Potassium-argon dating

Potassium-argon or K-Ar dating is a method used by archaeologists and geologists to ascertain the date of ancient mineral deposits.

The potassium isotope ⁴⁰K decays to ⁴⁰Ar with a half-life of 1.3x10⁹ years. As argon is a gas any traces of that element will escape from rocks 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 ⁴⁰K to ⁴⁰Ar can be analysed and a numerical date since the last molten state can be assigned.

Due to the long half-life, the technique is most applicable in archaeology to Lower Palaeolithic sites older than 100,000 years and has been used at Olduvai Gorge in dating the strata created from ancient lava flows found above and below the archaeological deposits there. It has also been indispensable in other early east African sites with a history of volcanic activity such as Hadar, Ethiopia.

References

• K-Ar N. Mex. Geochron. Lab
• Potassium-Argon Dating Univ. Cal.