A big Mt Taranaki eruption could launch devastating pyroclastic flows across a wider area than first thought - putting more people at risk.
A pyroclastic flow is a super-heated hurricane of gas, ash and rock particles that is fired from an erupting volcano and scorches everything in its path.
At Mt Taranaki and other volcanoes like it around the world, scientists have long assumed that these flows could spread only about 10km to 15km.
But a new study, led by University of Auckland researchers, has pushed this limit out to 25km, with serious implications for populated areas within the danger zones.
The new estimates put many Taranaki towns at risk, although the main areas of New Plymouth were protected by the edifice of the older Pouakai volcano.
Mt Taranaki was considered the most likely New Zealand volcano to cause national-scale impacts over our lifetimes, with a 50 per cent probability of an eruption in the next 50 years.
According to existing estimates, more than 85,000 people live within 30km of the mountain - and 40,000 in high-priority evacuation areas.
The fresh findings have been presented to the joint Taranaki Volcano Scientific Advisory Group and work was now underway to revisit local hazard plans.
The new study's leader, PhD student Geoff Lerner, was able to shed fresh light on this explosive history by applying methods of paleomagnetism – or the record of magnetic signals stored in volcanic rocks.
"Because of the difficulties of understanding what has caused the deposits far from volcanoes, scientists in the past at Taranaki had never been able to conclusively identify hot pyroclastic flow deposits," Lerner said.
"We tried a paleomagnetic method that no one had attempted at Mt Taranaki before, targeting some deposits with other characteristics consistent with hot flows."
When rocks cooled after being erupted from a volcano, they recorded a signal from the Earth's magnetic field at that time, leaving scientists some important clues about the direction the rocks had travelled in.
"If they tumble about after cooling, for example in a volcanic flood or lahar, the directions become jumbled," Lerner explained.
"If they travel while still very hot and are deposited together, such as within a pyroclastic flow, they then cool and all of the particles are magnetically lined-up.
"We can later test whether particles are lined up or not, which can tell us if the deposit was from a pyroclastic flow or not."
Using sophisticated lab equipment to heat and measure rock samples many times, Lerner and his colleagues were able to reconstruct the temperature at which the rocks were deposited.
"We found that the magnetic directions at several different sites between 15km to 20 km from the volcano did line up, which means that these were from pyroclastic flows."
The team also gathered new charcoal samples from the deposits for radiocarbon dating, which put them at around 11,500 years old – and linked them to one of Taranaki's biggest eruptions in 30,000 years.
"We estimate that such long pyroclastic flows are only possible in the largest 15 per cent of Taranaki eruptions, the sorts of events that occur on average every 1000 years."
Lerner's supervisor, Auckland University volcanologist Professor Shane Cronin, said a big large eruption would disrupt air and surface transport, tourism, farming, power and water supplies across the North Island.
Although an event could be expected in the relatively near future, the dormancy since Taranaki's last big blow, at around 1790, had been one of its longest.
The same research team recently revealed how Mt Taranaki was in its second-longest break between eruptions in more than 1200 years of records.
"Thus, we have no modern experience of its typically very long eruptions," Cronin said.
"Past research shows that once Mt Taranaki starts erupting, it continues for years, decades, or centuries."
The volcano began erupting about 130,000 years ago, with large eruptions occurring on average every 500 years and smaller eruptions about 90 years apart.
A recent estimate of the net losses in economic activity from a brief Taranaki eruption was crudely estimated at between $1.7 billion and $4 billion – or between $13 billion and $26 billion over a decade of volcanism.
Cronin, who is director of a national science collaboration aimed at making New Zealand more resilient to natural hazards, said a wider research programme at Taranaki was offering scientific insights that were also important for engineering and socio-economic issues.
"Using a novel integration of volcanic scientific knowledge, experimentation and advanced mathematical and economic simulation, we aim to radically cut down uncertainty that hinders decisive hazard and mitigation planning."
Cronin said the new study, about to published in the Geological Society of America Bulletin, had potential implications for other volcanoes in New Zealand and overseas.
Approximately 29 million people worldwide live within a 10 km radius of an active volcano, while 229 million live within a 30km radius.