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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 °C per km of depth (1 °F per 70 feet of depth) near the surface in most of the world.〔 Strictly speaking, ''geo''-thermal necessarily refers to the Earth but the concept may be applied to other planets. A line tracing the gradient through the planetary body is called a geotherm on Earth and other terrestrial planets. On the Moon it is called a selenotherm. The Earth's internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, and possibly heat from other sources. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. 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). 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,〔 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 not formed anymore today. == Heat sources == Temperature within the Earth increases with depth. Highly viscous or partially molten rock at temperatures between is postulated to exist everywhere beneath the Earth's surface at depths of , and the temperature at the Earth's inner core/outer core boundary, around deep, is estimated to be 5650 ± 600 kelvins. The heat content of the Earth is 1031 joules. * 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. * Heat of impact and compression released during the original formation of the Earth by accretion of in-falling meteorites. * Heat released 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 force on the Earth as it rotates; since rock cannot flow as readily as water it compresses and distorts, generating heat. * There is no reputable science to suggest that any significant heat may be created by electromagnetic effects of the magnetic fields involved in Earth's magnetic field, as suggested by some contemporary folk theories. In Earth's continental crust, the decay of natural radioactive isotopes has had significant involvement in the origin of geothermal heat. The continental crust is abundant in lower density minerals but also contains significant concentrations of heavier lithophilic minerals such as uranium. Because of this, it holds the largest global reservoir of radioactive elements found in the Earth.〔William, G. E. (2010). ''Geothermal Energy: Renewable Energy and the Environment'' (pp. 1-176). Boca Raton, FL: CRC Press.〕 Especially in layers closer to Earth's surface, naturally occurring isotopes are enriched in the granite and basaltic rocks.〔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.〕 These high levels of radioactive elements are largely excluded from the Earth's mantle due to their inability to substitute in mantle minerals and consequent enrichment in partial melts. The mantle is mostly made up of high density minerals with high contents of atoms that have relatively small atomic radii such as magnesium (Mg), titanium (Ti), and calcium (Ca).〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geothermal gradient」の詳細全文を読む スポンサード リンク
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