Dictionary of Human Evolution and Biology

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Potassium-argon Dating

Chronometric (absolute) dating technique that exploits the decay of an isotope of potassium (40K) into calcium (40Ca) plus an inert gas, argon (40Ar); especially useful when volcanic eruptions occur, as the argon present in the lava at that time is given up as a gas; thus a zero event occurs where new basalt (lava) contains almost no argon; eventually 89% of the potassium atoms will become Ca and 11% Ar, with a half-life of 1.25 billion years, making the technique useful for dating strata deposited from 100 kya to the very oldest rocks known.

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