Well, there are a few complications: Potassium 40 has a branched decay: only 1/9 of all K-40 atoms decay to Ar-40.
This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.
Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption.
Instead, other methods are used to work out a fossil’s age.
These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
The correction is simple: multiply the number of argon atoms by 9 to find the total number of potassium atoms that decayed.
Second, the resulting argon atoms are merely mechanically trapped in the crystal lattice, not bonded in any way.
A line whose slope is proportional to the age of the sample is called an isochron.
Any analyses with excess argon will plot off the isochron.
The heat from a volcanic eruption releases all the argon from the molten rock and disperses it into the atmosphere.
Argon then starts to re-accumulate at a constant rate in the newly formed rock that is created after the eruption.
Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.