Radiometric dating

Atoms of the same element can have different numbers of neutrons; these forms are called isotopes. Some isotopes are unstable and transform spontaneously into other isotopes. The original isotope is called the parent and the product is called the daughter. A half-life is the time required for half of a given quantity of a radioactive isotope to decay. Since decay proceeds at a characteristic rate, scientists can estimate a sample's age from the measured parent-to-daughter proportions, using the appropriate decay equation and analytical measurements.

Different isotope systems are useful for materials and timescales. Uranium-lead methods, including uranium-238 decaying to lead-206, are used to date old rocks and minerals; potassium-argon and argon-argon methods are widely used in volcanic geochronology. Radiocarbon dating measures carbon-14 and is used mainly for once-living material on much shorter archaeological and geological timescales; it does not directly date most rocks. The isotope system chosen depends on the sample, its age, and the event a researcher wants to date.

A radiometric age usually records an event involving a particular mineral or material, such as crystallization, eruption, metamorphism, or exposure at Earth's surface. It does not automatically give the age of every rock or event nearby. Geologists therefore examine how a sample formed and whether later heating, alteration, or loss or addition of elements may have affected its isotope system. Results are commonly interpreted together with stratigraphy, fossils, paleomagnetism, and other geochronological evidence.

Radiometric dating helps assign numerical ages to geological events and constrain the boundaries of intervals in Earth's history. The International Commission on Stratigraphy maintains the International Chronostratigraphic Chart, whose numerical ages are subject to continuing revision as evidence and calibrations improve. The chart notes that in the Phanerozoic and Ediacaran, formal unit boundaries are defined by Global Boundary Stratotype Sections and Points rather than by numerical ages alone.