Underwater volcano eruptions may alter the ocean soundscape

A steam plume rises from Bogoslof volcano as hot lava heats the seawater during an eruption in August 2017. Only small parts of the volcano are visible as island on the ocean surface. Photo: Dave Withrow (NOAA/Fisheries)

Their signals may cause anomalies in the monitoring of earthquakes, land-based volcanoes or even whale songs

Most volcanoes erupt beneath the ocean, but scientists know little about them. Yet they may have effects on life on our planet, e.g.: they may alter the ocean soundscape.

Depending on the size and depth of an underwater eruption, gas and ash may never break the ocean surface, or the gas and ash could create a volcanic plume with the potential to interfere with air travel. „The ocean is a big place so it’s pretty unlikely that you’re going to have a situation where a ship haphazardly wanders over an eruption, but there are a few that have come close,“ Gabrielle Tepp of the Alaska Volcano Observatory and the U.S. Geological Survey (USGS) says.

These unpredictable eruptions may also create a floating blanket of rocks, called a pumice raft, which can clog harbors and damage boats.

During the 174th Meeting of the Acoustical Society of America, held Dec. 4-8, 2017, in New Orleans, Louisiana, Tepp presented acoustic observations from two submarine volcanoes: Ahyi, a seamount in the Northern Mariana Islands in the Pacific Ocean, and Bogoslof, a shallow submarine volcano in the Aleutian Islands in the Bering Sea.

The volcanoes made very different sounds, suggesting that different processes occurred during eruption. In 2014, Ahyi erupted for two weeks, with short, repetitive gunshotlike explosions every few minutes. In 2016 and 2017, Bogoslof had more sustained eruptions, lasting minutes to hours, which occurred every few days.

Evidence of these eruptions showed up on distant seismometers, which measure waves passing through the ground to record earthquakes, and hydrophone arrays that pick up underwater sound to detect covert nuclear detonations. When volcanoes erupt directly into the water, the sounds can travel for thousands of miles before dissipating.

On November 26, 2010, Pakistani fishermen returned from a day at sea to report that a new island had emerged. The tiny dot of land was a mud volcano, and it was still visible on January 11, 2011, when the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite acquired the image. Yet there’s no need to change any maps: mud volcanoes mostly disappear again within a few months, washed away by the waves and currents in the sea. Photo: Nasa / Wikimedia Commons

Questions remain, however, such as if seismometers are sufficient for remote monitoring or if the more accurate information provided by cabled hydrophone arrays is worth the greater expense. Researchers are also interested in how the movement of waves from water into rock, and vice versa, affects signal detection.

Tepp and colleagues at National Oceanic and Atmospheric Administration and USGS recently deployed a hydrophone array in the Northern Mariana Islands. With those data, they will hope to determine where and how often local volcanoes erupt to see if the area needs better hazard monitoring.

Due to the long distances that eruption signals travel, they likely show up as anomalies on far-off monitoring devices used to study earthquakes, land-based volcanoes or even whale songs.

„Eruptions that create a loud enough sound, in the right location, can travel pretty far, even from one ocean to another,“ Tepp said. „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?“

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(09.01.2018, USA: 01.09.2018)