Pompeii's Destruction Lets Scientists Date Volcanic Eruptions to Within Decades
Using Pliny the Younger's eyewitness account of Vesuvius as a benchmark, Berkeley researchers sharpened argon-argon dating to 0.4% accuracy and halved the uncertainty on potassium-40's half-life.
Nearly 2,000 years after Mount Vesuvius buried Pompeii, the eruption is helping scientists fix the ages of rocks with a precision that was once out of reach for anything so recent.
In a study published this week in Science Advances, researchers from the Berkeley Geochronology Center, the University of California, Berkeley and Italy's University of Padua calibrated argon-argon dating, one of geology's workhorse clocks, against the eruption's known date of Aug. 24, 79 CE. That date comes from Pliny the Younger, whose uncle, Pliny the Elder, died in the disaster and who left a firsthand written account of it. Measuring eight samples of sanidine, a potassium-bearing volcanic mineral from Vesuvius, the team dated the eruption to 1,938 years, give or take 13, before the minerals were analyzed in 2025. The true figure was 1,946 years. That is a precision of 0.7% and an accuracy of 0.4%.
"If you want to put together the eruptive history of a volcano in relatively recent time, precision and accuracy really count," said study leader Paul Renne, director of the Berkeley Geochronology Center. "The study shows that you can achieve that kind of highly useful precision and accuracy into the historical realm."
Argon-argon dating works because potassium-40 in volcanic rock slowly decays into argon-40, a gas that is normally absent from minerals when they crystallize. Scientists bombard samples with neutrons to turn a stable form of potassium into argon-39, then compare the two argon isotopes: the more argon-40 relative to argon-39, the older the rock. By anchoring the method to a date known from history, the team also measured the half-life of potassium-40 more precisely. It is now 12.044 billion years, plus or minus 0.088 billion, twice as precise as the previous value from nuclear physics.
The key samples had sat on a shelf for decades. In 1998, co-author Andrea Marzoli collected unusually potassium-rich pumice from Oplontis, another Roman town buried by the eruption. Because magma chambers under volcanoes like Vesuvius stratify, with potassium-rich melt at the top, that material erupts first and ends up at the bottom of the ash layers. Several years ago, graduate students Caroline Hasler, Anthony Fuentes and Andy Tholt, led by postdoctoral fellow Jack Carter, pulled out the 30-year-old samples and ran them on an improved mass spectrometer. Hasler also had to settle a historical argument: some scholars date the eruption to the autumn of 79 CE based on a coin found at Pompeii. After comparing it with other Roman coins, she concluded it was probably minted before September.
The payoff reaches well beyond Pompeii. More precise dates will help geologists reconstruct the histories of volcanoes that still threaten crowded cities such as Naples, Mexico City and Yogyakarta, Indonesia, and help cross-check radiocarbon and uranium-lead dating. "This lets us more precisely infer causality between events in the geologic record, for example a meteor impact structure and a mass extinction," Renne said. A decade ago, his team used the same method to show that the dinosaur-killing impact and massive volcanism in India occurred within a few tens of thousands of years of each other, 66 million years ago.
