Geochronology is a special branch of geochemistry,
related to isotope physics, that involves determining the time of
formation of rocks, minerals and fossils. Geochronologists examine
materials ranging in age from a few years to billions of years old,
mostly utilizing the principle that radioactive isotopes present
at the "birth", or formation, of a mineral will decay
at a certain fixed rate. Measurement of relative abundances of the
"parent" and "daughter" isotopes can determine
the age of the rocks.
One example of the use of radiogenic isotopes is the uranium-lead
(U-Pb) method of dating zircons. Zircon occurs in small quantities
in many crustal rocks and contains a small amount of U which decays
to Pb with a half-life (4,500 Ma) approximately equal to the age
of the Earth. By measuring the abundances and isotopic ratios of
U and Pb in zircon, the age of the rock can be determined with a
precision on the order of 2 to 5 Ma.