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Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is about 25–30 °C/km (72-87 °F/mi) of depth near the surface in most of the world.[1] Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets.

The 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 isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[2] At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa (3.6 million atm).[3] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes 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,[4] resulting in larger temperature gradients within the Earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are no longer formed.[5]

Heat sources


Temperature within the Earth increases with depth.

  • Much of the heat is created by decay of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements mainly located in the mantle.[4][8][9]
  • Gravitational potential energy released during the accretion of the Earth.
  • Heat released during differentiation, as abundant heavy metals (iron, nickel, copper) descended to the Earth's core.
  • Latent heat released as the liquid outer core crystallizes at the inner core boundary.
  • Heat may be generated by tidal forces on the Earth as it rotates. The resulting earth tides dissipate energy in Earth's interior as heat.
  • There is no reputable science to suggest that any significant heat may be created by the Earth's magnetic field, as suggested by some contemporary folk theories.

In Earth's continental crust, the decay of natural radioactive isotopes makes a significant contribution to geothermal heat production.

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.

Heat flow


Heat flows constantly from its sources within the Earth to the surface.

The heat of the Earth is replenished by radioactive decay at a rate of 30 TW.[17] The global geothermal flow rates are more than twice the rate of human energy consumption from all primary sources.

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 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%.[10] 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.[10] 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.[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 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.[19] 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.

In areas of Holocene uplift and erosion (Fig. 1) the shallow gradient will be high until it reaches an inflection point where it reaches the stabilized heat-flow regime. If the gradient of the stabilized regime is projected above the inflection point to its intersect with present-day annual average temperature, the height of this intersect above present-day surface level gives a measure of the extent of Holocene uplift and erosion. In areas of Holocene subsidence and deposition (Fig. 2) the initial gradient will be lower than the average until it reaches an inflection point where it joins the stabilized heat-flow regime.

A long time before anything was known about the floors of the oceans a certain notable from the University of Sydney had been reporting a thermal gradient of about 30 °C per vertical mile in the mines[20] (ca 0.01864 Kelvin per meter) and an estimate of the underworld combining such a gradient with the Stephens measurement on Basalt[21] would find a rate of about (1.7 ⋅ 0.01864) 0.03 watts per square meter instead of the 0.06 W ⋅ m−2 like the one found in a 1971 Encyclopedia.[22] Before then it was actually measured like that three times over from a drilled hole at the Oak Ridge National Laboratory in Tennessee and it was 1963 when they reported it as 0.73±0.04/μcal/cm−2 sec.[23] That is 0.0305 ± 0.0017 W/m2. Now more recently most any unauthorized person may notice that the ice on top of Lake Vostok would have to have a very high thermal conductivity (for ice) to be conducting heat at the estimated planetary average of 0.06 watts per meter2 (1971 Encyclopedia) or 0.087 W ⋅ m2 (this article) through any temperature gradient that could be reconciled with the Wiki articles on Lake Vostok and Vostok Station where we find that the ice is about three and a half or four kilometers thick and the temperatures on the top where the station is have been getting around from about -31.9 °C to about -68 °C and the heat does not seem to be up to standard at that location either.[24]

The more recent estimates would more nearly fit a 1980 study of about 500 American wells showing most typical values of less than 30 °C/km and also commonplace values substantially higher than that throughout much of the country [Kron and Heiken 1980].

A variation in surface temperature induced by climate changes and the Milankovitch cycle can penetrate below the Earth's surface and produce an oscillation in the geothermal gradient with periods varying from daily to tens of thousands of years and an amplitude which decreases with depth and having a scale depth of several kilometers.[27][28][27]

If the rate of temperature increase with depth observed in shallow boreholes were to persist at greater depths, temperatures deep within the Earth would soon reach the point where rocks would melt.

See also


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