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Moment that mattered: The Hunga Tonga-Hunga Ha’apai volcano erupts

An image from Nasa’s GOES-17 satellite showing the eruption of the Hunga Tonga-Hunga Ha’apai volcano from above

An image from Nasa’s GOES-17 satellite showing the eruption of the Hunga Tonga-Hunga Ha’apai volcano from above. Photo: Nasa

In June 2017, professor Shane Cronin published an article in the journal of the American Geophysical Union, entitled ‘New Volcanic Island Unveils Explosive Past.’ It was an analysis that he and his colleagues had carried out of an undersea volcano 65 kilometres north of Tonga after significant but limited eruptions took place there in 2014. The evidence they gathered suggested that the Hunga Tonga-Hunga Ha’apai volcano, which belongs to a chain that stretches from Fiji to New Zealand, had undergone ‘huge explosive eruptions’ in the past. “At that point we were sort of saying, ‘okay, this volcano is capable of doing some very big things’,” says Cronin.

When the volcano began erupting again at the tail end of 2021, however, Cronin didn’t think it would result in anything significant. “We were watching it closely, but we were of the view that it was going to be similar to the pattern of the previous eruptions [in 2014],” he says. “The eruption started on December 20th 2021, and then by about 5th January 2022 it had pretty much decided to stop.”

The volcano threw a cloud of dust, water vapour and gas half the size of France 55 kilometres above the surface of the Earth”

And then, just after 5pm local time on Saturday 15th January, the volcano began what would become the most powerful eruption the world had seen since Krakatoa 140 years ago. Hunga Tonga-Hunga Ha’apai exploded with an intensity described by Nasa as “hundreds of times more powerful than the atomic bomb dropped on Hiroshima,” throwing a cloud of dust, water vapour and gas half the size of France 55 kilometres above the surface of the Earth. The pressure wave it created went on to circle the planet four times over the following week. Up on the outer edges of space it generated 450-mile-per-hour winds and briefly disrupted the equatorial electrojet (the stream of electric currents in the ionosphere), flipping its course from east to west.

Part of the reason that Cronin and his fellow volcanologists were taken by surprise by the 15th January blast was that, despite Tonga being in a region of high volcanic activity, the nation had no early warning systems in place. “At the time there was not a single sizeable seismograph working in Tonga itself,” says Cronin ruefully. “The nearest one was 750 kilometres away in Fiji. Frustratingly, there were eight seismometers in Tonga, but they hadn’t been installed.”

When the eruption was taking place, Cronin was at home in Auckland, 2,000 kilometres away. “I’d been in touch with Tonga, I knew what was going on,” he says. He saw live video on social media of a tsunami hitting Tonga and ash starting to fall. Within half an hour of the eruption, however, the force of the volcano cut Tonga’s domestic internet cable. An hour later the international cable that connects it to the rest of the world also went down. “It broke at about the time at which the big plume [of hot ash and gas] had already started tracking over Tonga,” says Cronin. “Everything went dark, we had no information at all from the eruption.”

As Cronin frantically tried to track down satellite data about what was going on, reports of sonic booms came in from around the planet. “People ended up hearing these booms all the way up to Alaska,” says Cronin. “And then there was a strange tsunami that came down to New Zealand and damaged a whole bunch of boats in a small harbour north of here.” The effects were felt as far away as northern Peru, where high waves caused by the eruption left two people drowned.

Destroyed beach resorts in the Hihifo district of Tongatapu

Destroyed beach resorts in the Hihifo district of Tongatapu. Photo: Mary Lyn Fonua / Matangi Tonga / AFP

Cronin met with the other members of the New Zealand Volcano Science Advisory Group to pool their limited knowledge and try to work out what they thought might be happening on and around Tonga. “We were thinking there were going to be hundreds if not thousands of people killed by the scale of this event,” he says. “Then about 12 hours later there were a few people with satellite phones that were able to get through. And we were astonished that at that time there was only one confirmed casualty and two other suspected casualties.” The final death toll in Tonga still stands at three.

As a scientist, Cronin’s biggest priority was to get his hands on some ash. “I asked the New Zealand Defence Force, which was flying up there for a relief mission, to collect some off the side of the runway,” he recalls. Within a week of the eruption they sent him three little bottles of ash that he then distributed to laboratories across New Zealand for analysis. “We were in Covid lockdown, but we managed to get permission to come into the lab, so we rushed around and got the first information back on health and agricultural assessments within a week.”

There are bits of building material and cars wrapped around trees like tin foil. Everything has been completely wiped out”

On Tuesday 8th March Cronin flew out to Tonga himself, to gauge the fallout from the eruption and build a timeline of how it had unfolded. On the Kanokupolu peninsula, located on the west of the main island of Tongatapu, he saw the devastation it had wrought. “There were a series of little beaches and resorts and now it’s basically like one long stretch of beach,” he says. “All the trees have been completely stripped off, all the resorts have been stripped down to concrete foundations and there are bits of building material and cars wrapped around trees like tin foil. Everything has been completely wiped out.” Cronin could see that the waves had reached as high as 18 metres and had crossed the entire peninsula in places.

As part of his investigations, Cronin snorkelled over the underwater volcano, diving down to collect rock samples and using a depth sounder to see how much deeper the caldera (the hollow at its top) had sunk into the ocean after the eruption. After becoming frustrated at the lack of progress by international teams in charting the caldera, he had ocean-mapping equipment sent to him by courier from New Zealand, strapped it onto a boat and did it himself.

He also travelled around Tonga gathering samples and taking measurements, collecting photos and videos taken by locals and speaking with them about their flight from the tsunami. “Most people’s stories are of being stuck in a traffic jam in the middle of Tongatapu, because they drove into the middle of the island, that’s what saved a lot of lives,” he says. People were interested in what Cronin was doing. “They were very keen to ask – What should we expect next? Is it going to erupt again?” he says.

After completing his research, Cronin presented it to Tongan prime minister Siaosi Sovaleni and his cabinet. He showed his timeline, new caldera map and analysis of ash deposits which suggested that they were “likely to be a positive benefit for soils after the initial crop burning.” He suggested ways Tonga could repurpose its land use to offer greater protection against tsunamis. The prime minister’s key question, however, was the same as everyone else’s in Tonga – will this happen again?

“My answer was that the fact that it’s wringed out the upper part of the volcano and now has a very deep caldera means that it’s almost impossible for it to reproduce an eruption of the magnitude that we’ve seen, until it can rebuild the magmatic system,” says Cronin. “That will take anything from 500 to 900 years based on the cycle of activity that we’ve had in the past. We expect no large-scale activity from Hunga for the next few centuries.”

The Tongan government and its people should not necessarily breathe easily, however. “There are other volcanoes in the chain which are equally capable of large-scale events,” says Cronin. “There’s one which looks exactly like Hunga, called Tofua.” Tofua, located 165 km north of Tonga, would potentially be capable of causing even greater effects than Hunga, because its caldera sits above sea level. “I’ve done a lot of radiocarbon dating and stratigraphy on Tofua and it seems to erupt on about a 400-year cycle,” says Cronin. “It’s been about 400 years since the last eruption.”

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