Scientists have reconstructed 350,000 years of volcanism across the North Island, to reveal evidence of hundreds of ancient eruptions that would cause widespread havoc if they happened today.
Their new study, just published in the Bulletin of Volcanology, marks the first analysis of its kind and puts the spotlight on smaller and more common blows than better known "super-eruptions" at the Taupō and Okataina calderas.
The Massey University scientists focused on the centre of the Taupō Volcanic Zone (TVZ), at the heart of the North Island, to build the first-ever "spatio-temporal" model of volcanic hazard of its kind for the region.
The model brought together a mass of data gathered from previous studies and terrain information, which was then fed into a geographic information system (GIS) database.
That allowed the scientists to explore patterns among 432 different vent sites from past eruptions - and to also tease out what influence seven large caldera volcanoes in the region had on them.
"This inventory has not only identified the location of these volcanoes, but also collated their most up-to-date information about their age, style of eruption, inferred duration of their eruptions and their chemical signatures," said the study's lead author, Dr Szabolcs Kósik, of Massey-based Volcanic Risk Solutions.
Based on known eruptions, these smaller blows occurred at an average rate of one to two events every 1000 years.
The two most recent ones happened within the Okataina caldera, near Rotorua.
The Kaharoa eruption, about 700 years ago, blasted ash from Mt Tarawera across north and east of the North Island, while the more famous Tarawera eruption in 1886 killed more than 100 people and destroyed the iconic Pink and White Terraces.
Kósik and his fellow researchers, professors Mark Bebbington and Karoly Nemeth, were taken aback that such a high number of small eruptions had played out over 350,000 years - a short period in geological terms.
"In the Auckland Volcanic Field, we've seen the birth of about 54 volcanoes over 250,000 years of course in an area of about 100 times smaller – still, the total number is surprisingly high," he said.
"We were also surprised that the spatial distribution of vents was impacted differently by the seven calderas in the region."
He said the smaller, more frequent eruptions in the area had been overlooked in the past, and the new insights put their risk in context with their "big brothers" – devastating caldera-forming events like Taupō's Oruanui eruption around 25,400 years ago, which essentially blew out the hole today filled by Lake Taupō.
The smaller bangs tended to spew less material into the atmosphere and unfolded over short periods – typically days to a month.
Sometimes, they proved to be one-off explosions that coated surrounding areas with relatively thin layers of tephra.
While these eruptions would "just slightly" modify the landscape we know today, they'd still be big enough to cause "significant" disruption, Kósik said.
"Volcanic impact could be on transportation routes such as the main logistic artery of the North Island - State Highway 1 - or the main trunk line," he said.
"Air traffic between major centres such as Auckland, Wellington, Palmerston North, Taupō or Napier would likely be under ongoing and unpredictable threats of ash fall.
"Major tourist hubs such as Taupō and Rotorua would also be affected strongly."
His team had a wider aim of pin-pointing those places where new volcanic vents could open, and saw the new study as a starting point.
"We recognised that far more refined data on the age pattern of these small volcanoes across the whole Taupō Volcanic Zone is needed in the future," he said.
"While we worked hard to include every location of volcanoes in our analysis, the level of information about their geology is uneven, and that needs to be smoothed by future geological mapping.
"We also learned that there have been many hidden, sub-surface events, whose records are buried under thick young ignimbrite sheets, sometimes more than 100m thick – and hence, we can't access them directly."
The new study comes amid a wave of major new projects in the wider TVZ - an area that stretches north-eastward from Taupō, out to the country's rowdiest volcano, Whakaari/White Island in the Bay of Plenty.
The $8.2m MBIE-funded ECLIPSE project aimed to understand the future risk of a Taupo "super-eruption", while another, supported by a Marsden Fund grant and led by Victoria University's Dr Jenni Hopkins, also sought to shed more light on the TVZ's fiery past.
While the zone is thought to have been active around 1.6 million years, unleashing some of the most catastrophic eruptions in the planet's history, current knowledge was still skewed toward the past 50,000 years.
This period was generally well preserved by the geologic record, yet more than 90 per cent of picture remained missing.