How a mighty volcanic eruption sent enormous pressure waves all around Earth

A formidable underwater volcano created the twin islands of Hunga Tonga and Hunga Ha‘apai. Then, on Jan. 15, that volcano destroyed them.

Located in the South Pacific, the eruption was one of the most powerful ever captured on satellite. The size and fury of the resulting plume made a jaw-dropping scene on Earth’s surface and amazed scientists. Unfortunately, the eruption has had disastrous and deadly local consequences: A tsunami nearly 50 feet high pummeled some of Tonga’s populated islands.

The historic eruption was also energetic enough to create powerful shock or pressure waves that rippled through the atmosphere and all over the globe. It’s not too different than a rock tossed in water.

“Think of the ripples that you see when you throw a rock into a calm pond,” explained Ryan Torn, chair and professor at the University at Albany Department of Atmospheric and Environmental Sciences. “Immediately, you see circular ripples, which are waves emanating from where the rock enters the pond.”

When the Tonga volcano erupted, these waves (also technically called “acoustic gravity waves”) traveled through the air at around the speed of sound, 343 meters per second, or some 760 mph. The blast’s wave of energy knocked against air molecules, and they bump into each other. The animation below shows the global event well.

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The atmosphere acted like a fluid because the atmosphere is actually a fluid. It’s not as dense as a liquid, but gas particles react to temperature, pressure, and things flowing through in similar ways, explained Phil Blom, an expert in acoustics and geophysics research at Los Alamos National Laboratory.

And out in the open atmosphere, there’s not much friction to stop these waves. “It will circle the globe multiple times,” said Blom. An atmospheric scientist at the University of Miami, Brian McNoldy, recorded six waves (as of Jan. 19) passing over the region. The waves traveling through the atmosphere are picked up by weather stations that record changes in the atmosphere’s pressure, but are imperceptible to us. Eventually, the waves dissipate in the air.

“It will circle the globe multiple times.”

Only a profoundly energetic blast creates such world-traveling waves. Though the scientific investigation has just started, volcanologists suspect that seawater interacting with the volcano’s magma (molten rock) beneath the surface ultimately provided this eruption with the pressure for such a massive explosion. Water converted into steam creates intense pressure. “That’s what gave this [eruption] outsized energy, we think,” explained Josef Dufek, a volcanologist at the University of Oregon.

Eventually, like opening a shaken-up soda can, there’s a great pressure release, which is the eruption. Blown-apart pieces of magma, known as volcanic ash, are thrust high into the air. This ash plume reached over 22 miles (35 kilometers), noted Dufek, and may have even topped some 30 miles (50 kilometers).

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The resulting pressure waves even stoked meteotsunamis in Europe. Whereas tsunamis are long waves of displaced water (often by earthquakes), meteotsunamis are driven by momentous changes in air pressure, like from a storm, or a blast from a volcano. The changes in air pressure are transmitted to the water and can drive a surge of water. Sea levels went up by some eight inches (20 centimeters) in parts of Spain.