how is the Earth's surface classified on the basis of temperature gradient
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Answer:
Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth's interior. As a general rule, the crust temperature is rising 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 most of the world.[1] However, in some cases the temperature may drop with increasing depth, especially near the surface, a phenomena known as inverse or negative geothermal gradient. Strictly speaking, geo-thermal necessarily refers to Earth but the concept may be applied to other planets.
Temperature profile of inner Earth, schematic view (estimated).
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 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 (6,730 °C; 12,140 °F) and the pressure could reach 360 GPa (3.6 million atm).[3]