A Mt Taranaki eruption could send fast-moving pyroclastic flows packed with hot gas and ash across kilometres of surrounding countryside - and ash falls could spread even further.
That's according to a first-of-its-kind study, just published in prestigious journal Geological Society of America Bulletin, that sought to model how an eruption at the volcano might play out.
Mt Taranaki last erupted with a medium-sized ash eruption around 1755AD, although smaller volcanic events - including the formation of a lava dome in the crater and its later collapse - occurred over the 1800s.
Its most recent big blow happened around 1655AD, and today, the mountain is considered sleeping and likely to erupt again, with significant potential hazards from lahars, pyroclastic flows, lava flows, debris avalanches, floods and ashfalls.
According to estimates, more than 85,000 people live within 30km of the mountain - and 40,000 in high-priority evacuation areas - and there were threats to the region's dairy farming and petrochemical industries, including reticulated supply to the North Island.
In the new paper, Massey University's Rafael Torres-Orozco - along with Professor Shane Cronin, Natalia Pardo and Dr Alan Palmer, respectively of the University of Auckland, Massey and Colombia's University of Los Andes - investigated Mt Taranaki's 17 largest recent eruptions.
"This study used painstaking, fine-scale observations of deposits on the upper volcano flanks, mainly covering the last 5000 years of eruption," said Cronin, who currently directs one of New Zealand's national science challenges, dedicated to natural hazards.
"This is the first time such modelling of scenarios has been carried out at Taranaki - or indeed to such detail in all of New Zealand volcanoes."
"We have looked at broad scenarios of different scale for economic studies, or we have designed one-off scenarios for testing emergency response, but this is the first time we are able to make our own scenarios from such detailed volcano-deposit studies."
The work essentially revealed three main types of large eruption.
One scenario was characterised by blows from Fanthams Peak - the secondary cone on Taranaki's south side - and this was also the simplest, with a rapid onset and progression to a major eruption, mainly producing a widespread fall of tephra.
"The second type starts with the gradual growth of a lava dome at the volcano's summit, before this is blasted away in violent explosions — suddenly leading to hot pyroclastic flows and then a major eruption with widespread tephra fall," Cronin said.
The third type was a sputtering-style that alternated between slow dome growth, with bursts of explosive events that may be large, but short lived.
"This third type may last for many months and have slow onsets to large events and produces many destructive and deadly pyroclastic flows."
In every case, pyroclastic falls would cover the most populated areas, at 20 to 30km from the crater, with 10cm-thick deposits, while pyroclastic density currents could threaten farmlands and urban locations within 15km to 18km.
The researchers said these scenarios highlighted the major role that pyroclastic flows currently played in evaluations of volcanic hazards around Taranaki and other similar "andesitic" volcanoes.
"Identifying eruption scenarios are important to help Taranaki CDEM, GeoNet and volcano scientists to plan for the next major events at Mt Taranaki."
GNS Science monitors Taranaki with a web camera and nine seismographs, and regularly briefed local officials.
Mt Taranaki: five facts
• Its classically shaped symmetrical peak stands in isolation to the west of the Central North Island volcanoes. At 2518m, it is the second highest peak in the North Island after Ruapehu, and is New Zealand's largest mainland volcanic cone by volume.
• It's a stratovolcano - also called a composite cone volcano - made of layers of mostly andesite lava flows and pyroclastic deposits, or tephra. The summit crater is filled with ice and snow and has a lava dome in the centre. Volcanic debris from lahars and landslides covers the plains around the volcano.
• Past huge landslides have reached as far as 40km from the cone, with lava reaching 7kms and pyroclastic flows 15km from the vent. Volcanic ash has been weathered and mixed with the soil to produce rich, fertile farmland.
• Taranaki is the youngest, largest and only active volcano in a chain that includes the Kaitake and Pouakai Ranges, Paritutu and the Sugar Loaves. All these are now eroded remains of what were once large volcanoes.
• Taranaki began erupting about 130,000 years ago, with large eruptions occurring on average every 500 years and smaller eruptions about 90 years apart.