Radio dating techniques
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Their result of 212–230 million years did not agree with the age of the fossils (elephant, pig, ape and tools) so they rejected the date.
Tests by other scientists using paleomagnetism and fission tracks confirmed the lower date.
So by 1980 there was a new, remarkably concordant date for the KBS tuff, and this became the one that was widely accepted.
For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14.