A volcano erupted sans warning. Now, scientists know why

Last year, one of the most dangerous volcanoes in Africa erupted without warning. In a way, Nyiragongo, a vertiginous volcano in the Democratic Republic of Congo, is always erupting: The mountain is crowned by a rare, persistent lava lake constantly fed by churning magma below. But on May 22, 2021, its molten innards found another route to the surface. They oozed from fractures on the volcano’s flanks toward the metropolis of Goma, leading to the deaths of at least 31 people, injuring 750 others, displacing thousands more and leaving behind a trail of destruction.

Now, in a new study, Delphine Smittarello, a geophysicist at the European Center for Geodynamics and Seismology in Walferdange, Luxembourg, and her colleagues articulated how the eruption managed to ambush everyone. Most sufficiently monitored volcanoes offer warning signals before erupting. Magma forcing its way through rock generates distinctive types of earthquakes, deforms the land as it ascends and unleashes noxious gases. Some volcanoes are so active that they are always creating noticeable chaos, but a distinct change in their usual, or “background,” behavior betrays their eruptive ambitions. Not so for Nyiragongo in 2021. To any expert’s eyes, it was business as usual. “We were not able to detect any dramatic change that could tell us that an eruption will occur,” Dr. Smittarello, the lead author of the new study, said.

Her team suspects that, before the paroxysm, magma intruded below Nyiragongo’s flank — but then, it waited. Not only does immobile magma stay silent, but the molten mass was already so close to the surface that should the flank have broken apart, it would have immediately erupted without the usual precursory clamour.

And it was only a matter of time. On May 22, the flank — weakened over time by earthquakes, and jostled and seared by incursions of magma — yielded. For six hours, the volcano wept from its freshly opened wounds. This sort of unannounced eruption offers scientists a harsh lesson: For every paradigm-shifting secret they extract from their mountainous subjects, “there are always things that we don’t understand,” said Emily Montgomery-Brown, a geophysicist at the U.S. Geological Survey’s Cascades Volcano Observatory who was not involved in the study. “It’s a good reminder not to get cocky.”

With its unusually fluid, fast-moving lava and its ability to suffuse suffocating carbon dioxide gas into its surroundings, Nyiragongo is an extraordinarily perilous volcano that frequently endangers both Goma, in Congo, and Gisenyi, a contiguous Rwandan city.

Nyiragongo’s flank eruptions in 1977 and 2002 killed hundreds, but both were preceded by signs that magma was about to invade the surface: large earthquakes, strange lava lake convulsions and the eruption of the nearby Nyamulagira volcano, the subterranean magmatic pathways of which are partially entwined with Nyiragongo’s.

Since 2015, a new seismic array has been established in the region to listen to Nyiragongo’s magmatic music. Partly thanks to its endlessly bubbling lava lake, its soundtrack is as interminable as it is loud. Trying to pick out unusual changes from the cacophony is akin to identifying a new voice in a gigantic crowd of people talking — not impossible, but extremely difficult. But it’s possible that we will never become perfect prophets of our volcanic futures. “There may be things we will never be able to forecast,” Dr. Montgomery-Brown said.