Hundreds of volcanic explosions underwater near Kīlauea. discovered

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AGU autumn conference 2021

On July 16, 2018, a lava bomb from the KÄ«lauea volcano hit the roof of a tour boat off the Big Island of Hawaii. Onlookers on the boat described how lumps of molten rock hit the boat out of nowhere, injuring 23 passengers and breaking a woman’s leg.

Now scientists have worked with those on the boat to locate the acoustic fingerprint of lava explosions. The discovery helped researchers identify hundreds of explosions in the area that year.

A map of the southeastern flank of the KÄ«lauea Volcano on Hawaii’s Big Island. Red lines trace the lava flow when it hits the sea, the gray star shows the position of the excursion boat that was hit, and the yellow triangles are the underwater hydrophones placed by the researchers. Photo credit: Puja Banerjee and Yang Shen

Shortly after KÄ«lauea entered a new phase of eruption in May 2018, the geophysicist Yang Shen and his colleagues installed seabed instruments in eleven locations near the southeast flank of the volcano. The sensors, located between 700 and 5,000 meters deep, recorded underwater signals from earthquakes and lava eruptions into the sea.

Ocean lava water explosions are steam-powered explosions that, in some cases, shoot clumps of lava hundreds of meters into the air, called lava bombs. These bursts also have unique underwater sound signatures. “It’s very different from underwater earthquakes or landslides,” says Shen, professor of oceanography at the University of Rhode Island (URI).

The lava bomb that hit the excursion boat showed up in spectrograms of the sound of the sea on July 16, 2018. The spectrogram describes the frequency changes in Hertz over 50 seconds. Arrows indicate the explosion at four different hydrophones underwater near the volcano. The sound waves arrived at slightly different times depending on the location of the sensor. Photo credit: Puja Banerjee and Yang Shen

Using time and location data from photos taken on the excursion boat, URI Ph.D. The student Puja Banerjee used four underwater microphones or hydrophones to determine the acoustic fingerprint of the lava bomb on July 16. Then she combed through the acoustic data from that summer and found that from early July to early August when the eruption ended, there were at least 644 lava explosions in the area. She will present the unreviewed research on December 14th at the AGU’s autumn meeting 2021.

The work “has important implications for containing future volcanic hazards,” said Robert Dziak, an acoustician at NOAA’s Pacific Marine Environmental Laboratory who was not involved in the research. The study reveals a coastal hot spot where most of the hydrovolcanic explosions have occurred.

Particles emitted by volcanoes can range in size from dust and ash (millimeters) to lapilli (“small stones” several centimeters) to bombs (6 centimeters to several meters).

Lava explosions don’t always shoot lava into the air, but when they do the result can be fatal. In 1992 one person was killed and three others were seriously injured when lava bombs exploded in Marapi, Indonesia. In 2018, in Hawaii, a bowling ball-sized lava bomb hit a man in the leg in front of his house.

Land eruptions can also fire lava bombs. The explosions are caused by dissolved gas or steam in lava that comes out of solution and forms bubbles that then explode.

More than 80% of volcanic eruptions occur in the ocean, Shen said. “We could possibly use these natural acoustic signals to study how acoustic waves propagate in the ocean.”

Scientists could use the relationship between sound wave speed and sea water temperature to study how climate change is affecting sea water, Yang said. The brief, clear sound explosions of lava water explosions may be easier to decipher than more complex acoustic signals of underwater quakes.

Previous research suggests that the sound of lava-water explosions is widespread: hydrophones placed more than 5,000 kilometers from KÄ«lauea recorded explosions, according to a 2001 study.

—Jenessa Duncombe (@jrdscience), Employed author

Quote: Duncombe, J. (2021), Hundreds of Underwater Volcanic Explosions Discovered at KÄ«lauea, Eos, 102, https://doi.org/10.1029/2021EO210666. Published on December 14, 2021.
Text © 2021. The authors. CC BY-NC-ND 3.0
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