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.
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.
This is different to relative dating, which only puts geological events in time order.
Most absolute dates for rocks are obtained with radiometric methods.