transports heat primarily by convection, leading to a geothermal gradient that is determined by the mantle adiabat, rather than by the conductive heat transfer processes that predominate in the lithosphere, which acts as a [[Thermal boundary layer thickness and shape|thermal boundary layer]] of the convecting mantle.{{Citation needed|date=August 2018}}

== Heat flow ==
{{Main|Earth's internal heat budget}}

Heat flows constantly from its sources within Earth to the surface. Total heat loss from Earth is estimated at 44.2 TW ({{nowrap|4.42 × 10<sup>13</sup> Watts}}).<ref name=Pollack>[http://anquetil.colorado.edu/EPP3/readings/Pollack_etal_1993_Rev_Geophys.pdf Pollack, Henry N., et.al.,''Heat flow from Earth's interior: Analysis of the global data set,'' Reviews of Geophysics, 31, 3 / August 1993, p. 273 ] {{webarchive|url=https://web.archive.org/web/20110811133919/http://anquetil.colorado.edu/EPP3/readings/Pollack_etal_1993_Rev_Geophys.pdf |date=2011-08-11 }} {{doi|10.1029/93RG01249}}</ref> Mean heat flow is 65&nbsp;mW/m<sup>2</sup> over [[continental crust]] and 101&nbsp;mW/m<sup>2</sup> over [[oceanic crust]].<ref name=Pollack/> This is 0.087 watt/square metre on average (0.03 percent of solar power absorbed by Earth<ref>{{cite web|title=Climate and Earth's Energy Budget|url=http://earthobservatory.nasa.gov/Features/EnergyBalance/page1.php|publisher=NASA|date=2009-01-14}}</ref>), but is much more concentrated in areas where the lithosphere is thin, such as along [[mid-ocean ridge]]s (where new oceanic lithosphere is created) and near [[mantle plume]]s.<ref>{{cite journal|author1=Richards, M. A.|author2=Duncan, R. A.|author3=Courtillot, V. E.|date=1989|title=Flood Basalts and Hot-Spot Tracks: Plume Heads and Tails|journal=Science|volume=246|issue=4926|pages=103–107|bibcode=1989Sci...246..103R|doi=10.1126/science.246.4926.103|pmid=17837768|s2cid=9147772}}<!--| access-date=2007-04-21--></ref>
Earth's crust effectively acts as a thick insulating blanket which must be pierced by fluid conduits (of magma, water or other) in order to release the heat underneath. More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. Another major mode of heat loss is by [[thermal conduction|conduction]] through the [[lithosphere]], the majority of which occurs in the oceans due to the crust there being much thinner and younger than under the continents.<ref name=Pollack/><ref name="heat loss">{{cite journal
 | doi=10.1029/JB086iB12p11535 | title=Oceans and Continents: Similarities and Differences in the Mechanisms of Heat Loss
 | date=1981 | last1=Sclater | first1=John G
 | journal=Journal of Geophysical Research | volume=86
 | issue=B12 |pages=11535
 | last2=Parsons
 | first2=Barry
 | last3=Jaupart
 | first3=Claude | bibcode=1981JGR....8611535S
}}</ref>

The heat of Earth is replenished by radioactive decay at a rate of 30 TW.<ref name="sustainability">{{Cite news | last =Rybach | first =Ladislaus | date =September 2007 | title =Geothermal Sustainability | periodical =Geo-Heat Centre Quarterly Bulletin | location =Klamath Falls, Oregon | publisher =Oregon Institute of Technology | volume =28 | issue =3 | pages =2–7 | url =http://www.oit.edu/docs/default-source/geoheat-center-documents/quarterly-bulletin/vol-28/28-3/28-3-art2.pdf | issn =0276-1084 | access-date =2018-03-07 | archive-date =2018-03-08 | archive-url =https://web.archive.org/web/20180308041805/http://www.oit.edu/docs/default-source/geoheat-center-documents/quarterly-bulletin/vol-28/28-3/28-3-art2.pdf | url-status =dead }}</ref> The global geothermal flow rates are more than twice the rate of human energy consumption from all primary sources. Global data on heat-flow density are collected and compiled by the International Heat Flow Commission (IHFC) of the [[International Union of Geodesy and Geophysics#Structures|IASPEI]]/[[International Union of Geodesy and Geophysics|IUGG]].<ref>[http://www.ihfc-iugg.org www.ihfc-iugg.org] IHFC: International Heat Flow Commission - Homepage. Retrieved 18/09/2019.</ref>

