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Building on this, the general concept is radiometric dating, which can rely on any of numerous decay chains, with half-lives ranging from decades to 100s of billions of years. Specific decay chains are effective clocks for differing lengths of time. Radiocarbon dating is often used on recent human or organic artefacts or remains, within the past 50,000 years or so as noted. It was uranium-lead decay which first demonstratively showed that the Earth was not "young", whether the few thousand years of theological textual analysis or the 30--300 million years that various geological estimates had suggested, but at least 1 billion years old, established by Ernest Rutherford in the early 20th centuries. The present value of 4.54 +/- 0.05 billion years was arrived at by the 1950s, based on the ratios of lead to uranium within zircon crystals, the latter of which cannot form in the presence of lead, and hence, any lead present necessarily being the result of radioactive decay. The measurement process also demonstrated the extent to which modern industrial activity was creating pervasive lead contamination virtually everywhere, largely the result of leaded gasoline, one of Thomas Midgley's lethal legacies. More generally, geochronology is based on numerous methods of which radiometric dating is but one, though a quite powerful one. <https://en.wikipedia.org/wiki/Radiometric_dating> <https://en.wikipedia.org/wiki/Age_of_Earth> <https://en.wikipedia.org/wiki/Geochronology> |