When a South Pacific volcano called Hunga Tonga-Hunga Ha’apai exploded on the afternoon of 15 January 2022, most of the world was taken by surprise. Few, in fact, had even heard of the mountain because it is mostly underwater, in the Kingdom of Tonga.
But the eruption was more than a curiosity. It produced a giant thump heard at least as far away as Fiji, 700 kilometres north-west. The same thump was later picked up worldwide by instruments designed to monitor for nuclear tests.
“This is the largest infrasound (ultra-low frequency) event we’ve ever seen,” R. J. Le Bras of the Comprehensive Nuclear Test Ban Treaty Organisation said last month at a meeting of the Seismological Society of America (SSA) in Seattle, US.
But how big exactly was it?
Vulcanologists rate eruptions on an eight-point explosivity index (VEI), in which each one-point increase represents a 10-fold increase in power. The eruption of America’s Mt St Helens in 1980 was VEI-5. The 1991 eruption of Mt Pinatubo in The Philippines was VEI-6. Ditto for the 1883 eruption of Krakatoa in Indonesia. The biggest in recorded history, the 1815 eruption of Mt Tambora, also in Indonesia, was VEI-7.
As for Hunga Tonga-Hunga Ha’apai? Scientists are still struggling to be sure, but it looks to have been somewhere in the VEI-6 range, and probably the biggest since Tambora.
Part of the difficulty in determining its exact magnitude comes from its remoteness. But a bigger complication is the fact it occurred underwater.
Hunga Tonga-Hunga Ha’apai is the crest of a giant submarine volcano which in 2014–15 briefly drew attention when a much milder eruption caused it to break the surface and create what was then the world’s newest island.
The 2022 eruption, however, was entirely beneath the waves. That creates a problem, because one of the ways to determine the VEI is by measuring the amount of lava accompanying the explosion. In this case, while much of that lava was blown out of the water and into a giant plume of volcanic ash, much also presumably remained underwater. That makes it hard to determine exactly how much there was, in total.
Luckily, measuring the amount of lava isn’t the only way to estimate an eruption’s power. It can also be done by looking at the height and size of the ash plume, and on that metric, Hunga Tonga-Hunga Ha’apai was off the chart. Instruments on NASA satellites estimated it to have to reached as high as 58km.
“That’s above the top of the stratosphere,” reported Larry Mastin of the US Geological Survey’s Cascade Volcanic Observatory in Vancouver, Washington, at the SSA meeting. To put that in perspective, he said, the plume from Mt Pinatubo rose only 35–40km high – about two-thirds the height of Hunga Tonga-Hunga Ha’apai’s.
Once in the upper air, Hunga Tonga-Hunga Ha’apai’s plume expanded into an “umbrella cloud” that within an hour grew to a diameter of 200km. That, Mastin said, was faster than the rate of spread of the umbrella cloud from Mt Pinatubo, suggesting that the explosion at Hunga Tonga-Hunga Ha’apai was blowing about three times as much ash into the air as Pinatubo had done.
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 below water, where steam created by the hot lava might have contributed to the power of the rising plume.
But scientists are finding other evidence that this was a truly dramatic explosion.
To start 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. That makes it about 1,000 to 2,000 times more powerful than the bomb that levelled Hiroshima in 1945.
All told, said Steve NcNutt, a geophysicist at the University of South Florida, US, whatever happened at Hunga Tonga-Hunga Ha’apai was the type of thing that only occurs once a century, if that often. It was the biggest eruption in at least 138 years, and possibly in 207 years.
Even in Florida, 11,500km away, he said his team – like the teams from the Comprehensive Nuclear Test Ban Treaty Organisation – heard it on infrasound, and to hear a volcanic blast from that distance, he told the SSA meeting, “is extraordinary”.
Another of the factors that puts it off the scale, McNutt said, is the amount of lightning it produced in and around the plume. “There were 400,000 lightning flashes,” he said. “The peak hour had 200,000.”
Consider that for a moment: 200,000 lightning flashes in an hour is 55 per second. During that hour, McNutt said, “Hunga Tonga-Hunga Ha’apai was producing 80% of all the lightning in the world – an unbelievable amount.”
Meanwhile, the explosion created a tsunami that literally circled the world, forcing tsunami warning centres to adapt on the fly to figure out who was and wasn’t at risk from a never-before-seen type of wave.
“This was a model breaker,” said Summer Ohlendorf of America’s National Tsunami Warning Centre in Alaska.
The worst effects of the tsunami, not surprisingly, were felt on nearby islands in Tonga, where waves reportedly reached heights of 15 metres and several people died. But ultimately, there was damage as far away as harbours in California, US, and even two deaths in Peru. “It is rare for a volcano-generated tsunami to reach beyond 1,000km,” Ohlendorf said at the SSA meeting.
One early hint that this wasn’t normal, she said, came well before the waves reached the North American and South American coasts, when the island of Vanuatu, 2,000km away from the explosion, was hit by 1.4m surges.
At one point, she said, 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 thing as a volcanic explosion, but something they at least understood. From that, she said, “we could at least get an idea” how the wave might spread.
Emile Okal, a geophysicist at Northwestern University, US, said she believed that the most likely explanation for why the tsunami spread so broadly was that the blast created an atmospheric air wave that propagated outward at about the speed of the spreading tsunami, feeding its power rather than letting it dissipate. “We could think of this as a tsunami of the atmosphere,” he told the SSA scientists.
The air wave was so strong that the part of it going westward jumped across Africa to the Atlantic Ocean, while the part going eastward jumped across the Americas. Once in the Atlantic, the two air waves stirred opposite-direction tsunamis that eventually converged.
“There were east and west-propagating tsunami waves,” Stuart Weinstein, deputy director of the Pacific Tsunami Warning Centre, Hawaii, told the SSA scientists. Not that they were enormous, but in the Leeward Islands in the Caribbean, he said, they hit 60cm – a rather dramatic effect from a tsunami that originated on the far side of two continents, half a globe away.