== Direct application ==
Heat from Earth's interior can be used as an energy source, known as [[geothermal energy]]. The geothermal gradient has been used for space heating and bathing since ancient Roman times, and more recently for generating electricity. As the human population continues to grow, so does energy use and the correlating environmental impacts that are consistent with global primary sources of energy. This has caused a growing interest in finding sources of [[Renewable energy|energy that are renewable]] and have reduced greenhouse gas emissions. In areas of high geothermal energy density, current technology allows for the generation of electrical power because of the corresponding high temperatures. Generating electrical power from geothermal resources requires no fuel while providing true baseload energy at a reliability rate that constantly exceeds 90%.<ref name="Geothermal" /> In order to extract geothermal energy, it is necessary to efficiently transfer heat from a geothermal reservoir to a power plant, where electrical energy is converted from heat by passing steam through a [[turbine]] connected to a generator.<ref name="Geothermal" /> The efficiency of converting the geothermal heat into electricity depends on the temperature difference between the heated fluid (water or steam) and the environmental temperature, so it is advantageous to use deep, high-temperature heat sources. On a worldwide scale, the heat stored in Earth's interior provides an energy that is still seen as an exotic source. About 10 GW of [[geothermal electric]] capacity is installed around the world as of 2007, generating 0.3% of global electricity demand. An additional 28 GW of direct [[geothermal heating]] capacity is installed for district heating, space heating, spas, industrial processes, desalination and agricultural applications.<ref name="IPCC" />

== Variations ==
The geothermal gradient varies with location and is typically measured by determining the bottom open-hole [[temperature]] after borehole drilling. Temperature logs obtained immediately after drilling are however affected due to drilling fluid circulation. To obtain accurate bottom hole temperature estimates, it is necessary for the well to reach stable temperature. This is not always achievable for practical reasons.

In stable [[tectonic]] areas in the [[tropics]], a temperature-depth plot will converge to the annual average surface temperature. However, in areas where deep [[permafrost]] developed during the [[Pleistocene]], a low temperature anomaly can be observed that persists down to several hundred metres.<ref>[http://www.pgi.gov.pl/pgi_en/index.php?option=news&task=viewarticle&sid=107 The Frozen Time, from the Polish Geological Institute] {{webarchive|url=https://web.archive.org/web/20101027200436/http://www.pgi.gov.pl/pgi_en/index.php?option=news&task=viewarticle&sid=107 |date=2010-10-27 }}</ref> The [[Suwałki]] cold anomaly in [[Poland]] has led to the recognition that similar thermal disturbances related to Pleistocene-[[Holocene]] [[climatic]] changes are recorded in boreholes throughout Poland, as well as in [[Alaska]], [[northern Canada]], and [[Siberia]].
[[Image:300px-Geothermgradients.png|right|thumb|500px]]

In areas of Holocene [[Tectonic uplift|uplift]] and [[erosion]] (Fig. 1) the shallow gradient will be high until it reaches a point (labeled "Inflection point" in the figure) where it reaches the stabilized heat-flow regime. If the gradient of the stabilized regime is projected above this point to its intersection with present-day annual average temperature, the height of this intersection above present-day surface level gives a measure of the extent of Holocene uplift and erosion. In areas of Holocene [[subsidence]] and [[Deposition (sediment)|deposition]] (Fig. 2) the initial gradient will be lower than the average until it reaches a point where it joins the stabilized heat-flow regime.

Variations in surface temperature, whether daily, seasonal, or induced by [[Climate change (general concept)|climate change]]s and the [[Milankovitch cycle]], penetrate below Earth's surface and produce an oscillation in the geothermal gradient with periods varying from a day to tens of thousands of years, and an amplitude which decreases with depth. The longest-period variations have a scale depth of several kilometers.<ref name="Stacey">{{cite book|last=Stacey|first=Frank D.|title=Physics of the Earth|edition=2nd|date=1977|publisher=John Wiley & Sons|location=New York|isbn=0-471-81956-5}} pp. 183-4</ref><ref name="Sleep">{{cite book|last=Sleep|first=Norman H.|author2=Kazuya Fujita|title=Principles of Geophysics|date=1997|publisher=Blackwell Science|isbn=0-86542-076-9}} pp. 187-9</ref> Melt water from the [[polar ice caps]] flowing along ocean bottoms tends to maintain a constant geothermal gradient throughout Earth's surface.<ref name="Stacey"/>{{dubious|date=August 2021}}{{verify source|date=August 2021}}

