How can a rock tell you its age?

A rock tells its age through the radioactive atoms trapped inside it when it formed. Those atoms slowly turn into other elements, called daughter products, at a steady, known rate. Measure how much of the original atom is left and how much daughter has piled up, and you can work out how long the clock has been running. That's radiometric dating.

The rhythm of the clock is the half-life: the time it takes for half of a batch of radioactive atoms to decay. Nobody can predict when one atom will go, but a crowd of them behaves with clockwork regularity. Better still, for almost all such atoms the half-life is unaffected by heat, pressure or chemistry, so a clock buried deep in the Earth keeps the same time as one in a lab.

Geologists' favourite clock is a tiny crystal called zircon. When it grows, it happily takes in uranium but strongly rejects lead. So any lead found inside it later must have come from decaying uranium. Even better, each zircon contains two clocks: uranium-235 turning into lead-207 with a half-life of about 700 million years, and uranium-238 turning into lead-206 with a half-life of about 4.5 billion years. If both agree, the date is solid. Zircons from Jack Hills in Western Australia have been dated to 4.404 billion years, the oldest minerals known on Earth.

The method's champion was the English geologist Arthur Holmes, who dated a Norwegian rock at 370 million years while still a student, and later helped show that Earth is billions of years old.