8.5 How hot is the interior of Earth?

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Geothermal gradients measured near the surface are higher than in the interior. Temperature is ~1000-1200oC at 100-150 km depth, perhaps 3000-4000oC at the core-mantle boundary, and greater than 5000oC in the inner core.
Heat sources within Earth include (a) radioactive decay of elements found in the mantle and crust and (b) crystallization of iron in the core.
The shape of the geothermal gradient relates to (a) the loss of heat at the surface of the hydrosphere, atmosphere, and space; (b) heating of the lower mantle by the core; and (c) increasing temperature with compression of rock at high pressure.