When Yves Moussallam trekked around Vanuatu’s Ambrym volcano in the winter of 2018, the ground was blanketed in green, and five incandescent lakes of molten rock burbled in the volcano’s caldera. Just two weeks later, though, he found himself in a landscape devoid of color. Gray ash coated each rock and crevice, and the lakes sat empty, their lava vanished like water swirled down a drain.

“It looked like everything was in black-and-white,” says Moussallam, a volcanologist at Columbia University who is also associated with France’s Laboratoire Magmas et Volcans. “The whole caldera area had completely changed.”

This transformation came in the wake of an extraordinary eruption that surprised scientists with its progression. While some of the lava spurted up from nearby cracks, the vast majority moved underground—a slug of magma big enough to fill 160,000 Olympic swimming pools. As the team reports in Scientific Reports, the process cracked the earth, sending coasts soaring into the air, and brought lava burbling up onto the ocean floor.

“It’s kind of a negative eruption, in a way,” says volcanologist Clive Oppenheimer of the University of Cambridge, who was not on the study team. “It’s not stuff coming out of the ground, it’s the magma migrating beneath the ground.”

View Images Lava roils in one of Ambrym's lakes before the 2018 eruption. Lava lakes can act like windows to the deep, giving clues to what's happening deep beneath the surface. Photograph by Dan Tari, Vanuatu Meteorology and Geohazards Department

The new study, conducted in collaboration with the Vanuatu Meteorology and Geohazards Department, provides a rare and detailed portrait of Ambrym’s activity above and below, which can help geologists unravel the myriad processes that contribute to volcanic activity.

“As volcanologists, we’re always trying to understand what’s going on kilometers beneath our feet, and that can be difficult because we don’t have direct access to the magmatic reservoirs,” says Tara Shreve, the new study's lead author and a Ph.D. candidate at the Institut de Physique du Globe de Paris. But the new study combines an array of clues to better understand the events conspiring deep underground, providing important details about Ambrym’s volcanic capabilities—and the variety of hazards such eruptions can present.

“It’s not like a lab science, where you can go and do the same experiment over and over and over again,” says Emily Montgomery-Brown, a geodesist at U.S. Geological Survey’s California Volcano Observatory who was not part of the study team. “We learn so much from every single eruption.”

A chance sighting

Moussallam initially ventured to Ambrym as part of a study analyzing the prodigious gasses puffing from volcanoes across the Vanuatu arc, a project funded by the National Geographic Society. They monitored gasses at three of Ambrym’s lava lakes before heading on their way. Two weeks later, they were prepping for their flight back home from Vanuatu’s capital city, Port Vila, when they got the news: Ambrym was erupting.

View Images As magma squeezed through the subsurface during Ambrym's 2018 eruption, it caused the landscape above to fracture and crack. This was especially evident in Pamal village, eight miles from the crater's rim. Photograph by Bernard Pelletier, Géoazur

The team caught a helicopter back to the island and gaped at the difference. The molten lakes had disappeared. A lava flow cooled in the distance. Nearby trees crackled with flames. Connecting the dots, they at first assumed that magma had burst to the surface, draining the system.

“We thought that was the story,” Moussallam says. But, as they later discovered, the eruption was still playing out deep under their feet.

Intense earthquakes began rocking the island, and hefty fractures cut through the ground, forming steps in the landscape. In the coastal village of Pamal, eight miles from the caldera’s rim, roads were cleaved in two and houses were thrust feet into the air. The ground split under one building, leaving part of the structure hanging in mid-air.

“Clearly something was still going on,” Moussallam says. “It was really surprising it was so far away from where the eruption had begun.”

Pairing satellite analyses with on-the-ground observations, the team later learned that this was all part of a multi-day saga, as 14 billion cubic feet of magma shifted eastward, squeezing through deep cracks under the island for more than 10 miles.

This sudden addition of subsurface material shoved the coasts upward some six and a half feet, exposing a vast expanse of coral and red algae to deadly sunlight, says Géoazur’s Bernard Pelletier, a study coauthor who surveyed the coasts post-eruption. The loss was also felt at the volcano’s gaping summit caldera, which sunk by roughly eight feet.

On December 18, four days after the eruption began, volcanic pumice washed up on the island’s eastern shore—likely the result of magma finally oozing out from the subsurface into coastal waters.

Peering inside Earth

This type of draining through deep fissures in the ground, known as rift zone volcanism, is not unheard of, but Ambrym is an unlikely candidate.

Rift zone volcanism is most common in places where tectonic plates are separating, and extension in the crust pulls the land apart. Take, for example, the deep fissures found in Iceland’s volcanoes, which frequently line up with the pair of tectonic plates separating beneath the island country. Rift volcanism is also responsible for much activity at Kilauea which, along with the underlying flanks of Mauna Loa, is slowly sliding into the sea, Montgomery-Brown explains.

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By contrast, Vanuatu sits near the tectonic collision zone between the Pacific and Indo-Australian plates, which compresses the region. However, the latest analysis suggests that Vanuatu’s pressure-packed position isn’t a problem. The rift that drained the magma is oriented so that the two sides separate in the direction of least compression, allowing the fracture to inflate “like a whoopee cushion,” Montgomery-Brown says. The team’s modeling suggests that the pocket of magma inside the rift likely bulged more than 13 feet across in some spots.

One lingering curiosity is what happened to the volcano’s gas, says Philipson Bani, a volcanologist at France’s Institute of Research for Development who was not on the study team. Ambrym has been one of the greatest natural emitters of carbon dioxide and other volcanic gasses around the world for many years. How it maintained such activity remains a mystery, he says. Then the eruption happened and, almost overnight, the gaseous factory seemed to turn off.

“How can you just shut off the pipe?” Bani says. “On Ambrym, we have more and more and more gas in the past, and then boom. It stops.”

Magmatic budgets

Still more clues to Ambrym’s eruption may continue to emerge, Moussallam notes. He’s currently looking into the chemistry of the lavas, which seem to be of at least two different compositions, likely originating from separate reservoirs. While more research is required to confirm the find, it hints that the eruption’s ignition spark might have been the formation of a new connection between the pair of reservoirs.

Detailed analyses of volcanic systems, like this latest Ambrym paper, are important in understanding the mechanics of volcanic eruptions. Such work might even help give clues to a volcano’s magmatic budget, revealing how much molten rock might be available for future eruptions, Mongomery-Brown says.

Just months before Ambrym drained, Kilauea’s lava lakes in Hawaii were making their own fiery exit from deep cracks on the volcano’s flanks. But Montgomery-Brown and her colleagues recently found that Kilauea’s extensive eruption and the collapse of its summit crater came from the release of a mere 11 to 33 percent of its shallow magma reservoir. The find sparked many questions, including why the eruption stopped at all.

In these ways, both eruptions provide a vital look into the dynamic and varied ways volcanoes work, says Matthew Patrick, a geologist with the United States Geological Survey’s Hawaiian Volcano Observatory, who was not involved with the new study.