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).

If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept.

In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1).

In a rock twice as old there will be one 235U atom left for every three 207Pb atoms (Pb/U = 3), and so forth.

By Andrew Alden 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.

Uranium comes in two common isotopes with atomic weights of 235 and 238 (we'll call them 235U and 238U).

Both are unstable and radioactive, shedding nuclear particles in a cascade that doesn't stop until they become lead (Pb).

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).

The 235U–207Pb cascade has a half-life of 704 million years and the 238U–206Pb cascade is considerably slower, with a half-life of 4.47 billion years.

So when a mineral grain forms (specifically, when it first cools below its trapping temperature), it effectively sets the uranium-lead "clock" to zero.