radiometric dating

Method of dating rock by assessing the amount of radioactive decay of naturally occurring isotopes. The dating of rocks may be based on the gradual decay of uranium into lead. The ratio of the amounts of ‘parent’ to ‘daughter’ isotopes in a sample gives a measure of the time it has been decaying, that is, of its age. Different elements and isotopes are used depending on the isotopes present and the age of the rocks to be dated. Once-living matter can often be dated by radiocarbon dating, employing the half-life of the isotope carbon-14, which is naturally present in organic tissue.

Radiometric methods have been applied to the decay of long-lived isotopes, such as potassium-40, rubidium-87, thorium-232, and uranium-238, which are found in rocks. These isotopes decay very slowly and this has enabled rocks as old as 3,800 million years to be dated accurately. Carbon dating can be used for material between 1,000 and 100,000 years old. Potassium dating is used for material more than 100,000 years old, rubidium for rocks more than 10 million years old, and uranium and thorium dating is suitable for rocks older than 20 million years.