Isotope dating method
With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same.If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column). First, its chemical structure likes uranium and hates lead.
This means the clock is truly set at zero when zircon forms.
Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable.
Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence.
The two cascades are different—235U becomes 207Pb and 238U becomes 206Pb.
What makes this fact useful is that they occur at different rates, as expressed in their half-lives (the time it takes for half the atoms to decay).