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Potassium-argon or K-Ar dating is a method used by archaeologists to ascertain the date of ancient mineral deposits.
The potassium isotope K40 decays to argon40 with a half-life of 1,300,000,000 years. As argon is a gas any traces of that element will escape from rocks only when they are molten. Therefore, any argon found in solid rocks must have been produced since that molten state ended and the rock solidified. The ratio of K40 to A40 can be analysed and a numerical date since the last molten state can be assigned.
Due to the long halflife, the technique is most applicable with Lower Palaeolithic sites older than 100,000 years and has been used at Olduvai Gorge in dating the strata created from ancient lave flows sandwiching the archaeological deposits there. It has also been indispensable in other early east African sites with a history of volcanic activity such as Hadar.
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