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Tuesday, 22 October 2019

An underwater volcano generates bubbles over 400 meters in diameter

A plume of ash and gas rises from the Bogoslof volcano in 2017. | Dave Schneider / Alaska Volcano Observatory & US Geological Survey

At the beginning of the 20th century, sailors sailing near Alaska reported seeing black bubbles appearing to escape from the sea, each "at least the size of the dome of the United States Capitol (Washington)", according to certain statements. Surprisingly, they were not the only ones who reported the strange phenomenon. And they were not wrong, except on one detail: the bubbles are actually even bigger than they had estimated.

According to a new study, when the Bogoslof volcano, located in the Aleutian Islands and being largely submerged, erupts, it produces giant underwater bubbles up to 440 meters in diameter. These bubbles are filled with volcanic gas and create clouds up to several kilometers in altitude, says lead author of the study, John Lyons, geophysicist at the Alaska Volcano Observatory, Institute of Geological Survey of the States -United.

Images of volcanic clouds were captured by satellite after the last eruption of the volcano in 2017. However, the bubbles themselves have never been photographed to date.

During the eruption, a low buzzing sound spread through the air. Something emitted low-frequency signals, called infrasounds (sounds of a frequency lower than that perceptible by Man). These could last up to 10 seconds.

Lyons and his team, who regularly monitor active volcanoes in Alaska, have detected these signals in their data, thanks to adapted microphones. However, it took them a while to determine what it really was.

Map of Bogoslof volcano, with two satellite images of the top and crater partially submerged during an eruption. Credits: Lyons et al / Nature

It was only after going through the literature that the team proposed its hypothesis, arguing that the sound was the "murmur" of giant gas bubbles developing in the magma of the erupting volcano. They then proposed a computer model simulating the phenomenon.

The volcano erupted more than 70 times in 9 months. A distinct grunt (the "whisper") of a second, preceded each eruption, found the researchers. The computer models developed have shown that the vibration corresponds to the frequency that the eruptive bubbles generated as they stretched, dilated and exploded. The results were published Oct. 14 in the journal Nature Geoscience.

In their model, a bubble gushes from the column of magma, under water, and begins to grow. Once it reaches the surface of the water, part of the bubble emerges as a dome and continues to grow even faster. The pressure outside the bubble then becomes larger than the inside, and the bubble begins to contract. Eventually, his film becomes unstable and breaks, causing it to burst.

When it explodes, volcanic gas (composed of water vapor, sulfur dioxide and carbon dioxide) is partially released into the water, where it interacts with the lava, which breaks it down and produces ashes as well as volcanic clouds.

The team hypothesized that the low-frequency buzz came from the growth and oscillation of each bubble, and that the high-frequency signal represented the burst.

Diagram summarizing the formation of a bubble around a submerged eruption point. Gas flows from the column of magma to give birth to a bubble. The latter then continues to grow until it collapses. At one point, the bubble reaches its maximum radius (where the pressure is the lowest): at that moment, it contracts and eventually burst, giving rise to ash and volcanic clouds that can reach miles 'altitude. Credits: Lyons et al / Nature 

These shallow explosive underwater eruptions are very rare,  " Lyons said. " There is a lot of underwater volcanism, but most of it occurs under a large body of water (great depths), and all that extra pressure tends to suppress the incredible eruption  ."

Nevertheless, there are still outstanding issues and the results are limited by the research team's methodology, which is based on a number of assumptions. For example, it is difficult to know exactly what the water around the bubble looks like - whether it is normal seawater or whether the consistency is doughier, and so on. " It would be nice to be able to record these data elsewhere on Earth, and thus ensure that our methodology is sound, " Lyons said.

Sequence of recording the beginning of an eruption on March 8, 2017. Also audible are more than 100 consecutive bubble signals. (Each peak in the shape of the wave is a signal generated by a bubble). Credits: John Lyons


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