Tonga volcanic eruption the most powerful in more than a century

Scientists have begun to piece together what happened during the January 15 eruption of the undersea Hunga Tonga-Hunga Ha’apai volcano about 65 kilometers (40 miles) north of Tonga’s capital that killed at least three people. The eruption has defied easy explanation and updated scientists’ understanding of this type of volcano.
The volcanic eruption rarely observed pressure waves around the globe for six days and unleashed an unexpected type of tsunami wave, according to two new studies published on Thursday in the journal Science. The huge plume of gases, water vapor and dust also created hurricane-strength winds in space, NASA said in a separate study published this week.

Early data in the aftermath of the explosion suggested it was the biggest since the 1991 Mount Pinatubo eruption in the Philippines, but the Science studies, which involved 76 scientists in 17 countries, had suggested that the pressure waves it unleashed were similar to those generated by the cataclysmic 1883 Krakatoa eruption and 10 times larger than those from the 1980 Mount St. Helens eruption in Skamania County, Washington.

The Tonga eruption was “unusually energetic,” the Science study researchers wrote. The low-frequency atmospheric pressure waves, called Lamb waves, detected after the eruption circled the planet in one direction four times and in the opposite direction three times, they revealed.

A relatively rare phenomenon, these waves travel at the speed of sound. They aren’t detectable by humans and are slower than shock waves, as they have been mistakenly described sometimes, said study author Quentin Brissaud, a geophysicist at the Norwegian Seismic Array in Oslo. Lamb waves were also observed during the Cold War after atmospheric nuclear tests.

“It’s quite rare. So Lamb waves are really related to large air volume displacements. And they mostly propagate along the Earth’s surface,” said coauthor Jelle Assink, senior geophysicist at the seismology and acoustics department at the Royal Netherlands Meteorological Institute.

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Moving across the surface of multiple oceans and seas, Lamb pressure waves from the explosion created a fast-moving spate of scattered tsunamis.

Traditional tsunamis are usually linked with sudden changes in the ocean floor such as during an earthquake. Crucially, these so-called meteotsunamis travel much faster than traditional tsunamis, arriving two hours earlier than expected, and last longer, which could have implications for early warning systems.

And because an atmospheric pressure wave generated them, the tidal waves appeared to “jump continents,” with tsunamis recorded from the Pacific to the Atlantic, said coauthor Silvio De Angelis, professor of volcano geophysics in the department of Earth, ocean and ecological sciences at the University of Liverpool in the United Kingdom.

The research also revealed that audible sound from the eruption was detected more than 10,000 kilometers (6,000 miles) from the source in Alaska — where it was heard as a series of booms. The 188 Krakatoa eruption was heard 4.800 kilometers (2.98 miles) away, although it was one less study3 reported than the Tonga

A lithograph illustrates clouds pouring from the Krakatoa volcano during the cataclysmic 1883 eruption in southwestern Indonesia.

The researchers said more data was needed to understand the mechanism of the eruption.

It’s thought that one of the reasons for such an energetic explosion — creating an umbrella cloud 30 kilometers high (about 19 miles) and a plume some 58 kilometers high (36 miles) — was because “hot and gas-charged magma entered into contact with the (seawater) very rapidly,” De Angelis said via email. “The rapid transfer of intense heat between hot magma and the cold water causes violent blasts capable of tearing the magma apart.”

Space disturbance

Another study, published Tuesday in Geophysical Research Letters, found the Tonga volcano also created havoc in space, spurring hurricane-strength winds, based on data from NASA’s Ionospheric Connection Explorer, or ICON, mission and the European Space Agency’s Swarm satellites.
The giant plume of gases, water vapor and dust pushed into the sky by the eruption created large pressure disturbances in the atmosphere, leading to strong winds, NASA said in a statement. As these winds expanded upward into thinner layers of the atmosphere, they began moving faster.

“Upon reaching the ionosphere and the edge of space, ICON clocked the windspeeds at up to 450 mph — making them the strongest winds below 120 miles altitude measured by the mission since its launch,” NASA said.

(From left) Satellite images from January 6 and January 18 show the volcanic eruption's impact near Tonga.

In the ionosphere, where the Earth’s atmosphere meets space, the extreme winds also buffeted electric currents, flipping particles from their usual east-flowing electric current — called the equatorial electrojet — to a westward direction for a short period, and the electrojet surged to five times its normal peak power.

“It’s very surprising to see the electrojet be greatly reversed by something that happened on Earth’s surface,” said Joanne Wu, a physicist at the University of California, Berkeley, and a coauthor of the new Geophysical Research Letters study.

“This is something we’ve only previously seen with strong geomagnetic storms, which are a form of weather in space caused by particles and radiation from the sun.”

Brian Harding, a physicist at UC Berkeley and lead author, said the Tonga eruption was “allowing us to test the poorly understood connection between the lower atmosphere and space.”

He added, “The volcano created one of the largest disturbances in space we’ve seen in the modern era.”

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