Volcanic mountains

The significant impact of the Tonga volcanic eruption in 2022

When a South Pacific volcano called Hunga Tonga-Hunga Ha’apai exploded on the afternoon of January 15, 2022, most of the world was taken by surprise. Few, in fact, had even heard of the mountain as it is mostly underwater, in the Kingdom of Tonga.

But the eruption was more than a curiosity. It produced a giant thud heard at least as far away as Fiji, 700 kilometers to the northwest. The same noise was later picked up around the world by instruments designed to monitor nuclear testing.

“This is the largest infrasound (ultra-low frequency) event we have ever seen,” RJ Le Bras of the Comprehensive Nuclear Test Ban Treaty Organization said last month during a briefing. meeting of the Seismological Society of America (SSA) in Seattle, USA. .

But what exactly was his size?

Volcanologists rate eruptions on an eight-point explosiveness index (VEI), in which each one-point increase represents a 10-fold increase in potency. The 1980 eruption of US Mt St Helens was VEI-5. The 1991 eruption of Mount Pinatubo in the Philippines was VEI-6. Ditto for the 1883 eruption of Krakatoa in Indonesia. The largest in recorded history, the 1815 eruption of Mount Tambora, also in Indonesia, was VEI-7.

Read also: The Hunga Tonga-Hunga Ha’apai eruption: what’s next?

As for Hunga Tonga-Hunga Ha’apai? Scientists are still struggling to be sure, but it appears to have been somewhere in the VEI-6 range, and probably the largest since Tambora.

Part of the difficulty in determining its exact scale comes from its remoteness. But a bigger complication is the fact that it happened underwater.

Part of the difficulty in determining its exact scale comes from its remoteness. But a bigger complication is the fact that it happened underwater.

Hunga Tonga-Hunga Ha’apai is the crest of a giant undersea volcano which in 2014-2015 briefly caught the eye when a much milder eruption caused it to shatter the surface and create what was then the newest island in the world.

The 2022 eruption, however, was entirely under the waves. This creates a problem, because one of the ways to determine the VEI is to measure the amount of lava accompanying the explosion. In this case, while much of this lava was blown out of the water and into a giant plume of volcanic ash, much of it likely remained underwater. It is therefore difficult to determine exactly how many there were, in total.

Fortunately, measuring the amount of lava is not the only way to estimate the power of an eruption. This can also be done by looking at the height and size of the ash plume, and on this metric Hunga Tonga-Hunga Ha’apai was off the chart. Instruments lit Nasa the satellites estimated it to be as high as 58 km.

“It’s above the top of the stratosphere,” Larry Mastin of the US Geological Survey’s Cascade Volcanic Observatory in Vancouver, Washington, reported at the SSA meeting. To put that into perspective, he said, the Mount Pinatubo plume was only 35-40 km high, about two-thirds the height of Hunga Tonga-Hunga Ha’apai.

Satellite image after the Hunga Tonga-Hunga Ha’apai volcano erupted on January 17, 2022. Credit: Maxar/Contributor/Getty

Once in the upper air, the Hunga Tonga-Hunga Ha’apai plume developed into an “umbrella cloud” which, within an hour, reached a diameter of 200 km. This, Mastin said, was faster than the rate of spread of Mount Pinatubo’s umbrella cloud, suggesting that the explosion at Hunga Tonga-Hunga Ha’apai was blowing about three times as much ash into the air as Pinatubo had. do.

There are, of course, uncertainties in Mastin’s calculations, starting with the fact that Pinatubo was high and dry when it exploded, while Hunga Tonga-Hunga Ha’apai was underwater, where steam created by the hot lava could have contributed to the power of the rising plume.

But scientists are finding other evidence that it was a truly dramatic explosion.

To begin with, it produced a magnitude 5.8 earthquake – roughly equivalent to a 12 to 32 megaton atomic bomb, said Gene Ichinose, a geophysicist at Lawrence Livermore National Laboratory, US, who also spoke at the SSA meeting. This makes it about 1,000 to 2,000 times more powerful than the bomb that leveled Hiroshima in 1945.

All told, said Steve NcNutt, a geophysicist at the University of South Florida, USA, everything that happened in Hunga Tonga-Hunga Ha’apai was the kind of thing that only happens once a century, if not often. It was the biggest eruption in at least 138 years, and possibly 207 years.

Even in Florida, 11,500 miles away, he said his team – like teams from the Comprehensive Nuclear Test Ban Treaty Organization – had heard it through infrasound, and heard a volcanic explosion at that distance , he told the SSA meeting, “is extraordinary”. .

Another factor that puts it off scale, McNutt said, is the amount of lightning it produced in and around the plume. “There were 400,000 flashes,” he said. “Rush hour had 200,000.”

Consider this for a moment: 200,000 flashes in an hour is 55 per second. During this hour, McNutt said, “Hunga Tonga-Hunga Ha’apai produced 80% of all the éclairs in the world – an incredible amount.”

“Hunga Tonga-Hunga Ha’apai produced 80% of all the éclairs in the world – an incredible amount.”

Steve NcNutt, Geophysicist, University of South Florida

Meanwhile, the blast created a tsunami that literally circled the globe, forcing tsunami warning centers to adapt on the fly to determine who was and wasn’t threatened by a type of wave. never seen before.

“It was a pattern breaker,” said Summer Ohlendorf of the US National Tsunami Warning Center in Alaska.

The worst effects of the tsunami, unsurprisingly, were felt on the neighboring islands of Tonga, where waves reportedly reached heights of 15 meters and several people died. But ultimately there was damage as far away as ports in California, USA, and even two deaths in Peru. “It’s rare for a volcano-generated tsunami to reach more than 1,000 km,” Ohlendorf said at the SSA meeting.

A first hint that this was not normal, she said, came long before the waves reached the North and South American coasts, when the island of Vanuatu, 2,000 km from the explosion, was hit by surges of 1.4 m.

Tonga volcano eruption
Satellites have captured some of the effects of the January 2022 Hunga Tonga-Hunga Ha’apai eruption felt around the world. Credit: Mary Pat Hrybyk-Keith / NASA Goddard Space Flight Center

At one point, she says, her team tried to match what they were seeing with what might happen from a giant earthquake in the nearby Tonga Trench – not the same as a volcanic explosion. , but something they at least understood. From there, she said, “we could at least get an idea” of how the wave might spread.

There was damage as far away as ports in California, USA, and even two fatalities in Peru.

Emile Okal, a geophysicist at Northwestern University, US, said she thought the most likely explanation for the tsunami spreading so widely was that the blast had created an atmospheric wave that traveled roughly at the speed of the tsunami’s spread, fueling its power. rather than letting it dissipate. “We could think of this as a tsunami of the atmosphere,” he told SSA scientists.

The air wave was so strong that the westward portion of it skipped across Africa to the Atlantic Ocean, while the eastward portion skipped across the Americas. . Once in the Atlantic, the two waves of air caused tsunamis from opposite directions which eventually converged.

“There were tsunami waves propagating east and west,” Stuart Weinstein, deputy director of the Pacific Tsunami Warning Center, Hawaii, told SSA scientists. Not that they were huge, but in the Leeward Islands of the Caribbean, he said, they reached 60cm – a rather dramatic effect of a tsunami which originated on the other side of two continents, half a globe away.