The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.
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.
Variations in this data may point to errors anywhere in the process, which is why all the steps of preparation are recorded in detail.
K-Ar analyses cost several hundred dollars per sample and take a week or two.
Finally, the argon atoms are counted in a mass spectrometer, a machine with its own complexities.
Three argon isotopes are measured: Ar is determined by comparison to it.
These effects must be corrected, and the process is intricate enough to require computers.