The basic logic behind radiometric dating is that if you compare the presence of a radioactive isotope within a sample to its known abundance on Earth, and its known half-life (its rate of decay), you can calculate the age of the sample.

What is half-life and how does it relate to radiometric dating?

A useful application of half-lives is radioactive dating. It takes a certain amount of time for half the atoms in a sample to decay. It then takes the same amount of time for half the remaining radioactive atoms to decay, and the same amount of time for half of those remaining radioactive atoms to decay, and so on.