Warning sign that appeared the day before Tonga's volcanic eruption

·News Editor
·4-min read

Scientists have uncovered a “warning sign” that emerged the day before the powerful volcanic explosion in Tonga that sent shockwaves around the world.

Shane Cronin, Professor of Volcanology at the University of Auckland, spent the last two months in Tonga researching the Hunga-Tonga Hunga-Ha’apai volcano and mapping its caldera – and what they discovered blew them away.

Images comparing the islands before and after the explosion on January 15 show most of the land mass has disappeared, having collapsed into the volcano during the eruption.

Satellite images showing before and after the Hunga Tonga volcano eruption on January 15.
Satellite images show most of the land mass collapsed into the volcano during the eruption on January 15. Source: Getty

But one feature disappeared the day before the eruption, and it could help scientists with future eruptions from other volcanoes.

“This cone actually disappeared on the day before the big eruption,” Professor Cronin said at a presentation of his findings on Tuesday in the country’s capital of Nuku’alofa.

"So we know that this caldera collapse probably now began a day before the big eruption and it was probably part of the process leading to the big eruption of the 15th.

“So this is quite an important observation to tell us what kind of warnings we should be looking out for in the future from similar volcanos.”

Professor Cronin’s research with the Tonga Geological Services team revealed for the first time the enormous impact the eruption had on the volcano itself.

A previous research expedition in 2015 showed the deepest part of the caldera to reach about 150m “at the most”, Professor Cronin said, with a basin volume of 1.2 cubic kilometres.

An ocean survey map shows the new depth of Hunga Tonga's volcano caldera to be more than 800 metres.
The new survey of Hunga's caldera shows the small land areas left, and the internal depth of more than 800 metres. Source: Shane Cronin/University of Auckland/Tonga Geological Services
A survey map showing the new depth of Hunga Tonga's volcano. The centre reaches more than 800m, compared to 150m in 2015.
Professor Shane Cronin estimates the real depth could be another 100m below the recorded depth due to a high amount of debris in the caldera. Source: Shane Cronin/University of Auckland/Tonga Geological Services

After Hunga’s violent eruption – which unleashed a tsunami on Tongatapu that reached all the way to California – the caldera was measured at 4km wide and more than 860m deep.

“When we first saw it on the boat it was really astonishing to us, we were absolutely blown away by how deep and how fundamental the change has been on Hunga. But it helps us to understand why the eruption was so very, very large,” Prof Cronin said.

“Our preliminary calculations tell us around 6.5km³ of material has been removed from the central part of this caldera.

“If we scraped all of Tongatapu down to sea level just to fill in this hole, we would only get about two-thirds of the way there.”

Because the bottom of the caldera is filled with debris and deposits, it's likely the real base could be about 100m deeper, Prof Cronin estimates.

The explosion sent an ash plume 100,000 feet into the upper atmosphere and tsunami waves as far as California. Source: Goes-West Satellite/NOAA/Cover Images
The explosion sent an ash plume 100,000 feet into the upper atmosphere and tsunami waves as far as California. Source: Goes-West Satellite/NOAA/Cover Images

Hunga unlikely to erupt again for several hundred years

Through their research, Prof Cronin and his team discovered Hunga’s explosion was even larger than the eruption of Pinatubo in the Philippines 31 years ago.

“We now know that the rise and spread of this eruption was up to two times faster than the next biggest one that was on record from Pinatubo in 1991,” he said.

“The Hunga one was much faster.. faster growth and height and also width of the plume.”

“There was more lightning detected during this event than any other event around the world so it’ not a surprise that people are still frightened by thunder and lightning storms following this eruption.”

The eruption sparked an “unusually large” tsunami that reached more than 18m high on the western side of the island.

The impact flattened homes, resorts and businesses that were already struggling to survive through the pandemic, and tragically claimed the lives of three people.

Hunga’s massive eruption was a once-in-a-lifetime event, with Prof Croning saying it’s very unlikely to see a large-scale eruption again for the next few hundred years at least.

“The volcano needs to build back up again to a shallow level to be able to be having that great big potential,” he said.

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