# What are two sources of heat in the earth's interior?

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Feb 2, 2016

Actually, there are three sources: (1) Primordial heat, (2) Gravitational contraction, and (3) Radioactive decay.

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(1) Primordial heat left over from the formation of the Earth: Much of earth's primordial heat, from when the planet first accreted and developed its core, has been retained. Additionally, the amount of heat that can arise through simple accretionary processes, bringing small planetesimals together to form the proto-earth, is large. Extraterrestrial collisions are still occurring today. The very large amount of kinetic energy inherent in these moving bodies is instantly converted to heat energy upon impact, thus providing a component to the earth's internal heat source.

(2) Gravitational contraction: In the early stages of accretion, the earth was much less compact than it is today. The accretionary process led to an increasingly greater gravitational attraction, forcing the earth to contract into a smaller volume, resulting in the conversion of gravitational energy into heat energy. Additionally, frictional heating was caused by denser iron-rich core material sinking to the center of the planet.

(3) Radioactive decay of isotopes in the mantle and crust: It involves the disintegration of natural radioactive elements inside earth. This process produces subatomic particles that zip away, and later collide with surrounding material inside the earth. Their energy of motion is converted to heat. In its early stages of formation, the earth had greater amounts of radioactive elements, but many of these are short-lived and have decayed to near extinction. Others with a more lengthy rate of decay are still undergoing this radioactive process, thus still releasing heat energy.

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