Two types of relative dating
These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.
Because of their unique decay rates, different elements are used for dating different age ranges.
Each isotope is identified with what is called a ‘mass number’.
When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.