These break down over time in a process scientists call radioactive decay.Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.
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.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14.
Because of their unique decay rates, different elements are used for dating different age ranges.
For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.
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.
This is different to relative dating, which only puts geological events in time order.
Most absolute dates for rocks are obtained with radiometric methods.