How the Tonga volcano generated a shock wave around the world

The Hunga Tonga volcano in the southwest Pacific erupted explosively on Saturday evening local time, producing a tsunami, sending ash 100,000 feet high and generating an atmospheric shock wave that rippled around the globe. The eruption was heard in Alaska, about 5,000 miles away, while an area the size of New England was blanketed by the ashen smoke plume.

The volcano is about 40 miles north of Tonga’s main island, Tongatapu, near the international date line. Tonga, home to 105,000 people, can be found northeast of New Zealand and southeast of Fiji.

“We have a nightmare situation of an isolated community experiencing the effects of a large volcanic ash plume producing significant volcanic lightning, as well as a tsunami,” Janine Krippner, a volcanologist at the Smithsonian Global Volcanism Program, wrote in a Twitter direct message. “Seeing that ash plume, that volcanic lightning, and that tsunami leave me feeling sick thinking about the people being impacted by this large eruption.”

In addition to the more immediate and striking atmospheric effects resulting from the volcano, some have speculated that the volcano could affect Earth’s climate. Although experts remain skeptical, scientists are still collecting more data.

Hunga Tonga, an underwater volcano, erupted in 2009 and in late 2014. Renewed eruptions ensued Dec. 21, 2021, with occasional spurts of activity during the following weeks. A particularly explosive eruption occurred Jan. 15, resulting in arguably the most remarkable and striking display of volcanic power captured by a weather satellite.

The plume towered to about 100,000 feet, roughly three times the altitude at which commercial aircraft fly. The tropopause is the highest level of Earth’s atmosphere. Thunderstorms are flattened at this point. Hunga Tonga’s plume was buoyant enough to be able penetrate the layer of cloud mass and reach the stratosphere, before pockets in air and ash disappeared once more. The bulge in the middle of the cloud mass where this occurs is known as the “overshooting top.”

Satellite imagery captured “gravity waves” rippling outward from where the plume punctured this ceiling-like layer in the lower atmosphere — like wavelets surrounding a stone tossed in a pond.

A prolific volcanic thunderstorm

Within six hours of the initial blast, the ash and smoke plume from Hunga Tonga covered an area larger than New England. Even though night was falling, the plume of ash and smoke from Hunga Tonga would have blocked the sunlight. The plume’s static discharges were twice as tall as Earth’s strongest thunderstorms and produced prolific bursts of lightning.

Lightning detection networks and satellites tallied more than 60,000 strikes in 15 minutes following the volcano’s initial blast, corresponding to nearly 70 lightning strikes per second. It was a feat that few other conventional thunderstorms can match.

Notice, too, the bull’s eye-like pattern that emerges in the lightning data. This is due to the gravity waves. The waves’ upward motion increases local lightning speeds by enhancing the movement of air. Air sinks in their wake and suppresses lightning activity.

The blast was powerful enough to generate a tsunami of several feet in Tonga and prompted tsunami advisories across Hawaii, Alaska, British Columbia and much of North America’s West Coast, including in Washington, Oregon and California.

A four-foot spike in water levels was observed in Port San Luis, Calif., and Arena Cove, Calif., reported a 3.5-foot jump. Crescent City in California saw a spike of 2.7 feet and King Cove, Alaska, saw a tsunami measuring 2.8 feet.

In addition to an uptick in water levels, tsunamis can produce dangerous and erratic currents. They can travel across the oceans much faster than commercial jetliners.

Volcano’s explosion heard in Alaska

Experts at the National Weather Service in Anchorage and the University of Alaska Fairbanks confirmed that audible booms heard in the state early Saturday morning local time originated from the volcano. That means the sound traveled more than 5,000 miles.

“Dog and I woke up suddenly at 3: 30 and now I know why,” tweeted Shan Cole, a writer based in Anchorage.

That means the sound traveled close to 800 mph, and instruments confirmed that much of the noise produced did fall within the spectrum of what humans can hear. The fact that no sound was detected in Hawaii suggests that the atmosphere near Alaska may have played an important role in reflecting sound onto the surface.

An atmospheric shock wave

In the initial satellite imagery surrounding the volcano, it’s easy to spot a ring of white radiating rapidly outward far ahead of the volcanic plume. This is the atmospheric shock wave.

That shock wave traveled around the world, also moving faster than the speed of sound. For example, in Florida it might be detected by an anomaly of air pressure just after 9 AM. This is because the shock wave traveled around the globe, moving faster than sound speed.

Daryl Herzmann of Iowa State University compiled air-pressure data from sensors across the Lower 48 to illustrate the wave rolling across the country.

Potential climate impacts

Volcanic eruptions can release enormous amounts of sulfur dioxide and aerosols that, in large enough quantities, can cool the planet and work to snuff out a La Nina pattern. Although there were initial theories that Hunga Tonga’s material could cause a similar effect to Hunga Tonga, experts quickly pointed out the fact that its magnitude was too small to have any significant impact on climate.

Simon Carn, a professor at Michigan Tech, tweeted that the sulfur dioxide “columns do not appear to be extreme” so far. It would need to be five to 10 times as dense to begin to have a measurable climate impact.

Alan Robock, a professor in the Department of Environmental Sciences at Rutgers, noted that the amount of sulfur dioxide needed to cool Earth would be immense.

“Only if the eruption injects a lot of SO2 into the stratosphere, at least 1000 [kilotons, or thousands of tons] or more, will there be a climate impact,” he wrote in an email.

An underwater volcano off Tonga erupted on Jan. 15, triggering a tsunami warning for several South Pacific island nations. (Reuters)

Satellite measurements show SO2 quantities from the latest eruption were 400 kilotons. Robock said the eruption will “produce about 1/50 of the impact of the 1991 Pinatubo eruption,” or about 0. 02 degrees (0. 01 degree Celsius) average cooling.

Still, experts point to continued eruptions as something to monitor.

“We have no way of knowing when this eruption will be over,” the Smithsonian’s Krippner wrote.

correction

A previous version of this article incorrectly referred to the Smithsonian Global Volcanism Program as the Smithsonian Global Volcano Program and incorrectly rendered Port San Luis as Port St. Luis. This article has been updated.

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