The details of the 2021 Nyiragongo eruption using infrasound

Eruptions at continental basaltic volcanoes can take and combine various forms, including lava lakes, lava flows and fountaining, explosions or structural collapses. Recording seismicity is widely recognized as essential for tracking magma movements at depth but must be complemented with other observations for monitoring eruptions, which are by essence atmospheric processes. Aside from a few well-instrumented cases worldwide, accurately reconstructing the precise eruptive mechanisms and chronology is hampered by the lack of detailed visual observations in space and time. However, because they emit low-pitched inaudible sounds, called infrasounds, any changing and potentially hazardous eruptive activity can be inferred with specialised microphones.

On 22 May 2021 in D.R. Congo, the drainage of Nyiragongo’s long-lived and world’s largest lava lake was accompanied by lava flows from eruptive fissures toward a one-million urban area composed of the cities of Goma (D.R. Congo) and Gisenyi (Rwanda). After 1977 and 2002, this was the third known flank eruption and the first one adequately monitored with seismic and geodetic instruments to understand magma movements at depth. A probable scenario supported by these geophysical observations is the rupture of the edifice, starting around 15:57 UTC, draining the lava lake during a short-term (~6 hours) flank eruption and initiating a week-long magmatic intrusion (dyke) in the Earth’s crust.

Using acoustic numerical modeling, we converted infrasound records from local distance (< 20 km) up to Kenya (more than 800 km away from Nyiragongo) into high-resolution time-lapse observations of this catastrophic lava-lake drainage. The emitted infrasounds also provided unprecedented insights into the timing of fissure openings and lava eruptions on the volcanic flank, occurring simultaneously with the lava lake drainage. This striking example highlights how decoding each specific volcano’s acoustic signature provides unique information inaccessible to other ground-based instruments, which can be integrated to monitoring and multi-hazard early warning systems.