In May 2021, Mount Nyiragongo in Congo, one of the most dangerous volcanoes in the world, erupted without warning. Lava spewed from the fissures and flowed from the mountain to the city below, killing or missing hundreds and injuring hundreds.
Now, using data from a monitoring station installed near the volcano in 2015, researchers have pieced it together How did that eruption happen suddenly?The data also suggest the event could have been more deadly — and underscore the urgent need to better understand the volcano’s particular hazards ahead of its next eruption, volcanologist Delphine Smittarello and colleagues said in their Sept. 1 report. nature.
“What makes Nyiragongo unique is that 1 million people live at the foot of the volcano,” said Smittarello of the European Centre for Geodynamics and Seismology in Luxembourg. Located near the eastern border of Congothe looming Congolese city of Goma, with a population of about 700,000, and the Rwandan city of Gisenyi, with a population of about 83,000 (Serial Number: 12/2/14). “There are so many people so close to a very dangerous place.”
Nyiragongo’s last two eruptions were in 1977 and 2002, both of which heralded several days of distinct seismic rumbles strong enough to be felt by those nearby. But even sensitive monitoring stations near the volcano appear to have no clear warning signs of magma moving underground ahead of the May 22, 2021 eruption.
The volcano’s summit crater has a huge lake of lava boiling: by 2021, the lake has risen near the top of the crater. But usually lake levels alone aren’t enough to indicate an impending eruption, Smittarello said. Levels have risen and fallen intermittently over the years since 2002, as magma moved around pipes deep in the volcano. By 2021, the lake is still 85 meters below its 2002 level.
So Smittarello and her colleagues looked again at the seismic and acoustic data from the monitoring stations. This time, the analysis identified the rumble of a small earthquake that started 40 minutes before the actual eruption. Half an hour later, just ten minutes before the lava eruption, a sound signal was detected— low frequency “infrasound” waves – began to increase, suggesting that the volcano was about to erupt (SN: June 25, 2018).
The trigger for the actual eruption may have been a tiny rupture that formed in the volcanic cone, the researchers said, due to pressure from the magma inside and the build-up of heat over time. This is enough to let the lava pass through.
The researchers believe that the brief lag time between the signal and the eruption may be because the magma is already very close to the surface. “We’re monitoring magma movement, not the presence or absence of magma,” Smittarello said. Because the magma travels a short distance, there is no warning.
The eruption itself lasted about six hours, but it was followed by 10 days of heavy seismic activity, suggesting that the magma is now moving. The data from these real-time monitoring shows something disturbing – magma is moving underground, away from the top of the mountain, meandering below the city of Goma and the nearby Lake Kivu.
As the magma migrated, scientists and local residents worked together to track the formation of surface fissures, which can indicate magma and lateral channels through which it moved beneath the surface. A similar lateral channel formed during the eruption of Hawaii’s Kilauea volcano, Smittarello said.In this case, magma Migrate to nearby along the lower eastern rift belt of the volcano before the eruption (SN: July 6, 2018).
Based on the magma’s likely path, Goma city officials issued evacuation orders for tens of thousands of people who may have been in the magma’s path. Meanwhile, scientists are anxiously watching for potential “eruption along the lake“In Lake Kivu – a rare disaster in which a toxic cloud of dissolved gases such as carbon dioxide and methane erupted suddenly from the deep lake, suffocating nearby creatures (SN: 4/2/94). Gas-rich magma seeping into the lake bottom may have triggered such eruptions.In either case, “if [the magma] It’s a disaster to find a way to the surface,” Smittarello said.
Fortunately, no disaster happened, Smittarello said. “It was a lucky situation. But we don’t know why.”
This is especially fortunate given the fact that the magma is closer to the surface than was thought at the time, the team reports in the new study. That means the residents above are closer to a bigger catastrophe than people realize.
Reanalysis of subsequent seismic data allowed the researchers to determine the actual location of the underground levee. The team found that a dike beneath Goma was as deep as 450 meters. This was especially surprising since such shallow magma channels were expected to release a mixture of volcanic gases from cracks in the ground.
It’s not unheard of for volcanic levees not to give an indication of their presence, said Michael Poland, a geophysicist with the U.S. Geological Survey, who runs the Yellowstone Volcano Observatory in Vancouver, Washington. The magma may have lost a lot of gas as it circulated to the mountaintop lava lake; it may have degassed as it pushed into the underground passage.
But Poland, who was not involved in the new study, said the situation was worrying because there was one less warning sign that the aforementioned communities could be at risk. It also raises new questions—for example, how this gas-poor magma would interact with Lake Kivu if it were to flow into it.
Poland added that the 2021 eruption clearly shows that scientists need to investigate these issues to better understand Nyiragongo’s specificity — and adjust monitoring and hazard warnings accordingly.
All volcanoes have their own personalities, and scientists need to better understand any warning signs that may exist. In this case, for example, this could include lava lake levels, he said. “In Nyiragongo, traditional methods are less reliable.”