Uranium series dating
The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.
So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.The straight line takes the zircons off the concordia. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched.The results from these zircons therefore plot along that straight line, establishing what is called a discordia. If a 1500-million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance.These methods — some of which are still used today — provide only an approximate spot within a previously established sequence: Think of it as ordering rather than dating.Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.