Isotope preferred dating very old rocks
For an example, consider the potassium-argon dating method. Potassium-39 and potassium-41 are stable, but potassium-40 undergoes a form of decay that turns it to argon-40 with a half-life of 1,277 million years.
Thus the older a sample gets, the smaller the percentage of potassium-40, and conversely the greater the percentage of argon-40 relative to argon-36 and argon-38.
So we knew about "deep time," but exploring it was frustrating.
For more than a hundred years the best method of arranging its history was the use of fossils or biostratigraphy.
The work of geologists is to tell the true story of Earth's history—more precisely, a story of Earth's history that is ever truer.
The other method relies on actually counting the atoms of each isotope, not waiting for some of them to decay. It involves preparing samples and running them through a mass spectrometer, which sifts them atom by atom according to weight as neatly as one of those coin-sorting machines.
For instance, the uranium-to-lead decay cascade is really two—uranium-235 decays to lead-207 and uranium-238 decays to lead-206, but the second process is nearly seven times slower.
(That makes uranium-lead dating especially useful.) Some 200 other isotopes were discovered in the next decades; those that are radioactive then had their decay rates determined in painstaking lab experiments.
That only worked for sedimentary rocks and only some of those.
Rocks of Precambrian age had only the rarest wisps of fossils.
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Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves.