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White Island blew just as scientists were part-way through a million-dollar study investigating whether they could ever warn of such quick-fire, steam-driven eruptions.
Despite their devastating force – as dramatically shown at the Bay of Plenty volcano yesterday afternoon – hydrothermal, or phreatic, eruptions are almost impossible to predict.
AtWhite Island, hydrothermal explosions triggered by super-heated steam and gas could be triggered without warning.
Often, this steam and gas built up behind a rock and mineral seal, and when the strength of that seal was exceeded by the gas pressure, an explosive eruption was unleashed.
The gas driving yesterday's short-lived eruption likely came from a deeper source of magma, but the magma itself was not thought to have been directly involved.
"The expansion of water into steam is supersonic in speed - and the liquid can expand to 1,700 times its original volume," University of Auckland volcanologist Professor Shane Cronin said.
There was evidence to show that more than 60 of these blasts had occurred at New Zealand volcanoes within the last century.
Along with yesterday's episode, recent examples included the 2012 blows at Mt Tongariro's Te Maari Crater; the 2007 explosion at Mt Ruapehu that cost Auckland man William Pike his right leg; and a 2006 eruption on Raoul Island that killed conservation worker Mark Kearney.
"These are all volcanic systems that can produce sudden, steam-burst eruptions – and they're some of the most dangerous ones," Cronin said.
"In my mind, if there was ever going to be an event that caused multiple mortalities in New Zealand, it was going to be one of these – I just didn't think it would be as awful as the situation unfolding at the moment."
Thirteen people are believed killed and others are fighting for their lives with serious burns after the devastating White Island eruption - and experts are warning of more volcanic activity.
In a new Government-funded research programme, Cronin and his colleagues have been trying to tease out what processes might have driven them.
"We are basically creating mini-hydrothermal explosions in the lab, and we're also looking at what the most susceptible fields in New Zealand are to these events," he said.
"The hotspots we know of at the moment are White Island and the Mt Ruapehu Crater Lake; secondary sites like Mt Tongariro; and then to a lesser extent, Waiotapu and Waimangu, along with geothermal fields used for energy production."
At sites like White Island, the biggest challenge involved was not being able to directly monitor such factors as pressure build-up deep within the system.
Eruptions that rocked Mt Tongariro in 2012 were steam-powered. Photo / Supplied
"Unless you can do that kind of thing, you can't actually tell what's going on below ground," he said.
"What we're at least trying to do with these experiments is to identify some kind of pressure or temperature trigger point, so we can put more proactive monitoring systems around it."
At White Island, GeoNet monitored a range of parameters of volcanic activity.
It had been increases in sulphur dioxide gas, along with tremors, that led scientists to lift the Alert Level from 1 to 2 just ahead of the eruption.
In any case, scientists have pointed out the ever-present danger that exists at the volcano – with one calling it a "disaster waiting to happen".
Emeritus Professor Ray Cas, of Australia's Monash University said he'd visited the island twice and always thought it was too dangerous to allow daily tour groups onto.
Cas noted that many of the island's eruptions had been hydrothermal.
Another overseas scientist, Dr Jessica Johnson of the University of East Anglia in the UK, said the eruption was "unfortunate, but not completely unexpected".
Over the next 24 hours, scientists estimate an equal likelihood of either no eruption or a smaller or similar sized eruption that would impact the Main Crater floor.
"There is a high level of uncertainty associated with this estimate," GNS Science volcanologist Brad Scott said.
"We also estimate that the least likely scenario is a larger eruption."
