Listen to an Underwater Volcano Burp 750-Foot Bubbles

On July 7, 1908, the cutter Albatross was cruising the volcanic islands of Alaska when the sea began to swell into a dome the size of the nation’s Capitol building, as one account noted with admirable specificity. It bulged and bulged some more until it ruptured, releasing a plume of gas and steam that spewed from the sea with such intensity that “the spellbound spectators began to fear they would be engulfed in a terrific cataclysm,” wrote William Thornton Prosser in his essay “Nature Turned Sorceress.”

The Earth that day had burped—big time. An underwater volcano released a cloud of carbon dioxide, sulfur dioxide, and water vapor that rose to the surface as a bubble before destabilizing and exploding, giving the officers of the Albatross a show few humans had ever witnessed—and few have witnessed since.

Photograph: Getty Images

That is, until late 2016, when a nearby underwater volcano named Bogoslof (its summit peeks just above the water, forming Bogoslof Island), began acting up, and fancy microphones 40 miles away happened to be listening. By analyzing low-frequency infrasounds (which our own ears can’t pick up) coming from the area, researchers have determined that for over half a year, the volcano released gas in the same variety of massive bubbles the Albatross crew spied, over and over. How massive? Try an average of 750 feet across, each one holding 180 million cubic feet of gas. The biggest one was a quarter-mile wide, uberbubbles that swelled to the surface in great domes, ruptured, and threw clouds of volcanic muck seven miles high.

Bogoslof is one of the tinier specks of land in the Aleutian Islands, but the bit above ground isn’t our concern—the vent that blew those bubbles is just off the island’s coast, submerged in maybe 100 feet of water. In late 2016, Bogoslof began bubbling up magma from the sea floor. That lava would cool and form a kind of cap over the vent, sealing in gases like carbon dioxide and sulfur dioxide. But eventually the pressure would grow too great and the lava scab would fracture to release a burst of gas. Then it would seal once more with lava, until that too would break and belch another bubble.

As the bubbles ascended to the surface, they didn’t disconnect from the vent as a nice clean sphere. Instead, the vent continuously fed the bubbles with gas as they rose. Think of it as more like those big bubble wands that trail one long bubble, as opposed to the little spherical bubbles that might stream out of a smaller wand. And when one of these underwater bubbles reached the surface, it grew like the bulge the Albatross witnessed over a century ago.

A bubble flexes in size trying to reach equilibrium, the point at which the pressure on the inside of the bubble and its outside—whether atmosphere or water—are in balance. Because the gas inside is compressible, the giant bubble will expand but overshoot its equilibrium, at which point either the atmosphere or the water will push on the gas to compress it again, making the bubble undershoot its equilibrium. “It's this push and pull between the pressure on the outside of the bubble and the pressure on the inside of the bubble,” says US Geological Survey geophysicist John Lyons, lead author on a new paper in Nature Geoscience, describing the findings.

This is known as oscillation, and this chaotic dance of pressure produces the infrasound that sensors on nearby Umnak Island could pick up. Because the microphones were spaced several hundred feet apart, the sensors detected bursts of infrasound at slightly different times, which algorithms processed using the speed of sound as a reference to determine what direction the noise was coming from. The same thing is going on in your own brain. “The sound arrives at one ear, either faster or slower, by just a little bit than the other ear," says Lyons. "Our ears act like a two-element array.”

Previous work has shown how much smaller bubbles produce infrasound when they oscillate, so Lyons and his colleagues could calculate the characteristics of these massive volcanic bubbles to show how they formed, how big they grew, and how they destroyed themselves all over the course of about 10 seconds, before that magma cap formed again, and broke again, releasing another bubble. (You can hear the audio adjusted for human ears below. The infrasound is sped up 300X, so each of the spikes is actually a separate bubble signal.)

This clip captures the start of an eruption on March 8, 2017 and then goes into a sequence of more than 100 consecutive bubble signals. (Each spike in the waveform is a bubble signal.) VIDEO: JOHN LYONS

And oh, how the bubbles destroyed themselves. When a bubble breached the surface, it formed a dome of water perhaps several feet thick on top of itself. But gravity pulled on that water, causing it to rush down the sides of the bubble dome, destabilizing the whole weird structure until ... pop. A 750-foot-wide bubble ruptures, launching a jet of hot gas and ash miles into the air. “Imagine the violence of a normal volcanic eruption, but then you add a bunch of water to it," says Lyons.

What these explosive moments looked like, no one can say for sure. But a Coast Guard ship happened to be in the general area at the time, and it sent Lyons a grainy photo of a volcanic lightning cloud and even some incandescence from bits of lava.

All heady stuff, but it’s easy to forget how dangerous even a remote volcano can be. There’s no indication that Bogoslof triggered any tsunamis with these eruptions, but submarine volcanoes can and do send out massive waves. Indeed, 17 percent of fatalities from volcanic eruptions above and below the water are due to resulting tsunamis.

“The more we can apply these modern sensing and modeling techniques to understand the eruption processes, the better we're going to be able to forecast it in the future,” says Bob Dziak, who studies submarine volcanoes and acoustics at NOAA, but who wasn’t involved in the new study.

Plus, we get to observe geological gastrointestinal distress of the highest order.

Photograph: Dave Withrow/NOAA