The atoms of some chemical elements have different forms, called isotopes.
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.
The table below shows characteristics of some common radiometric dating methods.
Geologists choose a dating method that suits the materials available in their rocks. Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock.
Imagine braving the desert heat for days or even weeks as you dig for dinosaur bones.