If the rate of temperature increase with depth observed in shallow boreholes were to persist at greater depths, temperatures deep within Earth would soon reach the point where rocks would melt. We know, however, that [[Earth's mantle]] is solid because of the transmission of [[S-waves]]. The temperature gradient dramatically decreases with depth for two reasons. First, the mechanism of thermal transport changes from [[Heat conduction|conduction]], as within the rigid tectonic plates, to [[convection]], in the portion of Earth's mantle that convects. Despite its [[solid]]ity, most of Earth's mantle behaves over long time-scales as a [[fluid]], and heat is transported by [[advection]], or material transport. Second, [[Decay heat|radioactive heat]] production is concentrated within the crust of Earth, and particularly within the upper part of the crust, as concentrations of [[uranium]], [[thorium]], and [[potassium]] are highest there: these three elements are the main producers of radioactive heat within Earth. Thus, the geothermal gradient within the bulk of Earth's mantle is of the order of 0.5 kelvin per kilometer, and is determined by the [[adiabatic]] gradient associated with mantle material ([[peridotite]] in the upper mantle).<ref name="T&S 187">{{cite book|last=Turcotte|first=D. L.|author2=Schubert, G.|title=Geodynamics|publisher=Cambridge University Press|location=Cambridge, England, UK|date=2002|edition=2nd|page=187|chapter=4|isbn=978-0-521-66624-4}}</ref>

== Negative geothermal gradient ==
Negative geothermal gradients occur where temperature decreases with depth. This occurs in the upper few hundreds of meters near the surface. Because of the low [[thermal diffusivity]] of rocks, deep underground temperatures are hardly affected by diurnal or even annual surface temperature variations. At depths of a few meters, underground temperatures are therefore similar to the annual average surface temperature. At greater depths, underground temperatures reflect a long-term average over past climate, so that temperatures at the depths of dozens to hundreds of meters contain information about the climate of the last hundreds to thousands of years. Depending on the location, these may be colder than current temperatures due to the colder weather close to the last [[ice age]], or due to more recent climate change.<ref>Lachenbruch, A. H., & Marshall, B. V. (1986). Changing climate: geothermal evidence from permafrost in the Alaskan Arctic. Science, 234(4777), 689-696.</ref><ref>Šafanda, J., Szewczyk, J., & Majorowicz, J. (2004). Geothermal evidence of very low glacial temperatures on a rim of the Fennoscandian ice sheet. Geophysical Research Letters, 31(7).</ref><ref name="Huang" />

Negative geothermal gradients may also occur due to deep [[aquifer]]s, where heat transfer from deep water by [[convection]] and [[advection]] results in water at shallower levels heating adjacent rocks to a higher temperature than rocks at a somewhat deeper level.<ref>Ziagos, J. P., & Blackwell, D. D. (1986). A model for the transient temperature effects of horizontal fluid flow in geothermal systems. Journal of Volcanology and Geothermal Research, 27(3-4), 371-397.</ref>

Negative geothermal gradients are also found at large scales in subduction zones.<ref name="Ernst, W.G. 1976">Ernst, W.G., (1976) Petrologic Phase Equilibria, W.H. Freeman, San Francisco.</ref> A subduction zone is a tectonic plate boundary where oceanic crust sinks into the mantle due to the high density of the oceanic plate relative to the underlying mantle. Since the sinking plate enters the mantle at a rate of a few centimeters per year, heat conduction is unable to heat the plate as quickly as it sinks. Therefore, the sinking plate has a lower temperature than the surrounding mantle, resulting in a negative geothermal gradient.<ref name="Ernst, W.G. 1976"/>

== See also ==
{{Portal|Earth sciences|Geophysics|World}}
* [[Temperature gradient]]
* [[Earth's internal heat budget]]
* [[Geothermal power]]
* [[Hydrothermal circulation]]
* [https://doi.pangaea.de/10.1594/PANGAEA.810104 PANGAEA Global Heat Flow Database] data set with geothermal gradients for large number of sites around the world

==References==
{{Reflist|2}}
{{Refbegin}}
{{cite web | title=Geothermal Resources | work=DOE/EIA-0603(95) Background Information and 1990 Baseline Data Initially Published in the Renewable Energy Annual 1995 | url=http://www.eia.doe.gov/cneaf/solar.renewables/renewable.energy.annual/backgrnd/chap9b.htm | access-date=May 4, 2005 }}
{{Refend}}

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[[Category:Geological processes]]
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