Underwater Volcano Eruptions Sound like Gunshots And Can Travel 10,000 Miles Through The Ocean

Piton de la Fournaise volcano
Representational image. Molten lava erupts from the Piton de la Fournaise, Réunion island, in 2015. Campi Flegrei in Italy has not erupted since the 1500s. Gilles Adt/Reuters

Buried in the depths of the oceans, over a million enormous underwater volcanoes puncture the seabed. Their violent eruptions are extremely powerful and very difficult to detect in the early stages—but new recordings may help scientists map these incredible events much more quickly.

Using hydrophones—special microphones designed for use underwater—scientists from the Alaska Volcano Observatory (AVO) and the U.S. Geological Survey (USGS) recorded the sounds of two volcanos erupting. Presenting their findings at the 174th Meeting of the Acoustical Society of America in New Orleans, the team, led by Gabrielle Tepp, from the AVO, show how different eruptions sound drastically different—while some explosions lasted hours, others ripped through the sea like gunshots.

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An eruption of the shallow Bogoslof volcano was captured on satellite images, Pacific Ocean, June 12, 2017. Kim Angeli/ Alaska Volcano Observatory/U.S. Geological Survey

"It sounded like bombs exploding"

Above the surface, submarine eruptions may go unnoticed. Underwater, however, they can send incredibly powerful shockwaves many miles. As Chip Young, leader of a scuba diving team swimming near the Ayhi underwater volcano at the time of its 2014 eruption, attests: "While we were diving, we could hear eruptions underwater. It wasn't casual, in fact, it sounded like bombs exploding with the concussion felt through your body.

"I don't know how close we were to the event, none of the…divers saw volcanic activity, but at least one explosion was so powerful, that it reverberated through the hull of the ship and the crew onboard thought that something had happened to the ship."

The Ayhi volcano sits in the Northern Mariana Islands in the Pacific Ocean. Its enormous eruption was just one of many that year. In fact, far more volcanoes erupt under the sea than on land. As well as terrifying local divers, they can produce blankets of floating rocks, blocking sea ports and damaging boats.

Traditional monitoring methods fall short

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Steam rises from the erupting Bogoslof volcano as hot lava heats the seawater in August 2017. Dave Withrow/NOAA Fisheries

Volcanic activity is usually monitored using seismology. Devices buried deep below the ground pick up vibrations. Seismologists can recognize and record the size and depth of volcanic activity using this data. Sound waves travel differently in vast bodies of water like oceans, so normal seismology can fall short in recording underwater eruptions.

"Unlike the land-based volcanoes, it's very rare that we detect increases of activity before an eruption begins. This doesn't mean the signs aren't there—they're likely just undetected by the sparse instruments that we have in the oceans," Gabrielle Tepp tells Newsweek.

Eruptions can sound drastically different

Using sophisticated hydrophones, the team recorded vastly different sounds from the Ayhi eruption and from the 2015 and 2016 eruptions of Bogoslof, a volcano near Alaska.

"The eruption styles of Ahyi and Bogoslof were different. This could be due to differences in the two systems, such as magma composition, amount of gas, and pressurization of the system," Tepp says. "Ahyi is also completely submerged at around 250 feet deep, while Bogoslof is very shallow with part of its top just peeking out of the water."

Ahyi produced thousands of short explosions over a two-week period. This resulted in sounds akin to gunshots, according to the research team. The Bogoslof recordings, however, show sustained eruptions lasting anything from minutes to hours. Earthquakes and tremors accompanied these eruptions, with plumes of ash breaking through the surface of the ocean.

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Ahyi lies below the surface of the Pacific Ocean. A U.S. Pacific Fleet ship is pictured May 3, 2017. U.S. Pacific Fleet/Flickr

Underwater sounds can travel thousands of miles

The powerful soundwaves can be carried thousands of miles through the sea. "We've found some of the Ahyi signals on hydrophones off the coast of Chile, about 9,600 miles across the Pacific," says Tepp. "Many volcanic sounds won't reach this far, however—either they're too weak or they may be blocked by topography, such as islands."

Just like noises above the sea, she explains, objects like walls can make it harder to listen to a sound.

Scientists can use both seismic and audio instruments to build a better picture of underwater volcano eruptions. According to a press release, the signs of an underwater volcanic eruption can even be picked up by whale song-monitoring devices.

Tepp said in the release, "It makes you wonder, how many of these signals have we seen on distant instruments where nobody knew what they were, and it's a submarine volcano from halfway around the world?"