When Pliny the Younger recorded his first-hand experience of the explosion of Mt. Vesuvius 2,000 years ago — a cataclysm that buried Pompeii and killed his uncle, Pliny the Elder — he provided enough detail about the precise date to allow scientists to benchmark methods for dating volcanic eruptions.

In a study published today (Sept. 25) in the journal Science Advances, scientists from the Berkeley Geochronology Center, UC Berkeley and the University of Padua in Italy report that argon-argon dating calibrated with the inferred date of the eruption — Aug. 24, 79 C.E. — is now significantly more precise, making it one of the most versatile dating methods available.

The improvement will allow geologists, paleontologists and archaeologists to attach more accurate dates to geologic events of the past, including past eruptions of volcanoes that still threaten dense urban areas like Mexico City, Naples and Yogyakarta in Indonesia. It will also help ground-truth other dating methods, such as carbon-14 dating of organic material and uranium-lead dating of billion-year-old rocks from the early days of planet Earth.

“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, a Berkeley professor in residence of earth and planetary science and director of the independent Berkeley Geochronology Center. “The study shows that you can achieve that kind of highly useful precision and accuracy into the historical realm.”

Accuracy refers to whether a result is correct, whereas precision is a measure of how reproducible the result is. The recalibrated argon-argon dating method, used on eight samples of a potassium-containing volcanic mineral from Vesuvius called sanidine, pegged the eruption at 1,938 ±13 years prior to when the minerals were analyzed in 2025. The actual age of the volcanic minerals was 1,946 years, according to analysis of Pliny the Younger’s writings. The estimate is equivalent to a precision of 0.7% and an accuracy of 0.4%.

“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, Renne used argon-argon dating to provide precise dates for a meteor impact, volcanic eruptions in India and the dinosaur extinction — all of which took place within a few tens of thousands of years 66 million years ago. These precise dates bolstered the idea that the impact triggered intensified volcanism, providing a one-two punch that led to the extinction of all non-avian dinosaurs.

Thanks to a deep dive into the recorded history of Vesuvius’ eruption by graduate student Caroline Hasler, the team was also able to validate the eruption date attributed to Pliny the Younger — August 24 — to within two months. This enabled the team to more precisely measure the half-life of the decay of potassium-40 to argon-40 that provides the basis for argon-argon dating. That half-life is now 12.044 billion years, give or take 0.088 billion — twice as precise as the previous value determined from nuclear physics.

Pumice from Oplontis

Renne and his team had previously analyzed sanidine from pumice ejected during the 79 C.E. eruption to ground-truth argon-argon dating. Based on that 1997 analysis, they predicted that they could do better, getting the precision down to less than 1%. He and his team have now achieved that goal, thanks to better pumice samples, an improved mass spectrometer and updated neutron irradiation techniques.

Argon-argon dating relies on the natural decay of potassium-40 in volcanic rock to argon-40, which is not normally present in minerals prior to eruption. Scientists irradiate rocks with neutrons to turn a non-radioactive isotope of potassium, potassium-39, into argon-39, and then compare the relative amounts of the two argon isotopes. The more argon-40 relative to argon-39, the older the sample.

In 1998, co-author Andrea Marzoli of the University of Padua obtained new, more potassium-rich pumice samples from Oplontis, another Roman town buried in ash. Unlike previous samples, the new ones came from the earliest stage of the Vesuvius eruption. Magma chambers under stratovolcanoes like Vesuvius tend to stratify, with iron and magnesium concentrating at the bottom of the chamber while soluble elements like potassium remain at the top. As a result, potassium-rich magma comes out first and ends up at the bottom of ash and pumice deposits. Marzoli’s pumice came from these bottom deposits.

Those samples were shelved and never analyzed, but several years ago, graduate students Hasler, Anthony Fuentes and Andy Tholt led by postdoctoral fellow Jack Carter in Renne’s lab suggested pulling out Marzoli’s 30-year-old samples and trying to improve on the 1997 results. The team had previously published a reconciliation of the two dating methods most commonly used for rocks — argon-argon and uranium-lead — and precisely calibrating argon-argon dating was a key component of that work.

“They came up with a Bayesian scheme — published in 2025 — to intercalibrate these two most important geochronometers that we have, which have not been giving us consistent results over the years,” Renne said. The Vesuvius results from 1997 were included in this intercalibration, but were not precise enough to have much weight in the result. The new results change that.

“It was really just a combination of better samples, instrumental advantage and a more concerted effort. All of those things came together,” Renne added. He credits co-author Bill Cassata with a major role in developing the analytical strategy and data analysis.

Before they could begin, however, Hasler had to resolve inconsistencies in the historical record. Some historians think the eruption took place later in the fall of 79 C.E., based on a coin found at Pompeii that they say could only have been stamped in September. Hasler compared the coin with other contemporary Roman coins and concluded that it was likely made before September. Adopting a liberal two-month uncertainty in the timeline turned out not to be important in calibrating the dating technique, but was key in establishing the half-life of potassium-40.

Renne said that more accurate and precise argon-argon dating will be invaluable in calibrating radiocarbon dating, which is the dominant method of attaching dates to materials such as wood that are younger than about 55,000 years.

“We’re hoping to really unify as many geologic dating methods as we can by using the same mathematics, the same Bayesian approach, and just bringing more data, more raw observations into that mix,” he said. “But argon-argon dating is always going to be a standard — it’s going to be an important calibrant in that sense.”

Besides contributing to the calibration of the argon/argon dating method, he said, the study establishes a new benchmark for the ability of the method to date very recent eruptions with decadal accuracy.

The work was funded by the National Science Foundation (2102788, 2030393), the Ann and Gordon Getty Foundation and the Berkeley Geochronology Center.