Up close to the Hunga Tonga-Hunga Ha'apai volcano

From Nine To Noon, 9:40 am on 20 April 2022

Volcanologist Shane Cronin has just returned from a Navy boat survey of Tonga's northern islands, trying to piece together what led to the largest volcanic explosion ever recorded there.

He is the first overseas scientist allowed into Tonga to investigate January's powerful undersea volcanic eruption and tsunami, which caused massive disruption and damage and claimed three lives.

So far, he's been able to inspect the ash and cruise about the area seeing first-hand the damage to coral.

Shane Cronin -  what is left of Hunga Tonga

Shane Cronin - what is left of Hunga Tonga Photo: Shane Cronin

When the University of Auckland professor spoke to Nine to Noon last month upon his arrival in Tonga, his hypothesis was that the side of the volcano had collapsed and expanded the explosive interaction between magma and water.

The run-ups – or the maximum height that a tsunami reaches onshore above sea level – around Hunga Tonga-Hunga Ha'apai indicated a large tsunami for a volcanic event.

From that information, Cronin initially thought it would mean the flanks collapsed.

But the professor and his fellow Tongan research team were in for a couple of surprises while traversing through the collapsed volcano, known as a caldera.

“It doesn’t look like the flanks have changed a great deal and instead … we learned that the caldera is incredibly deep.

“Where the former island was in 2015, it used to be about 140 metres above sea level, we were measuring over 850 metres below sea level was the new ground surface.

“So, we’ve had a huge drop of the central part of caldera, well in fact the entire caldera has dropped according to our single traverse.”

The good news about this for Tongans is that that the deepening of the caldera to that point would mean further eruptions would be effusive – or not so violent, he says.

But the mystery of this caldera deepens, Cronin says, with an enormous volume of about 10 cubic kilometres missing and little signs of ashfall on the east.

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 Samples of ash from the Hunga Tonga-Hunga Ha'apai eruption. Photo: Supplied / Shane Cronin, University of Auckland

“I’ve had some reports from American researchers who have been looking way out the west of the volcano on black smokers in the very deep ocean, and they’ve been finding a lot of ash out there.”

On an earlier trip around the edges of the hollow formation, Cronin was able to collect some new rock samples. He’s now sent back about 120 samples to New Zealand for assessment.

The pressure is on to provide as much information as possible to international scientists, who want to figure out why current models could not forecast the tsunami run-up, he says.

“In term so the geological aspects of this event and some of the things like the tsunami modelling, we will have a little bit more time to try and get it right. We are hoping to get an early paper out quite soon.

“There’s quite a cohort of people working on this from the University of Otago, Canterbury, Victoria, Massey and Auckland as well as GNS Science and NIWA.”

But the key takeaway has been that volcanoes are able to produce such large tsunamis even if we have seen them much before, Cronin says.

“This event seems to have produced them through a combination of sheer explosive power, and also collapse, and formation of a large hole the ground.”

While their research is concentrating on land information, there’s also a NIWA research vessel surveying the outside flanks of caldera.