The atoms of some chemical elements have different forms, called isotopes.
These break down over time in a process scientists call radioactive decay.
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.
Some measure the decay of isotopes more indirectly.
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.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14.
This is different to relative dating, which only puts geological events in time Most absolute dates for rocks are obtained with radiometric methods.
These use radioactive minerals in rocks as geological clocks.
All radiometric dating methods measure isotopes in some way.