In May 2021, Congo’s Mount Nyiragongo, one of the world’s most dangerous volcanoes, came to life without warning. Lava erupted from the fissures and flowed down the mountain toward the cities below, leaving hundreds dead or missing and hundreds more injured.
Now, using data from monitoring stations installed near the volcano in 2015, researchers have pieced together how did that eruption happen so suddenly. The data also suggests that the event could have been even more deadly, and highlights the urgent need to better understand this volcano’s particular dangers before the next eruption, volcanologist Delphine Smittarello and her colleagues report in the March 1 issue. September. Nature.
“Nyiragongo is unique in that 1 million people live right at the foot of the volcano,” says Smittarello, of the European Center for Geodynamics and Seismology in Luxembourg. The mountain is located near the eastern border of the Congolooming over the Congolese city of Goma, with about 700,000 inhabitants, and the Rwandan city of Gisenyi, with about 83,000 inhabitants (Serial number: 2/12/14). “There are so many people so close to a very dangerous place.”
Nyiragongo’s last two eruptions, in 1977 and 2002, were heralded by days of distinct seismic noises loud enough to be felt by people living nearby. But before the eruption on May 22, 2021, even sensitive monitoring stations near the volcano seemed to detect no clear warning signs of magma moving underground.
There was the great lake of molten lava bubbling up in the volcano’s summit crater: by 2021, that lake had risen to near the top of the crater. But usually, lake level alone isn’t enough to indicate an impending eruption, Smittarello says. Levels had risen and fallen intermittently over the years since 2002 as magma moved around the volcano’s deep pipes. And in 2021, the lake was still 85 meters below its 2002 level.
So Smittarello and his colleagues took another look at the seismic and acoustic data from the monitoring stations. This time, the analysis identified a rumble of small earthquakes that started just 40 minutes before the actual eruption. Half an hour later, just 10 minutes before the lava erupted, acoustic signal detections: low-frequency “infrasound” waves — began to rise, an indication that the volcano was about to erupt (Serial number: 06/25/18).
The trigger for the actual eruption was likely a small rupture that formed in the volcanic cone due to the build-up of stress over time from the pressure and heat of the magma inside it, the researchers say. That would have been enough to allow the magma to break through.
The short time lag between the signals and the eruption was likely because the magma was already extremely close to the surface, the researchers suggest. “What we monitor is the magma in motion, not the presence or absence of magma,” says Smittarello. Because the magma had very little distance to travel, there was also very little warning.
The eruption itself lasted about six hours, but then there was a lot of seismic activity that lasted another 10 days, suggesting that the magma was now in motion. That data, monitored in real time, indicated something worrying: magma was moving underground, moving away from the summit, winding beneath the city of Goma and nearby Lake Kivu.
As the magma migrated, scientists and local citizens worked together to track the formation of cracks in the ground, which can indicate spreading dikes, lateral pathways through which magma moves below the surface. Similar lateral pathways formed during the Kilauea eruption in Hawaii, says Smittarello. In that case, the magma emigrated to a neighborhood along the lower east rift zone of the volcano before it erupted (Serial number: 7/6/18).
Based on the possible path of the magma, Goma city officials issued evacuation orders for tens of thousands of people who might find themselves in the path of the magma. Meanwhile, scientists anxiously watched for signs of a potential “limnic eruption” in Lake Kivu, a rare type of disaster in which a noxious cloud of dissolved gases, such as carbon dioxide and methane, suddenly erupts from deep lakes and suffocates nearby living things (Serial number: 2/4/94). Gas-rich magma seeping into the bottom of the lake could have caused such an eruption. In any case, “if [the magma] finds a way to the surface, it’s a catastrophe,” says Smittarello.
No catastrophe occurred, fortunately, says Smittarello. “It was a lucky situation. But we don’t know why.”
It was especially lucky given the fact that the magma was closer to the surface than was thought at the time, the team reports in the new study. That means the residents above were even closer to a bigger disaster than previously thought.
Reanalysis of post-eruption seismic data allowed the researchers to determine the actual position of the underground dikes. The team found that a dam below Goma was 450 meters deep. That’s particularly surprising because such a shallow magma channel would be expected to emit a telltale cocktail of volcanic gases from cracks in the ground.
It’s not uncommon for volcanic dikes to provide no gaseous indicators of their presence, says geophysicist Michael Poland of the US Geological Survey, based in Vancouver, Washington, and directs the Yellowstone Volcano Observatory. The magma may have lost many of its gases as it circulated towards the summit lava lake; by the time it pushed into the underground channels, it was already potentially degassed.
But that scenario is concerning because there is one less warning sign of a possible danger to communities above, says Poland, who was not involved in the new study. It also raises new questions, such as how such gas-poor magma might interact with Lake Kivu if it flows into the lake.
What the 2021 eruption makes clear is that scientists need to investigate such questions to better understand Nyiragongo’s peculiarities and adapt monitoring and hazard warnings accordingly, adds Poland.
All volcanoes have their own personalities, and scientists need to develop a better appreciation of any warning signs that may exist. In this case, for example, that could include the level of the lava lake, he says. “The traditional approach is less reliable in Nyiragongo.”