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Pompeii helps scientists check a geological clock because the historically recorded eruption of Mount Vesuvius gives them an approximate known age to compare with a laboratory measurement. Researchers analyzed sanidine crystals in volcanic pumice—not Pompeii’s buildings or victims—and used argon-argon dating to estimate the minerals’ age. UC Berkeley’s September 25, 2026 report says eight samples yielded an estimate of 1,938 ±13 years before analysis in 2025.
How can Pompeii help date Earth’s history?
The link is a calibration check: a historical account provides an age benchmark, and scientists ask whether a geological dating method returns a result close to it. Mount Vesuvius’s 79 C.E. eruption buried Pompeii and nearby settlements, leaving pumice whose minerals can be measured. The event’s date is not beyond historical dispute, so it is an approximate anchor rather than a perfectly exact timestamp.
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The researchers examined sanidine, a potassium-containing mineral in pumice from the eruption. They did not use argon-argon dating on the city’s architecture, artifacts, or human remains. The laboratory clock is radioactive potassium decay; Pompeii supplies the historical reference against which the clock’s result can be compared.
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- Start with volcanic sanidine. The team analyzed mineral crystals from Vesuvius pumice, including samples collected at Oplontis that Berkeley describes as coming from early deposits.
- Use radioactive decay. Potassium-40 naturally decays into argon-40. According to Berkeley’s account, argon-40 is not normally present in the minerals before the eruption.
- Irradiate the sample. In the laboratory, neutron irradiation converts some potassium-39 into argon-39.
- Compare argon isotopes. Researchers measure argon-40 relative to argon-39. That ratio is used to calculate an age estimate.
The team revisited sanidine from the 79 C.E. eruption, improving on a 1997 analysis with a better mass spectrometer and updated neutron-irradiation techniques, according to UC Berkeley. The report also says the Oplontis samples contained more potassium than earlier samples.
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What age did the researchers calculate?
UC Berkeley reports that eight sanidine samples produced an argon-argon age estimate of 1,938 ±13 years before analysis in 2025. The university’s report gives the historical age of the minerals, calculated from Pliny the Younger’s account, as 1,946 years. These are the study figures as reported by Berkeley; the Science Advances paper could not be independently inspected for this article.
Berkeley reports 0.7% precision and 0.4% accuracy. Precision describes how consistently measurements can be reproduced; accuracy describes how close a result is to the correct value. They are different qualities, and neither percentage means that every argon-argon date in every setting will have the same performance.
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How certain is the eruption date?
The researchers used August 24, 79 C.E., an inferred date supported by their reading of the historical record, while allowing a two-month uncertainty. Some historians have argued for a later autumn date, citing a coin found at Pompeii. Berkeley reports that graduate student Caroline Hasler compared the coin with contemporary Roman coins and concluded it was likely made before September. The team’s two-month allowance did not affect the dating-method calibration, the university says, but did matter to its estimate of potassium-40’s half-life.
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Berkeley describes the result as a new benchmark for dating very recent eruptions, with decadal accuracy. That is a claim about the benchmark reported for this work, not a guarantee for every rock, eruption, or period of Earth’s history. A stronger reference point may help researchers compare volcanic events and refine geological timelines.
The university also identifies possible uses beyond this particular Vesuvius test: more precise argon-argon measurements could help geologists, paleontologists, and archaeologists date past events, including earlier eruptions at volcanoes near cities. They may also help check other dating approaches. These are potential applications, not separate tests established by this study.
| Method | Material or age context described by Berkeley | How it relates to this study |
|---|---|---|
| Argon-argon | Volcanic sanidine in this study | The method calibrated against the historically dated eruption |
| Radiocarbon | Organic materials younger than about 55,000 years, as characterized in Berkeley’s report | Potentially could be checked against improved geological dating; not tested across those materials in this study |
| Uranium-lead | Very old rocks, as characterized in Berkeley’s report | Another method the report says could potentially be checked; not tested in this study |
Study details and figures here are attributed to UC Berkeley’s September 25, 2026 report. Berkeley quotes study leader Paul Renne, a professor in residence of earth and planetary science and director of the Berkeley Geochronology Center: “If you want to put together the eruptive history of a volcano in relatively recent time, precision and accuracy really count.”
The report also gives a refined potassium-40 half-life estimate of 12.044 billion years, with an uncertainty of 0.088 billion years, and says it is twice as precise as the previous value determined from nuclear physics. This figure depends on the historical timeline and the team’s treatment of its uncertainty; it is a related result, not the age of Pompeii or the eruption.
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