More than 85,000 people live within 30km of Mt Taranaki - 40,000 of who are in high-priority evacuation areas. Photo / Alan Gibson
Mt Taranaki's next blow may be a moderate-sized event like recent bangs - or something else entirely if its "unusually long" slumber gives way to a new eruptive era.
It might be surprising that it's Taranaki - not Ruapehu, Tongariro or rowdy White Island - that is the most likely volcano to cause national-scale impacts in our lifetime.
Scientists currently put the chance of an eruption from the 2500m stratovolcano at about 25 per cent within the next 30 years, and 50 per cent within the next century.
In a new study, University of Auckland scientists took a deep look at its last 1000 years of activity to search for any patterns that might play out again.
We know from the past 30,000 years of Taranaki eruptions that these mostly occur in clusters, with very few isolated events present in the geological record.
Over such long stretches, stratovolcanoes like Taranaki could burst to life in a range of dramatic ways.
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"For example, over thousands of years, they are often capable of a range of eruption types and hazards, with eruptions that build lava domes, have large or small eruptions, produce pyroclastic flows or lava flows or lahars," study leader Dr Geoff Lerner explained.
"But over shorter times like a hundred or 1000 years, a volcano might only produce some of these hazards."
The most recent eruption period, called the Maero Eruptive Period, lasted between 1000 and 1790 and featured a dozen large eruptions that built up the present northwestern flanks of the volcano we know today.
By recreating the era using a range of lab and field based methods, Lerner and his colleagues revealed 11 separate active spells.
The first eruption, in 960AD, came from Taranaki's summit - unlike the previous bang a century early, which began from Fantham's Peak.
After that, Taranaki tended to erupt in several main ways.
There were "sub-Plinian" events that threw up medium-sized ash clouds, and single and repeated lava dome collapses, as happened in the most recent blow in 1790.
There were also blast eruptions, like one in 1350 that explains the partly open shape of Taranaki's summit crater rim.
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As a result of that event, Lerner said, pyroclastic flows had been pointed in a northwest direction, indicating that the shape of the summit had a big influence on these hot surges of gas and rock.
But there were no signs of the large-scale "Plinian" eruptions that Taranaki had produced at other times over the past 30,000 years - and also very few lava flows.
"A statistical look at the sequence of the last 1000 years shows that the time between eruptive periods was typically 10 to 100 years," he said.
"Eruptions were sometimes single events, but other times continuing periods of multiple eruptions that could have lasted over months or years."
The current period of rest - now stretching to about 230 years - happened to be the longest gap so far.
"If we are still in the Maero Eruptive Period, then we would expect the next activity to be similar to what we have seen recently, with more small to medium eruptions," he said.
"However, because the time since the last eruption is unusually long, it is possible we might be into a new volcanic regime containing new types of activity."
Whatever happened, he expected that most pyroclastic flow activity would again be directed to the northwest - unless the eruption was big enough to transform the summit.
Ash from eruptions might also affect areas to the east and north of the volcano, he added.
The findings, published in the Bulletin of Volcanology, come as scientists have just begun an $13.7m, five-year research programme investigating what would happen if Taranaki woke to become a regularly-erupting volcano.
A recent estimate of the net losses in economic activity from a brief Taranaki eruption was crudely estimated at between $1.7b and $4b – or between $13 billion and $26b over a decade of volcanism.
More than 85,000 people live within 30km of the mountain - 40,000 of whom are in high-priority evacuation areas.