The most violent eruption of the last 5000 years was undoubtedly that of Taupo, a volcanic cauldron found in New Zealand. It’s still an active volcanic system, so the more we know about it the better — but the exact age of that eruption has proved to be a puzzle for scientists.
The most precise dating methods had pegged it as happening in or close to the year 232. A recent Nature Communications study, using a rather clever dating method, has revealed that this devastating eruption occurred potentially hundreds of years more recently.
Taupo, located on the country’s North Island, is today a picturesque lake. It was also the site of the Oruanui eruption around 26,500 years ago, which was so unfathomably explosive and debris-filled that it registered as an eight on the Volcanic Explosivity Index (VEI) scale, making this a veritable supereruption, and Taupo a bona fide supervolcano.
Taupo’s Hatepe eruption, the focus of this paper, came in at a VEI 7. This means it wasn’t a supereruption, but holy hell, was it a whopper.
From the geological layers it left behind, it’s clear that the eruption was 100 times bigger than the paroxysm at Mount St Helens in May 1980. Nearby areas were smothered in about 100m of pyroclastic flow deposits. Forests at least 30km away were annihilated.
The eruption also produced an ash column 50km high. This not only blanketed the entirety of New Zealand in 1cm of ash, but it drifted across the planet, and it has been thought to have turned the skies red in Ancient Rome and China.
Taupo produced twice the amount of ash as the 1815 Tambora eruption, which blanketed the sky in the Northern Hemisphere for so long that 1816 was known as The Year Without Summer.
Fortunately, people hadn’t yet arrived on New Zealand to witness the Taupo cataclysm. That’s quite the relief: Pyroclastic flows, on average, travel at 80km/h, and their tumbling caches of gas and ash can reach temperatures of 700C.
If you get caught in one of these, your muscles will rapidly contract, the water in your skin will suddenly boil, your internal organs will quickly fail, and — as evidenced by the victims of Pompeii and Herculaneum back in 79 CE — your skull might even explode.
It’s a quick but gory demise. Study co-author Ben Kennedy, an associate professor of physical volcanology at the University of Canterbury, said that if he and his students were in New Zealand at the time, they would have seen an incredible explosion and the jaw-dropping expanding ash cloud. Soon after, they “would all get barbecued up by the devastating pyroclastic flows”, and any nearby towns would be “instantly vaporised”.
Study co-author Brendan Duffy, a lecturer in applied geoscience at the University of Melbourne, said that “the North Island would not have been a happy place”. The only problem with people not being there to witness it is that getting a precise date on the eruption has proven difficult, too.
Radiocarbon dating of tree rings is a commonly used method. Carbon has various isotopes, determined by how many neutrons they have. Carbon-13 and carbon-12 are the most common on Earth, but a rarer type, carbon-14, is produced in the atmosphere when cosmic rays interact with it. This carbon-14 then falls to the ground where living things can suck it up, including trees.
Tree rings show year-by-year changes in atmospheric carbon-14. Alone, these carbon-14 values don’t mean much; fortunately, researchers have developed standard baselines that match up certain carbon-14 values to set calendar dates. That way, organic matter containing the isotope can be given precise ages. When it comes to trees, the final (outer) ring represents the moment at which it was killed by the eruption.
Alas, it’s never quite so simple. Changes in solar activity, and, more commonly, changes in carbon reservoirs on the ground, mean the concentration of carbon-14 isn’t constant, but alternates over time. This needs to be considered when conducting radiocarbon dating, because if it isn’t, you can’t calibrate the local changes with the standard baselines.
The team suspected that, due to the wide range of dates given for the Hatepe eruption, some carbon-14 fiddling had been afoot around Taupo. The older ages certainly didn’t seem right, as there was no environmental record of the eruption at sites that had been radiocarbon dated.
Then, the study’s lead author, Richard Holdaway, an adjunct professor of paleobiology and isotope analysis at the University of Canterbury, had an idea: What if the volcano itself was cooking the books?
Volcanic systems contain plenty of dissolved gases, including CO2. Even when a volcano is not erupting, this gas can make it into ground and surface water, which is eventually absorbed by trees. Those nearest the volcano would take in more than those farthest away.
This magmatic CO2 includes carbon-13 and carbon-12. The more of these isotopes a tree takes up, the more diluted its concentration of carbon-14 will be, and the older the dates will be.
By carefully calibrating their radiocarbon dates of eruption-slaughtered trees near and far from Taupo, the team confirmed their suspicions: Trees near the volcano suggested the eruption happened in 36 CE, whereas those many kilometres away suggested 538 CE. This trend was so clear to see on a map of the area that when he first saw it, Duffy said he “nearly choked”.
Correcting for this contamination, the team concluded that the eruption is in fact anywhere from several decades to over two centuries younger than previously thought.
Nico Fournier, a senior volcano geophysicist at GNS Science who wasn’t involved in the research, said this study is a “welcome improvement of the chronology of eruptions at Lake Taupo”. It also underscores the value of using trees far from the blast site that have experienced minimal contamination when dating ancient eruptions.
The exciting thing about this work is not that Taupo’s eruption age is off, but perhaps many other historic eruptions are too, Kennedy said. Indeed, their paper’s data already suggests that the dating of at least two volcanoes elsewhere in the world, including Baitoushan on the North Korean-Chinese border and Papua New Guinea’s Rabaul, are also skewed by similar contamination.
“This means that global climate-changing events attributed to volcanoes may be wrong or simply attributed to the wrong volcano,” he added.
This detective work isn’t just about the past. Trees are constantly collecting volcanic data, and as Holdaway told Gizmodo, this means we can monitor “what is happening in the deep magma chambers through time”.
Although volcanologists already monitor volcanoes for signs of worrying levels of CO2 degassing, this study highlights that the carbon-14 levels in nearby tree rings become decidedly off-kilter several decades before a sizeable eruption. Perhaps, then, trees could help us to forecast future eruptions, too.