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Tonga tsunami impact spurs scientific review

The tsunami caused by the Hunga-TongaHunga-Ha’apai volcano was far larger than what scientists thought possible.

Ben Strang reports.

The eruption of the Hunga-TongaHunga-Ha’apai volcano in Tonga sent literal shock waves around the world.

It is understood to have been the most explosive eruption in 30 years and to have created a larger tsunami than experts thought possible.

Much is still unknown about the eruption, and scientists believe it’s far from over.

But already it has scientists reviewing the thinking around the impact such a volcanic explosion can cause.

The Beginning

Hunga-Tonga-Hunga-Ha’apai is a very active volcano. Located about 65 kilometres from Tonga’s main island, Tongatapu, Hunga erupted several times in the 20th century, and in 2009, and then again in December 2014 and January 2015.

The most recent eruption began on December 20. The explosion could be heard from 170km away, and a large plume was visible from the Tongan capital, Nuku’alofa.

Activity died down in early January, but kicked off again on January 13 when a large ash column was sent 17km into the atmosphere. Two days later, on January 15, a far larger eruption took place.

Explosive eruption

Professor Shane Cronin, a vulcanologist at the University of Auckland, said the power of the eruption was phenomenal.

‘‘From the explosivity, it’s looking like it was one of the most powerful eruptions over from the last 30 years,’’ Cronin said.

‘‘On the other side of the coin, it’s actually quite a short-lived eruption that only lasted for less than 10 minutes, so the volume of material and volume of magma was not huge on a global scale.’’

In terms of explosiveness, Cronin compared the HungaTonga-Hunga-Ha’apai eruption to the 1991 Mount Pinatubo eruption in the Philippines, and the Krakatoa eruption of 1883.

He said that was based on satellite imagery, and reports of where the shock wave and sound wave were felt or heard. The pressure wave was recorded on barometers 17,000km away in Switzerland, while the sound of the explosion was heard across New Zealand and in Alaska, more than 9000km away.

Cronin said the volume of the eruption could, and was likely to, increase over the coming days and weeks. He said it could end up rivalling the volume of the Pinatubo eruption.

Historical context

An eruption of this scale is to be expected. Cronin’s research of the Hunga-Tonga-Hunga-Ha’apai volcano, and of ash deposits on neighbouring islands, shows it has a major eruption every 900 to 1000 years.

‘‘There were two eruptions I can distinctively relate to this volcano,’’ Cronin said. ‘‘One was about AD1100, and one at about AD280.

‘‘Those past events were, in total, larger, and involved a series of eruption episodes. So this may not be over yet.’’

The tsunami

The eruption caused a significant tsunami that swept through the islands of Tonga.

Niwa hydrodynamics scientist Dr Emily Lane said the tsunami reached up to about two metres high at some Tongan islands.

But she said the force of the tsunami didn’t dissipate as would have been expected.

‘‘Prior to this, I would have said, yeah, a volcanic tsunami could cause damage maybe 200 or 300 kilometres away from the volcano,’’ Lane said.

‘‘But we’re seeing boats being destroyed in New Zealand, in Japan, there were cars and boats being washed away and boats being destroyed in the US.

‘‘These are all thousands of kilometres away from this volcano, so that’s an incredible amount of energy that’s been blown out to cause that.’’

According to The Washington Post, a 1.2m tsunami struck Port Saint Luis in California, and a tsunami of just under a metre was recorded in King Cove, Alaska.

Al Jazeera reported Peru closed 22 ports as a precaution, and Japan’s Pacific Coast registered a 1.2m tsunami.

But the tsunami wasn’t just recorded in the Pacific. Trinidad and Tobago’s weather centre recorded a 12cm tsunami at Mona Island, Puerto Rico, while smaller tsunamis were observed throughout the Caribbean.

What caused the tsunami?

Dr Lane said there may have been multiple factors which led to the tsunami.

‘‘Earthquake tsunamis are pretty straightforward, you know. You get an earthquake, it moves the ground under the sea, which moves the seawater and it radiates out, and the big ones of those are 50km wide by 200km long, so it’s a huge area.

‘‘With volcanoes it’s a far smaller area, so actually getting out enough energy to get all the way across the Pacific, it’s quite

flabbergasting to be honest.’’

Hunga-Tonga-Hunga-Ha’apai’s caldera – the chamber which holds magma – is only about 5km across, but Lane said many mechanisms could be at play in causing the tsunami.

There is an explosive eruption, and a phreatomagmatic eruption. Lane said when magma reacts with the water, it can vaporise the water above it and cause a tsunami.

Tsunami could also be caused by the caldera and the sea floor collapsing. Images taken by the Sentinel-1 Synthetic Aperture Radar satellites show much of the Hunga Tonga and Hunga Ha’apai islands either side of the caldera have been destroyed, suggesting a collapse is possible.

Lane said pyroclastic flows could also create a tsunami, as could the shockwave of the explosion.

‘‘Some of the stuff in the Caribbean was definitely caused by the atmospheric pressure waves because [the tsunami] couldn’t have reached the Caribbean that fast,’’ Lane said. ‘‘So what we’re seeing is probably a combination of a lot of these different mechanisms.’’

Ash fall

Based on the previous large eruptions, Cronin said up to 20cm of thick ash can be expected to fall on Tongatapu and the Ha’apai island group.

He said the ash shouldn’t be especially toxic. However, it can still cause acid rain and acid leachates, and water will be undrinkable. Volcanic lightning was also significant in the hours after the eruption.

Volcanic lightning usually forms in the ash column spewing from a volcano and is unlikely to threaten people, but could impact on communication systems.

National News

en-nz

2022-01-18T08:00:00.0000000Z

2022-01-18T08:00:00.0000000Z

https://fairfaxmedia.pressreader.com/article/281672553310772

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