Left image shows the rocky coast of Kīlauea, and right image shows a punctured steel boat roof.
Lava from Kīlauea formed a small new island on 13 July 2018 (left). A molten lava bomb busted through the roof of a Lava Ocean Tours boat 3 days later while tourists watched the lava (right). Credit: HVO (right), Hawaii Department of Land and Natural Resources (left)
AGU Fall Meeting 2021

On 16 July 2018, a lava bomb from the Kīlauea volcano tore through the roof of a tour boat just off the Big Island of Hawaii. Sightseers on the boat described blobs of molten rock smashing into the boat out of nowhere—injuring 23 passengers and breaking a woman’s leg.

Now, scientists have collaborated with those on the boat to pinpoint the acoustic fingerprint of lava explosions. The finding helped researchers identify hundreds of blasts in the area that year.

A map of the southeastern flank of the Kīlauea volcano on Hawaii’s Big Island. Red lines trace the flow of the lava as it met the sea, the gray star shows the location of the tour boat that was struck, and the yellow triangles are the underwater hydrophones placed by the researchers. Credit: Puja Banerjee and Yang Shen

Shortly after Kīlauea entered a new eruptive phase in May 2018, geophysicist Yang Shen and his collaborators installed ocean bottom instruments at 11 locations near the southeastern flank of the volcano. The sensors, placed between 700 and 5,000 meters deep, recorded underwater signals from earthquakes and the outpouring of lava into the sea.

Lava-water explosions in the ocean are steam-driven blasts that in some instances, shoot blobs of lava hundreds of meters into the air, called lava bombs. These outbursts have unique underwater sound signatures, too. “It’s very different from earthquakes or submarine landslides,” said Shen, a professor of oceanography at the University of Rhode Island (URI).

The lava bomb that hit the tour boat shows up in spectrograms of ocean noise on 16 July 2018. The spectrogram describes the changes in frequency in hertz over 50 seconds. Arrows denote the explosion at four different hydrophones underwater near the volcano. The sound waves arrived at slightly different times depending on the sensor’s location. Credit: Puja Banerjee and Yang Shen

Using time and location data from photographs taken on the tour boat, URI Ph.D. student Puja Banerjee determined the acoustic fingerprint of the 16 July lava bomb using four underwater microphones, or hydrophones. She then 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 research, which has not been peer reviewed, on 14 December at AGU’s Fall Meeting 2021.

The work “has important implications for mitigating future volcanic hazards,” said Robert Dziak, an acoustics scientist at NOAA’s Pacific Marine Environmental Laboratory who wasn’t involved in the research. The study reveals a hot spot along the coastline where the highest number of hydrovolcanic explosions occurred.

Particles ejected from volcanoes can range in size from dust and ash (millimeters) to lapilli (“little stones” of 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 deadly. In 1992, one person was killed, and three others were seriously injured when lava bombs exploded from Marapi, Indonesia. In Hawaii in 2018, a bowling ball–sized lava bomb hit a man on the leg outside his home.

Eruptions on land can launch lava bombs, too. The explosions are caused by dissolved gas or vapor in lava that comes out of solution and forms bubbles that then explode.

More than 80% of volcanic eruptions occur in the ocean, said Shen. “We might be able to use those natural acoustic signals to study how acoustic waves propagate in the ocean.”

Scientists could use the relationship between acoustic wave speed and seawater temperature to study how climate change is affecting ocean waters, said Yang. The short, clear sound bursts from lava-water explosions may be easier to decipher than more complex acoustic signals from underwater earthquakes.

Previous research suggests that sound from lava-water explosions travels far: Hydrophones placed more than 5,000 kilometers from Kīlauea recorded explosions, according to a 2001 study.

—Jenessa Duncombe (@jrdscience), Staff Writer

Citation: Duncombe, J. (2021), Hundreds of volcanic explosions detected underwater at Kīlauea, Eos, 102, https://doi.org/10.1029/2021EO210666. Published on 14 December 2021.
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