The May 22nd 2021 eruption of Nyiragongo Volcano, DRC: field observations and reconstruction of eruption dynamics

Activity at Mt Nyiragongo, situated in the Democratic Republic of Congo (DRC), has repeatedly threatened surrounding cities and villages. Lava flows in 2002 and 2021 reached parts of Goma, damaging infrastructure and displacing people. Its last eruptions in 1977, 2002, and 2021 were short (timescale of hours) and the immense population pressure in the region resulted in the rapid resettling of areas inundated by lava. As a result, detailed documentation of undisturbed lava deposits and observational data on lava effusion rates, fountain heights, and emplacement dynamics are scarce. The May 22nd 2021 eruption produced lavas that followed the general flow directions of those erupted in 1977 and 2002. We combine field observations and measurements of the May 2021 eruptive products with petrological and numerical models to constrain the volume of erupted lava and lava fountain heights. The reconstructed volume is ~ 10.2*10⁶ m³. Our analysis of fountain heights suggests an average of 7 to 14 m at the Lemera Makao (western) fissure and 26 to 55 m at the Shaheru (eastern) fissure. Timely fieldwork enabled detailed textural and compositional analysis to determine composition, crystal and bubble content, sizes, shapes, and their respective distributions, which are used to deduce the lava’s rheology. Effective medium viscosity modelling revealed that bubble loss and phenocryst growth had only moderate effects on lava viscosity, and rapid groundmass crystallization was the core factor governing lava flow arrest. These data may help inform modelling efforts to assess the risk associated with lava flows at Nyiragongo and volcanoes of similar compositions.