On 30 April 2018, Kilauea, the dozing 1247-meter-tall volcano on Hawaii’s Big Island, reawakened. Molten rock surged from a crack on Kilauea’s flank, marking the start of its largest eruption in centuries. It would eventually expel enough lava to fill half a million Olympic-size swimming pools and destroy 700 homes. As the lava flowed, the volcano also unleashed a series of rare explosive blasts that sent ashy plumes rocketing thousands of meters skyward.
Now, a team of earth scientists has analyzed those explosions and concluded they arose through a mechanism not seen in other eruptions. Essentially, the outbursts occurred as the mountain collapsed on itself, squeezing out explosive blasts of ash and gas, the researchers report today in Nature Geoscience. They liken the mechanism to a toy known as a Stomp Rocket, in which stomping on an air-filled plastic bellows shoots a foam rocket into the air.
“This is a very plausible explanation,” says Penny Wieser, a petrologist at the University of California (UC), Berkeley. Studying the mechanism driving these explosive blasts is a vital part of understanding Kilauea’s hazards, she says.
One of the most active volcanos on Earth, Kilauea erupts every 2 to 3 years on average. But these eruptions usually generate floods of lava, not towering plumes of volcanic ash and gas. The last time an intense explosive eruption rocked Kilauea was in 1924, when car-size blocks shot from the volcano’s maw. It’s not clear what causes these explosions, but scientists have long suspected that as the volcano’s subterranean magma chamber drained, groundwater encroached, flashing to steam with booming results.
Amid the volcanic turmoil of 2018, Hawaiian Volcano Observatory (HVO) scientists recognized the potential for a repeat of the 1924 outburst. The lava lake in Halema‘uma‘u crater at Kilauea’s summit rapidly drained, its surface disappearing from sight. If the subsurface magma level fell below the water table, scientists reasoned, groundwater flowing in from above it could spark a steam-driven eruption.
On 17 May, the first explosive blast rocketed from the peak. But then another followed. And another. Almost every day for weeks, ash-filled explosions blasted from the volcano with surprising regularity. Before each blast, a surge of intense earthquakes rattled the ground. How could groundwater drive such regular cycles?
It couldn’t, scientists later concluded. Modeling revealed the zone of hot rock around the magmatic system would require months or even years of cooling before water could flow in. Explosive eruptions can also come from the rapid rise of gaseous molten rock that blasts from the ground, like soda from a shaken bottle. But that also wasn’t the case for the 2018 explosions. Analysis of the erupted ash revealed it was likely pulverized rocks from the crater walls or reservoir, not fresh bits of molten rock as would be expected in a magmatic blast.
Instead, clues from the eruption pointed to another mechanism. By June 2018, the surface around the crater had visibly sunk. “We could see it happening,” says Josh Crozier, a geophysicist at Stanford University and an author on the new study. At the time, scientists suggested the collapse could be driving explosions, but the precise connection was not clear. For more clues, Crozier and his colleagues turned to geophysical data collected by HVO’s dense network of instruments across Kilauea for 12 explosions that May.
Seismometers catch the tiniest of volcanic shudders while other sensors track infrasound, which are waves with a frequency too low for human hearing. The data reveal subtle shifts in the surface and pressure changes in the magma reservoir below. Combining this information with radar measurements of plume height, the team developed a model to explain the observed behaviors.
In the model, as magma floods out of distant rifts, the shallow reservoir under the summit drains to create a growing gas pocket underneath an unsteady ceiling of rock. “At some point, it’s not supported enough. It suddenly fails,” Crozier says. Earthquakes rattle as a block of rock 1 or 2 kilometers wide sinks downward to fill the space below. The shift suddenly pressurizes the magma chamber, driving out the gas into the sky in an explosive burst. Then, as more magma drains away, a new gas pocket begins to emerge and the cycle starts anew.
This mechanism may have driven past explosive blasts at Kilauea, which has had at least six explosions in the past 1500 years powerful enough to send ash into the jet stream. Volcanic Stomp Rockets may happen at other volcanoes as well, although they’re likely rare, says Michael Manga, a volcanologist UC Berkeley. For example, in January 2022 Hunga Tonga-Hunga Ha’apai, an underwater volcano in the South Pacific Ocean, produced a blast so loud it was heard thousands of kilometers away. “It could be that the collapse enabled the eruption to be bigger than it otherwise would have been because it pressurized the magma underneath,” Manga says.
“Ultimately this paper will stimulate other people to think about the process,” says Michael Garcia, an emeritus volcanologist at the University of Hawaii at Manoa. Kilauea may still have other lessons to teach, he says. “I continue to be surprised by what new phenomena Kilauea shows us. She is full of surprises.”
More: https://www.science.org/content/article/hawaii-volcano
