One hundred years ago in June 1911 Arthur Holmes published the first determination of mineral ages on the basis of radioactive decay. It was known that uranium was radioactive, that helium and lead were produced as products of that radioactive decay, and that the age of a sample could be linked to its measured U/Pb ratio. The first ages ranged from 340 to 1640 Ma, providing critical evidence that the Earth was at least that old. These ages were obtained two years before the discovery of isotopes by Soddy in 1913.

The 100 years following those first determinations by Arthur...

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