Isotope dating method
For more than a hundred years the best method of arranging its history was the use of fossils or biostratigraphy.That only worked for sedimentary rocks, and only some of those.
(Anne Marie has also prepared this worked-out example of radiocarbon dating.)Most decay processes of geologic interest are too slow for decay-counting methods.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 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.Will you consider a small gift to support this site?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 more true.For that we can thank radioactivity, discovered at the turn of the last century.
A hundred years ago, our ideas about the ages of rocks and the age of the Earth were vague. Judging from the amount of rocks there are, plus the imperceptible rates of the processes forming them—erosion, burial, fossilization, uplift—the geologic record must represent untold millions of years of time.
Rocks of Precambrian age had only the rarest wisps of fossils.
No one knew even how much of Earth history was unknown!
We learned that some elements undergo radioactive decay, spontaneously changing to another type of atom while giving off a burst of energy and particles.
This process happens at a uniform rate, as steady as a clock, unaffected by ordinary temperatures or ordinary chemistry.
(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.