Resetting a clock from Earth’s rocks

A refinement in a widely used technique for determining the age of ancient rocks

opens up the possibility that Earth may have formed a crust as many as 200 million

years earlier than geologists thought.

Scientists can estimate the age of rocks by measuring the proportions of certain

radioactive isotopes in them. Most of these isotopes have a half-life–the amount

of time it takes for half of the unstable element to decay–between 100,000 and 1

trillion years, says Erik E.