For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes.
All radiometric dating methods measure isotopes in some way.
Most directly measure the amount of isotopes in rocks, using a mass spectrometer.
This is different to relative dating, which only puts geological events in time order.
Most absolute dates for rocks are obtained with radiometric methods.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14.
Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old.
Others measure the subatomic particles that are emitted as an isotope decays.
Some measure the decay of isotopes more indirectly.
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.
These are often characterised as the norm, rather than the exception.