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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 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.

The answers he brings may be different from anything you’ve heard before.

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).

This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life.

The oldest zircon yet found dates from 4.4 billion years ago.

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