Dating method using volcanic ash
A clastic sedimentary rock is made up of older rock and mineral fragments, and when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them.
Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at different times.
It has a half-life of 1.3 billion years, meaning that over a period of 1.3 Ga one-half of the Figure 8.14 The decay of 40K over time.
Each half-life is 1.3 billion years, so after 3.9 billion years (three half-lives) 12.5% of the original 40K will remain.
This is determined by drawing a horizontal line from 0.95 to the decay curve line, and then a vertical line from there to the time axis.
[SE] K-Ar is just one of many isotope-pairs that are useful for dating geological materials.
The red-blue bars represent 40K and the green-yellow bars represent 40Ar.