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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 plate boundaries, temperature rises in about 25–30 °C/km (72–87 °F/mi) of depth near the surface in the continental crust.[1] However, in some cases the temperature may drop with increasing depth, especially near the surface, a phenomenon known as inverse or negative geothermal gradient. The effects of weather, the Sun, and season only reach a depth of roughly 10–20 m (33–66 ft).
Strictly speaking, geo-thermal necessarily refers to Earth, but the concept may be applied to other planets. In SI units, the geothermal gradient is expressed as °C/km,[1] K/km,[2] or mK/m.[3] These are all equivalent.
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 nuclides in Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[4] The inner core is thought to have temperatures in the range of 4000 to 7000 K, and the pressure at the centre of the planet is thought to be about 360 GPa (3.6 million atm).[5] (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-lives had been depleted, Earth's heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago,[6] 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.[7]
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 ground temperature (MAGT) at a shallow depth of about 10-20 metres depending on the type of ground, rock etc.;[8][9] [10][11][12] it is this depth which is used for many ground-source heat pumps.[13] The top hundreds of meters reflect past climate change;[14] descending further, warmth increases steadily as interior heat sources begin to dominate.
- ^ a b Cite error: The named reference
IPCCwas invoked but never defined (see the help page). - ^ Jones, M. Q. W. (July 2018). "Virgin rock temperatures and geothermal gradients in the Bushveld Complex". Journal of the Southern African Institute of Mining and Metallurgy. 118 (7): 671–680. doi:10.17159/2411-9717/2018/v118n7a1. ISSN 2225-6253.
- ^ "Component parts of the World Heat Flow Data Collection". Pangaea. Global Heat Flow Compilation Group. 11 April 2013. doi:10.1594/PANGAEA.810104. Retrieved 2021-09-23.
- ^ Sanders, Robert (2003-12-10). "Radioactive potassium may be major heat source in Earth's core". UC Berkeley News. Retrieved 2007-02-28.
- ^ Alfè, D.; Gillan, M. J.; Vocadlo, L.; Brodholt, J.; Price, G. D. (2002). "The ab initio simulation of the Earth's core" (PDF). Philosophical Transactions of the Royal Society. 360 (1795): 1227–44. Bibcode:2002RSPTA.360.1227A. doi:10.1098/rsta.2002.0992. PMID 12804276. S2CID 21132433. Retrieved 2007-02-28.
- ^ Cite error: The named reference
turcottewas invoked but never defined (see the help page). - ^ Vlaar, N; Vankeken, P; Vandenberg, A (1994). "Cooling of the earth in the Archaean: Consequences of pressure-release melting in a hotter mantle". Earth and Planetary Science Letters. 121 (1–2): 1–18. Bibcode:1994E&PSL.121....1V. doi:10.1016/0012-821X(94)90028-0.
- ^ 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
- ^ 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,
- ^ "Groundwater temperature's measurement and significance - National Groundwater Association". National Groundwater Association. 23 August 2015. Archived from the original on 23 August 2015.
- ^ "Mean Annual Air Temperature - MATT". www.icax.co.uk.
- ^ "Ground Temperatures as a Function of Location, Season, and Depth". builditsolar.com.
- ^ Rafferty, Kevin (April 1997). "An Information Survival Kit for the Prospective Residential Geothermal Heat Pump Owner" (PDF). Geo-Heat Centre Quarterly Bulletin. Vol. 18, no. 2. Klmath Falls, Oregon: Oregon Institute of Technology. pp. 1–11. ISSN 0276-1084. Archived from the original (PDF) on 17 February 2012. Retrieved 2009-03-21. The author issued an updated version Archived 2013-02-17 at the Wayback Machine of this article in February 2001.
- ^ 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.