Earth’s Age Via Isotope
The heaviest elements are naturally radioactive. They readily shed to get down to lead, the heaviest steady-state metal.
For his 1948 dissertation project, American geochemist Clair Cameron Patterson determined the duration of Earth by measuring the age of meteorites found on the planet, using a technique developed by his academic mentor, American chemist Harrison Brown: counting lead isotopes in igneous rocks.
Patterson calculated Earth to be 4.55 billion years old, give or take 70 million years. The number stands still, though the margin of error has been whittled down to 20 million years.
