Nearly 2,000 years ago, Pliny the Younger documented what he witnessed when Mt. Vesuvius erupted, burying Pompeii and killing his uncle, Pliny the Elder. His account did more than preserve the story of one of history’s most famous volcanic disasters. It also recorded enough information about when the eruption occurred to give modern scientists an unusually precise historical reference point for testing volcanic dating techniques.
Researchers from the Berkeley Geochronology Center, UC Berkeley and the University of Padua in Italy now report in Science Advances that this historical benchmark has helped them significantly improve the precision of argon-argon dating. By calibrating the method against the inferred eruption date of Aug. 24, 79 CE, they have strengthened what is already one of the most widely useful tools for determining the ages of rocks and volcanic events.
The advance could help geologists, paleontologists and archaeologists assign more reliable dates to events in Earth’s past. That includes eruptions from volcanoes that continue to pose risks to heavily populated regions around Mexico City, Naples and Yogyakarta in Indonesia. The improved calibration could also help researchers verify other dating techniques, including carbon-14 dating of organic materials and uranium-lead dating of rocks that formed billions of years ago early in Earth’s history.
A More Precise Clock for Volcanic History
“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 describes how close a measurement is to the correct value, while precision reflects how consistently that measurement can be reproduced. To test the recalibrated method, the researchers examined eight samples of sanidine, a potassium-containing volcanic mineral from Vesuvius.
The argon-argon measurements placed the eruption 1,938 ±13 years before the minerals were analyzed in 2025. Based on the historical evidence preserved in Pliny the Younger’s writings, the minerals were actually 1,946 years old. That corresponds 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.
About a decade ago, Renne used argon-argon dating to establish precise ages for a meteor impact, major volcanic eruptions in India and the extinction of the dinosaurs. All three events occurred within a span of only a few tens of thousands of years about 66 million years ago. Those measurements bolstered the idea that the asteroid impact intensified volcanic activity, creating a one-two punch that contributed to the extinction of all non-avian dinosaurs.
Pliny’s Date Helps Refine a Radioactive Clock
Graduate student Caroline Hasler also closely examined historical records surrounding the Vesuvius eruption. Her work allowed the team to validate the August 24 date attributed to Pliny the Younger to within two months.
That historical constraint helped researchers improve their measurement of the half-life governing the decay of potassium-40 into argon-40, the process that underlies argon-argon dating. The revised half-life is 12.044 billion years, give or take 0.088 billion, making it twice as precise as the earlier value derived from nuclear physics.
Pumice From Oplontis Provides Better Samples
Renne and his colleagues had already used sanidine from pumice produced during the 79 CE eruption to test argon-argon dating. After their 1997 analysis, they predicted that future work could reduce the uncertainty to less than 1%.
They have now reached that target with the help of improved pumice samples, a more advanced mass spectrometer and updated neutron irradiation methods.
Argon-argon dating is based on the natural radioactive decay of potassium-40 in volcanic rock into argon-40. Argon-40 is generally not present in minerals before an eruption occurs. Researchers expose rock samples to neutrons, converting the non-radioactive potassium isotope potassium-39 into argon-39. They then measure the relative amounts of argon-40 and argon-39. A larger 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. These samples contained more potassium than those used previously and came from material released during the earliest part of the eruption.
Why the Earliest Pumice Matters
Magma beneath stratovolcanoes such as Vesuvius can separate into layers. Iron and magnesium tend to become concentrated near the bottom of a magma chamber, while more soluble elements such as potassium remain closer to the top.
Because of this structure, potassium-rich magma is generally among the first material expelled during an eruption. It therefore settles near the bottom of the resulting ash and pumice deposits. Marzoli’s samples came from these lower deposits, making them particularly valuable for argon-argon dating.
Despite their potential, the samples were placed in storage and remained unanalyzed for decades. Several years ago, graduate students Hasler, Anthony Fuentes and Andy Tholt, working with postdoctoral fellow Jack Carter in Renne’s laboratory, proposed returning to Marzoli’s nearly 30-year-old material to see whether it could improve upon the 1997 measurements.
The researchers had already published work aimed at reconciling argon-argon dating with uranium-lead dating, the two methods most widely used to determine the ages of rocks. Achieving a more precise calibration of argon-argon dating was an important part of that larger effort.
“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 measurements from 1997 were included in that earlier calibration, but their uncertainties were large enough that they had relatively little influence on the final result. The newly refined Vesuvius measurements carry much greater weight.
“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.
Reexamining the Date of the Vesuvius Eruption
Before the team could use the eruption as a dependable benchmark, Hasler needed to address a long-standing disagreement over exactly when Vesuvius erupted.
Some historians have proposed that the eruption occurred later in the fall of 79 CE. Their argument partly rests on a coin recovered from Pompeii that they believe could only have been produced in September.
Hasler compared that coin with other Roman coins from the same period and concluded that it was probably minted before September. The researchers nevertheless allowed for a two-month uncertainty around the eruption date. That uncertainty had little effect on the calibration of the dating technique itself, but it was important when determining the half-life of potassium-40.
Connecting Earth’s Different Dating Methods
Renne said the improved accuracy and precision of argon-argon dating should also make it especially useful for calibrating radiocarbon dating. Radiocarbon dating is the dominant technique for establishing the ages of organic materials such as wood when they 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.”
Along with improving calibration of the argon/argon dating method, Renne said the findings establish a new benchmark for how accurately the technique can determine the ages of relatively recent volcanic eruptions. The results show that argon-argon dating can now reach decadal accuracy for events that occurred within recorded history.
The work was funded by the National Science Foundation (2102788, 2030393), the Ann and Gordon Getty Foundation and the Berkeley Geochronology Center.
