Mt Taranaki could erupt with scant warning, scientists have found, raising a crucial need to understand what might next awaken the sleeping stratovolcano.
For more than two centuries, 2500m-high Taranaki has cut a dramatic but docile figure on the region’s skyline - yet it’s considered the most likely volcano to cause national-scale impacts in our lifetime, and is regularly monitored by GeoNet.
Based on an analysis of nearly 230 eruptions over the last 30,000 years, researchers have put the probability of a new Taranaki eruption at between one and 1.3 per cent each year.
While the probability is low, the risk is significant.
More than 85,000 people live within 30km of the mountain - with 40,000 in high-priority evacuation areas - and studies have pointed to Taranaki’s potential to unleash a wide range of eruptive effects, from lahars, lava flows and ashfall to debris avalanches and fast-moving pyroclastic surges.
One 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.
In a multi-million dollar new research effort, one of scientists’ most pressing questions has been why a quiet period has dragged on for so long ago at Taranaki, which began erupting around 130,000 years ago.
“Although Taranaki volcano has not erupted since about 1800 AD, it has been frequently active in the past, and the current quiet period is unusual,” University of Auckland volcanologist Associate Professor Phil Shane said.
Records from the past 30,000 years show its bouts have played out mostly in clusters, with few one-off bangs.
Scientists have found evidence for seven periods in the past few thousand years where tens to hundreds of individual eruptions unfolded, and the most recent episode, between 1000AD and 1790AD, included around a dozen events where ash was spewed across vast areas of the North Island.
Because none of Taranaki’s eruptions have been directly observed, Shane said, it fell to scientists to work out how much warning time we’d have ahead of its next burst.
In trying to solve this puzzle, they’ve turned to ancient “plagioclase” crystals grown within molten magma sent to the surface in past eruptions.
“As the crystals grow, microscopic patterns form just like tree-rings, but in response to changes in temperature, pressure, and magma composition beneath the volcano.”
Importantly, the crystals could indicate how much time it’d taken for magma to rise up through Taranaki’s innards.
What triggered activity in the first place, however, wasn’t clear.
While it’s typical for volcanoes to explode after fresh magma enters the system from beneath – something scientists attributed to Taupō's recent unrest – that hadn’t always been the case at Taranaki.
That left them to consider another primer – and one that came with potentially fewer warning signs – a build-up of volcanic gases.
In a new study, just published in the Bulletin of Volcanology, Shane and colleagues from the universities of Auckland, Otago and Paris Cité concluded this was a factor that urgently needed to be considered at Taranaki.
But, even in those events driven by rising magma – including those of the last millennium – Taranaki could shift to an eruptive state in as little as one day.
“Perhaps a cool magma body is residing beneath the volcano waiting to be re-heated into life at very short notice,” Shane said.
“But more extreme, we found that some eruptions had no sign of new magma being involved; this makes us wonder if there would have been any significant warning that an eruption was about to occur.
“We think several factors may be in play, and for some eruptions the build-up of volcanic gases from deep in the crust may have been sufficient to cause an explosion.”
At this point, he said, scientists could only speculate.
“We need to know more about the amount and types of volcanic gases, like carbon dioxide and sulphur dioxide, in previous eruptions so that we can work out what role they have played in triggering eruptions.”
With that ever-present risk in mind, scientists led by the University of Auckland’s Professor Shane Cronn have been working on a five-year, $13.7m MBIE-funded programme centred on Taranaki.
Useful advice on what to do during an eruption - and how to prepare - can be found on the National Emergency Management Agency’s website.