There are certain radioisotopes that occur in living things in a fixed ratio with stable isotopes of that element while the organism is alive because that is the ratio in the environment it is living in.Once the organism dies it decouples from its environment and no longer maintains that ratio.a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample.Radioactive dating uses the decay rates of radioactive substances to ...Potassium-40 is a radioactive isotope of potassium that decays into argon-40.Every radioisotope has a half life, in which half of it decays to another isotope (radioactive or not).This works best on igneous rock because it measures the time the crystals have been solid.
Though still heavily used, relative dating is now augmented by ...
By measuring the amount of the stable isotope in the fossil, using the known ratio, one can compute the original amount of the radioisotope then measuring the amount of the radioisotope still left it is possible to use the half life to compute the time that has passed since the organism died.
In more complex situations where no such ratio is known to exist more complex methods involving 2 or more radioisotopes can be used.
I will not visit these, a textbook on Radiochemical Dating will go into the detail needed to understand them.
Another technique used to date rocks that contain no fossils involves measuring the degree of alpha particle damage to individual crystals of the rock.