Geothermal gradient

Source: Wikipedia, the free encyclopedia.
Temperature profile of inner Earth, schematic view (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

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.

komatiites that are no longer formed.[7]

The top of the geothermal gradient is influenced by

mean-average ground temperature (MAGT) at a shallow depth of about 10-20 metres depending on the type of ground, rock etc.;[8][9]
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.

Heat sources

Earth cutaway from core to exosphere
Geothermal drill machine in Wisconsin, USA

Temperature within Earth increases with depth. Highly viscous or partially molten rock at temperatures between 650 and 1,200 °C (1,200 and 2,200 °F) 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 3,500 kilometres (2,200 mi) deep, is estimated to be 5650 ± 600

1031 joules.[1]
radiogenic heat from the decay of 238U and 232Th 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 lithophilic elements such as uranium. Because of this, it holds the most concentrated global reservoir of radioactive elements found in Earth.[19] Naturally occurring radioactive elements are enriched in the granite and basaltic rocks, especially in layers closer to Earth's surface.[20] 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).[19]

Present-day major heat-producing nuclides[21]
Nuclide Heat release

[W/kg nuclide]

Half-life

[years]

Mean mantle concentration

[kg nuclide/kg mantle]

Heat release

[W/kg mantle]

238U 9.46 × 10−5 4.47 × 109 30.8 × 10−9 2.91 × 10−12
235U 56.9 × 10−5 0.704 × 109 0.22 × 10−9 0.125 × 10−12
232Th 2.64 × 10−5 14.0 × 109 124 × 10−9 3.27 × 10−12
40K 2.92 × 10−5 1.25 × 109 36.9 × 10−9 1.08 × 10−12

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 of the convecting mantle.[citation needed]

Heat flow

Main article: 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 (4.42 × 1013 Watts).[22] Mean heat flow is 65 mW/m2 over continental crust and 101 mW/m2 over oceanic crust.[22] This is 0.087 watt/square metre on average (0.03 percent of solar power absorbed by Earth[23]), but is much more concentrated in areas where the lithosphere is thin, such as along mid-ocean ridges (where new oceanic lithosphere is created) and near mantle plumes.[24] 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 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.[22][25]

The heat of Earth is replenished by radioactive decay at a rate of 30 TW.[26] 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 IASPEI/IUGG.[27]

Direct application

Heat from Earth's interior can be used as an energy source, known as

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.[1]

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

climatic changes are recorded in boreholes throughout Poland, as well as in Alaska, northern Canada, and Siberia
.

In areas of Holocene

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

polar ice caps flowing along ocean bottoms tends to maintain a constant geothermal gradient throughout Earth's surface.[29][dubious – discuss][verification needed
]

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

adiabatic gradient associated with mantle material (peridotite in the upper mantle).[31]

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.[32][33][14]

Negative geothermal gradients may also occur due to deep aquifers, 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.[34]

Negative geothermal gradients are also found at large scales in subduction zones.[35] 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.[35]

See also

References

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  11. ^ "Mean Annual Air Temperature - MATT". www.icax.co.uk.
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  14. ^ a b 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.
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  17. ^ Anuta, Joe (2006-03-30). "Probing Question: What heats the earth's core?". physorg.com. Retrieved 2007-09-19.
  18. ^ Johnston, Hamish (19 July 2011). "Radioactive decay accounts for half of Earth's heat". PhysicsWorld.com. Institute of Physics. Retrieved 18 June 2013.
  19. ^ a b c d William, G. E. (2010). Geothermal Energy: Renewable Energy and the Environment (pp. 1-176). Boca Raton, FL: CRC Press.
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  27. ^ www.ihfc-iugg.org IHFC: International Heat Flow Commission - Homepage. Retrieved 18/09/2019.
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  33. ^ 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).
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"Geothermal Resources". DOE/EIA-0603(95) Background Information and 1990 Baseline Data Initially Published in the Renewable Energy Annual 1995. Retrieved May 4, 2005.

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