Radiometric dating

Radiometric dating works because radioactive isotopes decay into other elements at a steady, known rate. Each parent isotope has a half-life, the time it takes for half of a sample to turn into its daughter product, and this rate is not changed by heat, pressure or chemistry. By measuring the ratio of parent to daughter atoms in a sample, usually with a mass spectrometer, scientists can calculate how long the clock has been running. The result is an age in years, not just an order of events.

Different isotopes suit different time spans. Carbon-14, with a half-life of about 5,730 years, dates once-living material such as bone, wood and charcoal up to roughly 50,000 years old; organisms take in carbon-14 while they are alive, and the clock starts when they die. Older events need slower clocks, such as potassium-40 decaying to argon-40, or uranium-238 decaying to lead-206 with a half-life of about 4.5 billion years. Uranium-lead dating of tiny zircon crystals from Australia has shown that some minerals on Earth are nearly 4.4 billion years old.

Fossils usually lie in sedimentary rock, which cannot be dated this way, so paleontologists date layers of volcanic ash or lava above and below a fossil to bracket its age. Together with index fossils, these dates anchor the geologic time scale.