In May last year, plumes of ash launched miles into the air above the city of Goma in the Democratic Republic of the Congo (DRC). Cracks tore through the ground for hundreds of meters, releasing hot gas or flames. When Mount Nyiragongo, a volcano near the city, erupted, it destroyed about 3,600 homes and buildings, forced evacuations of 400,000 residents and killed at least 32 people, according to a report from the Smithsonian’s Global Volcanism Program, and it injured hundreds more.
But no one saw this destruction coming—in fact, the eruption took residents and seismologists entirely by surprise.
At the time, the nearby Goma Volcano Observatory didn’t notice any advance sign that the volcano was on the verge of spewing out lava. But now, in a study published last week in the journal Nature, researchers share how the imminent eruption managed to evade detection.
Mount Nyiragongo is near the DRC’s eastern border with Rwanda. It threatens both the Congolese city of Goma and the Rwandan city of Gisenyi, which are home to about 700,000 and 83,000 people, respectively, per Science News’ Carolyn Gramling.
“Nyiragongo is unique in that one million people are living just at the foot of the volcano,” Delphine Smittarello, the paper’s first author and a volcanologist at the European Center for Geodynamics and Seismology, tells Science News. “There are so many people so close to a very dangerous place.”
In both 1977 and 2002, eruptions from the mountain killed hundreds of people. But these disasters were preceded by large earthquakes, the eruption of a nearby volcano and other indicators of a looming explosion, per the New York Times’ Robin George Andrews. With these events, people in the area felt precursory earthquakes for days in advance, according to Science News.
On top of earthquakes, other signs of an impending eruption include land deformities and the release of harmful gases, per the Times. But as the researchers combed through observations taken before Mount Nyiragongo erupted, they were unable to find any of these warnings. “We were not able to detect any dramatic change that could tell us that an eruption will occur,” Smittarello tells the Times.
But the scientists did find subtle changes: They report that small quakes began 40 minutes before the eruption, and some acoustic signals occurred only 10 minutes in advance. The magma, which had already risen close to the surface of the volcano’s crater, likely produced a rupture in the volcanic cone and spewed out. Since the magma didn’t have to move far to erupt, the warning signals came at the last minute, according to Science News.
Mount Nyiragongo is a little unusual in that a lava lake sits atop it, and its magma isn’t tightly trapped inside, writes Alison Snyder for Axios. So, there was no accumulation of pressure that, in a different kind of volcano, would usually lead to signs of a coming eruption.
“This is a strange volcano,” Benoît Smets, a study co-author and a geohazards expert at the Royal Museum for Central Africa in Belgium, tells the Times. “You will not be able to detect such kinds of eruptions,” if this volcano is treated like a typical one.
Worldwide, there are about 100 observatories monitoring some 1,350 volcanoes, per Axios—meaning there’s not detailed data about the bulk of them, and many remain unmonitored. Scientists’ understanding of these volcanoes is based on a group of the most-studied ones, while many of the others are still shrouded in unknowns.
Seismologists need to develop a better understanding of Nyiragongo to improve monitoring for its eruptions, says Michael Poland, a geophysicist at the U.S. Geological Survey who did not contribute to the study, to Science News. “The traditional approach is less reliable at Nyiragongo,” he says to the publication.