Argon dating

First of all, the dating technique assumes that upon cooling, potassium-bearing minerals contain a very tiny amount of argon (an amount equal to that in the atmosphere).While this assumption holds true in the vast majority of cases, excess argon can occasionally be trapped in the mineral when it crystallizes, causing the K-Ar age to be a few hundred thousand to a few million years older than the actual cooling age.One out of every 10,000 Potassium atoms is radioactive Potassium-40 (K-40).These each have 19 protons and 21 neutrons in their nucleus.

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Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon.By comparing the proportion of K-40 to Ar-40 in a sample of volcanic rock, and knowing the decay rate of K-40, the date that the rock formed can be determined.Potassium (K) is one of the most abundant elements in the Earth's crust (2.4% by mass).For every 100 K-40 atoms that decay, 11 become Ar-40.The Washington Post article Scientists discover hundreds of footprints left at the dawn of modern humanity describes the geological dating of stratified layers of mud by analyzing and dating minerals within each layer.

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