Geochronology
Uranium-lead dating.  This technique measures the ratio of two lead isotopes (lead-206 and lead-207) to the amount of uranium in a mineral or rock.  Often applied to the trace mineral zircon in igneous rocks, this method is one of the two most commonly used (along with argon-argon dating) for geologic dating.  Uranium-lead dating is applied to samples older than about 1 million years.