In the early 1900s, sailors near Alaska reported seeing black bubbles appear to boil out of the ocean, each one the size of the dome of the Washington, DC metropolitan building. They were not the only sailors who reported the bizarre phenomenon, and they were not wrong, except for one thing … the bubbles were much larger.
When it mostly erupts under the volcano Bogoslof on the Aleutian Islands, it produces giant bubbles that can reach up to 1,444 feet (440 meters) above, according to a new study. These bubbles are filled with volcanic gas, so when they burst, the volcanic clouds create tens of thousands of feet in the sky, said lead author John Lyons, a research geophysicist at the Alaska Volcano Observatory of the US Geological Survey.
These volcanic clouds were captured in satellite images taken after the Bogoslof volcano last erupted in 201
Related: Photos: See Hawaii's New Underwater Volcano  During the eruption, a dull buzz hung in the air. Something gave off low-frequency signals called infrasound – sounds below the level that people can hear – that would take up to 10 seconds. Lyons and his team, which regularly monitor active volcanoes in Alaska, included these signals in their data. But "it took us a while to find out what they were," Lyons told Live Science.
It was only after searching the literature that the team came up with their hypothesis that the sound was the whisper of giant gas bubbles growing within magma from the erupting volcano. They then came up with a computer model for what happened.
In their model, a bubble springs from the column of magma under the water and begins to grow. When it reaches the sea surface it strikes out in the form of a hemisphere and continues to grow at an even faster rate in the lower density of the atmosphere. Eventually, the pressure outside the bubble exceeds the pressure inside and the bubble begins to collect; its film becomes unstable and bursts, causing the bubble to burst.
When it bursts, the volcano releases gas – water vapor, sulfur dioxide and carbon dioxide partially back into the water, where it interacts with the lava, pulls it into pieces and produces ash and volcanic clouds, Lyons said.  The team considered that low-frequency hum originates from the growth and oscillation of each bubble and the high-frequency signal represents the deficiency.
"These shallow explosive submarine eruptions are so rare," Lyons said. "There is a lot of underwater volcanism, but the majority of it happens under lots and lots of water very deep and all that extra pressure tends to suppress how explosive eruptions are."
But there are still open questions and the results are limited by their methodology, which relied on a number of assumptions, he said. For example, it is unclear how the water is around the bubble – whether it is as sea water or as wet cement. "It would be nice to be able to record it elsewhere and make sure our method is sound," Lyons said.
The study was published October 14 in the journal Nature Geoscience .
Originally published on Live Science .