At least 16 people have died, two are still missing, and two dozen were injured after one of New Zealand's most active volcanoes erupted on December 9.
Over the last month, New Zealand's geological monitoring agency had raised the volcanic-alert level on White Island, where the eruption occurred.
But according to experts, a blast like this one can't be easily predicted, since it was driven by steam, not magma.
New Zealand's most active volcano erupted on December 9, sending plumes of ash, volcanic rock, and scalding steam more than 12,000 feet into the air over White Island.
That debris then rained down onto unsuspecting tourists on the island, also called Whakaari, with fatal consequences. Sixteen people were killed, two are still missing, and two dozen were hospitalized with burns and injuries. The victims include New Zealanders and tourists from the US, China, Australia, Britain, and Malaysia, who were visiting the island from a cruise ship, the Associated Press reported.
Reconnaissance rescue flights over the island following the eruption found "no signs of life at any point," Prime Minister Jacinda Ardern said on December 10, according to the AP.
Barbara Barham, mother and mother-in-law of two of the blast victims, told the Washington Post that she was livid.
"There's been warnings about it … My son-in-law never would have booked the excursion if he knew there was any chance of them being injured," Barham said.
She's not the only person wondering why tourists were allowed anywhere near the island if such an eruption was a possibility.
But according to Shane Cronin, an Earth scientist at the University of Auckland, White Island is one of several volcanoes in New Zealand with a known potential to produce sudden explosive eruptions at any time. And unfortunately, they're very hard to predict.
"We don't normally see these eruptions coming, no matter how much we would like to," he wrote in The Conversation.
There were some warning signs
According to New Zealand's geological monitoring agency, GeoNet, the eruption was "an impulsive, short-lived event."
Ken Gledhill, a technical advisor at GeoNet, told the AP that "in the scheme of things, for volcanic eruptions, it is not large. But if you were close to that, it is not good."
According GeoNet, the volcanic-activity level on the island had been steadily increasing since October. One month ago, scientists raised the island's "Volcanic-Alert Level" from 1 to 2, citing continuous volcanic unrest and increases in activity in the crater.
"Hazards on the island are now greater than during the past few weeks," the report said.
Less than a week before the eruption, GeoNet reported that "substantial gas, steam, and mud bursts" were observed at one of the volcanic vents in the crater. But even then, the alert level remained at level 2 (out of a possible 5), only indicating "moderate to heightened volcanic unrest."
Tours at the private island reserve continued as usual. On the day of the eruption, 47 people were on White Island, authorities reported.
But it isn't possible to predict an eruption like this
The reason the White Island volcano is unpredictable is that the magma under this volcanic crater — which is in part filled by a lake — is very close to the surface.
So heat and gas from that magma can "suddenly and with little to no warning" release the super-hot water trapped in the pores of the rocks on the crater floor, Cronin said.
"Any external process, such as an earthquake, gas input from below, or even a change in the lake-water level can tip this delicate balance and release the pressure on the hot and trapped water," he added.
The resulting eruption, called a hydrothermal or phreatic eruption, is caused by steam, not magma. That makes it a lot harder to track with current volcanic monitoring systems.
A phreatic explosion happens when trapped water expands and turns into steam at supersonic speeds. The liquid can expand to 1,700 times its original volume, with catastrophic impacts. The steam-driven eruption is enough to "shatter solid rock, excavate craters, and eject rock fragments and ash" up to a distance of two miles, Cronin wrote in The Conversation.
He added that monitoring potential hydrothermal eruptions is challenging because the triggers for such events are poorly understood. And once such a trigger occurs, pre-warnings "are likely in the order of seconds to minutes," he wrote.
The best way to predict these events, according to Cronin, is to track potential vapor and liquid pressure under the crater during times of non-activity and identify patterns in how such volcanoes behave.
"Unfortunately there are no simple rules that can be followed and each hydrothermal system is different," Cronin said.
The last time the White Island volcano erupted was in April 2016. At that time, the volcanic-alert status was at level 1 — the lowest on the scale.
Other recent phreatic eruptions included a blast at Hawaii's Kilauea that sent a plume of ash 30,000 feet into the sky last year, and a 2014 eruption at Mount Ontake in Japan that killed more than 30 people.
'Stepping foot onto a live volcano'
Tour operators make the final decision about whether to take visitors to the privately owned White Island, where access is controlled through permits, according to the New Zealand Herald. Nearly 10,000 visitors a year travel to Whakaari.
The island's major tour company, White Island Tours, promises "amazing colors and the sense of awe you get by stepping foot onto a live volcano." According to the company's website, boats travel to the island "every day that the weather will let us, except for Christmas Day."
But White Island Tours does warn that "passengers should be aware that there is always a risk of eruptive activity regardless of the alert level."
That alert level often fluctuates without any eruption on White Island , according to Brad Scott, a volcanologist with research group GNS Science. He told the AP that there had not been any major problems with tourists visiting the island in the past, though there had been some close calls.
Cronin warned that following the December 9 eruption, more volcanic activity could be observed.
"The eruptions are short-lived, but once one happens, there is a high chance for further, generally smaller ones as the system re-equilibrates," he wrote.
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