Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14.
Each isotope is identified with what is called a ‘mass number’.
Most absolute dates for rocks are obtained with radiometric methods.
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.
Geologists often need to know the age of material that they find.
They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.