The calibration also produced a revised half-life for potassium-40, the radioactive isotope that decays into argon-40: 12.044 billion years, plus or minus 0.088 billion. That is twice as precise as the previous value derived from nuclear physics.

Why did Pliny's account matter so much?

Nearly 2,000 years ago, Pliny the Younger recorded the eruption that buried Pompeii and killed his uncle, Pliny the Elder. His writings preserved enough information about timing to give scientists an unusually precise historical anchor.

Graduate student Caroline Hasler examined the historical record and validated the August 24 date to within two months. Some historians had argued for a later, autumn date, citing a coin from Pompeii possibly minted in September. Hasler compared that coin with other Roman coins of the period and concluded it was probably minted earlier.

Where did the better samples come from?

A key breakthrough came from pumice collected in 1998 by co-author Andrea Marzoli of the University of Padua, from Oplontis, another Roman settlement buried by the eruption. The samples sat in storage, unanalyzed, for decades.

• The Oplontis pumice contained more potassium than earlier samples.
• It came from material expelled at the very start of the eruption.
• In stratovolcanoes like Vesuvius, potassium-rich magma sits near the top of the chamber and erupts first, settling at the bottom of ash deposits — making these samples ideal for dating.

A few years ago, graduate students Hasler, Anthony Fuentes and Andy Tholt, working with postdoctoral fellow Jack Carter in Renne's lab, proposed returning to the nearly 30-year-old material. Better samples, an advanced mass spectrometer and updated neutron irradiation methods together pushed uncertainty below 1% — a target the team had predicted was achievable back in 1997.

What can the improved clock do?

Renne said the refinement will let researchers infer causality between events in the geologic record more precisely — for example, linking a meteor impact with a mass extinction. About a decade ago, he used argon-argon dating to show that a meteor impact, massive volcanic eruptions in India and the dinosaurs' extinction all occurred within a few tens of thousands of years, about 66 million years ago.

The advance should also help calibrate other methods, including carbon-14 dating of organic materials younger than roughly 55,000 years and uranium-lead dating of rocks billions of years old. The team published a Bayesian scheme in 2025 to intercalibrate the two most important geochronometers, which Renne noted "have not been giving us consistent results over the years."

The practical stakes are current: the improved precision could sharpen eruptive histories of volcanoes that still threaten millions of people near Mexico City, Naples and Yogyakarta, Indonesia.

"It was really just a combination of better samples, instrumental advantage and a more concerted effort," Renne said. "All of those things came together."