Radiometric dating; all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes. Age in years for geologic materials are calculated by measuring the presence of a short-life radioactive element, e.g. carbon-14, or by measuring the presence of a long-life radioactive element plus its decay product, e.g. potassium-40/argon-40. See the USGS publication "Geologic Time" for more details.