Researchers from the Berkeley Center for Geochronology, the University of California, Berkeley, and the University of Padua in Italy have used this historic benchmark to improve the accuracy of argon-argon dating. Calibrating the method against the estimated eruption date of August 24, 79 AD strengthens one of the most widely useful tools for determining the age of rocks and volcanic deposits.

The advance could help geologists, paleontologists, and archaeologists assign more reliable dates to events in Earth’s past. That includes volcanic eruptions that continue to threaten densely populated areas near Mexico City, Naples, and Yogyakarta in Indonesia. Improved calibration could also help researchers compare and validate other dating techniques, including radiocarbon dating of organic material and uranium-lead dating of rocks formed billions of years ago.

A more accurate clock for understanding volcanic history

“If we want to piece together the history of relatively recent volcanic eruptions, accuracy and precision are very important,” said study leader Paul Renne, a professor of Earth and Planetary Sciences at Berkeley and director of the independent Berkeley Center for Geochronology. “This study shows that it is possible to achieve that level of accuracy and precision in the historical period.”

Accuracy describes how close a measurement is to the correct value, while precision reflects how consistently a measurement can be reproduced. To test the recalibrated method, the researchers analyzed eight samples of sanidine, a potassium-bearing volcanic mineral from Mount Vesuvius.

Argon-argon measurements indicated that the eruption occurred 1,938 ±13 years before the minerals were analyzed in 2025. Historical evidence preserved in the writings of Pliny the Younger indicates that the minerals were actually about 1,946 years old. The result corresponds to 0.7% accuracy and 0.4% precision.

“This allows us to more accurately infer causal relationships between events in the geological record, such as meteorite impact structures and mass extinctions,” Renne said.

About a decade ago, Renne used argon-argon dating to establish dates for a meteorite impact, a massive volcanic eruption in India, and the extinction of the dinosaurs. All three events occurred about 66 million years ago, within a span of tens of thousands of years. The measurements support the idea that the asteroid impact intensified volcanic activity, creating a one-two punch that contributed to the extinction of all non-avian dinosaurs.

How Pliny’s date improves radioactive dating

Graduate student Caroline Hassler examined the historical record surrounding the eruption of Vesuvius. Her work enabled the research team to verify the August 24 date attributed to Pliny the Younger within a two-month range.

This historical constraint helped researchers improve their measurement of the half-life governing the decay of potassium-40 into argon-40, the process underlying argon-argon dating. The corrected half-life is 12.044 billion years, with an uncertainty of 0.088 billion years. That makes it twice as accurate as the previous value obtained from nuclear physics.

Why Oplontis pumice produced better results

Renne and his colleagues had previously used argon-argon dating on sanidine from pumice produced during the 79 AD eruption. After their 1997 analysis, they predicted that future studies could reduce uncertainty to less than 1%.

They have now achieved that goal using improved pumice samples, more advanced mass spectrometers, and modern neutron-irradiation methods.

Argon-argon dating is based on the natural radioactive decay of potassium-40 into argon-40 in volcanic rocks. Argon-40 is typically not present in minerals before an eruption. Researchers expose rock samples to neutrons, converting the nonradioactive potassium isotope potassium-39 into argon-39. They then measure the relative amounts of argon-40 and argon-39. A higher proportion of argon-40 indicates an older sample.

In 1998, co-author Andrea Marzoli of the University of Padua collected new pumice from Oplontis, another Roman settlement buried by the eruption. The samples contain more potassium than the material used in earlier studies and came from deposits released during the eruption’s early stages.

Why the oldest pumice deposits matter

Magma beneath stratovolcanoes such as Vesuvius can separate into layers. Iron and magnesium tend to concentrate near the bottom of the magma chamber, while soluble elements such as potassium remain closer to the top.

Because of this structure, potassium-rich magma is usually among the first material released during an eruption. It therefore settles near the bottom of the resulting ash and pumice deposits. The Marzoli samples came from these lower deposits, making them particularly valuable for argon-argon dating.

Despite their potential, the samples remained in storage and unanalyzed for decades. A few years ago, graduate students Hassler, Anthony Fuentes, and Andy Torto, working with postdoctoral researcher Jack Carter in Renne’s lab, proposed reexamining Marzoli’s material from nearly 30 years earlier to determine whether it could improve on the 1997 measurements.

The researchers had already published a study designed to harmonize argon-argon dating and uranium-lead dating, two of the most widely used methods for determining the age of rocks. More accurate calibration of argon-argon dating was a key part of that broader effort.

“They devised a Bayesian scheme to mutually calibrate these two most important Earth chronometers, and they published it in 2025, but they hadn’t had consistent results over the years,” Renne said.

The 1997 Vesuvius measurements were included in the initial calibration, but their uncertainties were large enough that they had relatively little influence on the final results. The new measurements are substantially more precise and therefore carry much more weight.

“It was really just a combination of better samples, instrumental advantages, and a more collaborative effort. It all came together,” Renne said. He credited co-author Bill Cassata with playing a major role in developing the analytical strategy and data analysis.

Reconsidering the date of the Vesuvius eruption

Before the team could use the eruption as a reliable benchmark, Hassler had to address long-standing disagreements over its exact timing.

Some historians argue that the eruption occurred late in the fall of 79 AD. Part of their argument is based on the belief that coins recovered from Pompeii were produced only in September.

Hassler compared the coin with other Roman coins from the same period and concluded that it was probably minted before September. Even so, the researchers allowed for an uncertainty of two months before and after the estimated eruption date. Although that uncertainty had little effect on the calibration of the dating method itself, it was important for determining the half-life of potassium-40.

Connecting methods for dating Earth’s history

Renne said the increased accuracy and precision of argon-argon dating should also be useful for calibrating radiocarbon dating. Radiocarbon dating is a powerful technique for determining the age of organic materials such as wood that are less than approximately 55,000 years old.

“We want to really integrate as many geological dating methods as possible by using the same mathematics, the same Bayesian approach, and incorporating more data and more raw observations into the mix,” he said. “But argon-argon dating will always be the standard. In that sense, it will be an important calibration standard.”

Along with improving the calibration of argon-argon dating, the findings establish a new benchmark for how accurately the technique can date relatively recent volcanic eruptions. The results show that argon-argon dating can achieve decadal accuracy for events that occurred during recorded history.

This research was funded by the National Science Foundation (2102788, 2030393), the Ann & Gordon Getty Foundation, and the Berkeley Center for Geochronology.