We're unlikely to see another Taupo "super-eruption" within our lifetimes - but we still need to be prepared for the possibility. Photo / Ian Ashmore
We're unlikely to see another Taupo "super-eruption" within our lifetimes – but we still need to be prepared for the possibility, says a scientist behind a just-published analysis.
Lake Taupo essentially fills the hole left by one of those monster blows - the Oruanui eruption, more than 25,400 years ago, which spewed 530 cubic km of molten rock into the atmosphere and spread ash as far as Antarctica.
Taupo's most recent major episode - the devastating Hatepe event 1800 years ago - fired out more than 120 cubic km of material and obliterated the landscape surrounding the hidden caldera volcano.
The result of another one could be just as dramatic: scenarios explored in one recent EQC-funded study indicated Auckland could be blanketed in layers of ash centimetres thick.
Some scientists think of the whole area of volcanic activity between Kawerau and Tokaanu as one single but complex supervolcano system, which last saw a bout of unrest at Taupo in 2008.
Now, a new paper led by Mark Bebbington, a Professor in Geostatistics at Massey University, has looked at the probability of another Taupo eruption as large as the Hatepe event happening within the next 500 years.
Bebbington said Taupo and another vast caldera volcano - Okataina, near Rotorua - have been active in geologically recent times, with unrest every few decades and eruptions every few hundred years.
"These eruptions can be very small and short lived, or more rarely, enormous on a global scale."
Such "super-eruptions" are rare even in a global sense: but four of the 10 recorded in the past 2.8 million years have happened in the Central North Island.
Bebbington's study, published in Earth and Planetary Science Letters, ultimately aimed to find out whether we could use statistical models and existing past eruption records to forecast future eruptions.
An obvious challenge was, unlike with better-known cone volcanoes like Ruapehu and Taranaki, Taupo and Okataina weren't active enough to easily draw clear patterns.
"The main limitation is the limited number of eruptions, and also, larger eruptions tend to obscure evidence of previous eruptions."
Further, the data itself - largely encrypted in 25,000 years of geological, chemical and mineralogical records - was complex and difficult to analyse.
It called for an entirely new method to pick out patterns.
After developing and running a range of models, Bebbington selected one built from enough reliable data to make a detailed forecast.
Backing up findings of earlier studies, the new model put the annual probability of a Taupo eruption at any size at a very low chance of one in 800 – or at between 0.5 and 1.3 per cent within the next 500 years.
"So we're unlikely to see an eruption in our lifetimes," he said.
"On the other hand, the same could be said of winning Lotto, and sometimes the low probability comes up."
For the public, the big take-home was to be ready for anything.
"While you may be located far from a volcano, the ash from an explosive eruption could affect your area. You should also be familiar with natural hazard risks in general."
The study comes as New Zealand scientists, led by renowned Victoria University volcanologist Professor Colin Wilson, are working on an $8.2m, five-year study to clear up some of the wider uncertainties in risk posed by supervolcanoes.
Among more than 30 experts working on the Eclipse project, supported by a grant from the Government's Endeavour Fund, are a group of Italian scientists who have been researching Campi Flegrei, a major volcano near Naples that has behaved similarly to Taupo.
The team ultimately hope to create a new model allowing scientists to forecast whether unrest will lead to eruptions, to better estimate the size and timing of future events, and pinpoint how much warning might be given.
Bebbington noted earlier studies had concluded there was no simple relationship between sizes of eruptions and the time interval between them.
"Besides confirming this, the current study opens different windows of thought, showing that longer-term patterns - over thousands of years and tens of eruptions - can explain some of the uncertainty."
Meanwhile, more new research has widened what we know about Auckland's youngest volcano – and contradicted findings of an earlier study.
In 2016, a team of researchers who drilled into the offshore volcano concluded it was about 6000 years old - and may have begun erupting around that point.
That was in stark contrast to the long-held belief that Rangitoto is less than 700 years old and had erupted only once or twice, between 500 and 550 years ago.
The findings stemmed from radiocarbon dating of a layer of sedimentary material, indicating there had been 650 to 7400 years between basaltic lavas.
That volcanism at Rangitoto could have continued for so long raised potentially troubling questions about the risk posed to New Zealand's biggest city – and whether the eruptive behaviour of the Auckland volcanic field had changed.
But a new analysis which compared those samples to earlier ones retrieved from Rangitoto has effectively pulled the story of the volcano back to where it was.
Dr Lucy McGee, of the University of Adelaide, compared the isotopic compositions of Thorium and Radium between samples above and below the sedimentary material and another set, dated from at between 500 to 550 years ago, which she used in a 2011 paper.
Her report found the composition of the newer samples were either the same age, or up to 500 years older than her previously analysed samples – negating the claim that the volcano's lowermost lava could be 6000 years old.
"This shows we really need to do detailed studies at individual volcanoes to understand how they work, and the time scales that they operate on," she said.