Range of potassium argon dating
The site also must be geologically meaningful, clearly related to fossil-bearing rocks or other features that need a good date to join the big story.
Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.
What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
This technique allows the errors involved in K-Ar dating to be checked.
Argon–argon dating has the advantage of not requiring determinations of potassium.
With each increment in quality, more subtle sources of error have been found and taken into account.Thus, discretion and interpretation of age dating is essential.