Models of the Formation of the 29 February 2016 New Spatter Cone Inside Mount Nyiragongo

A new and unusual eruptive event occurred on 29 February 2016 within the summit crater of the Mount Nyiragongo volcano. Based on field campaigns performed between July 2015 and September 2017, and building on a previously published buoyancy‐driven bidirectional magma flow model explaining the progression of Mount Nyiragongo lava lake level, we provide the first quantitative estimations of volumes of erupted lava outpouring from the new spatter cone. Besides matching field data of the lava lake level covering the period December 2002 to September 2017, numerical solutions of the model reveal that the most probable dike path is one originating from the shallow magma reservoir, and not from the lava lake or branching from its feeding conduit. According to these simulations, the reservoir and erupted lava volumes are respectively estimated to the order of 10 km3and 20 M m3. Magma overpressure at the level of the shallow reservoir is estimated in the range 12 to 16 MPa, high enough to potentially initiate new erupting events. Mount Nyiragongo volcano contains the world's largest continuously active lava lake within its crater. This constitutes a major potential geological regional hazard to the Virunga volcanic region's inhabitants within the Democratic Republic of Congo and Rwanda, particularly the cities of Goma and Gisenyi with about 1.5 million people living at its foot. Within the last four decades, the lava lake has been twice (1977 and 2002) drained out catastrophically through a fracture network. A new and unusual eruptive event occurred on 29 February 2016 proximity to the lava lake, forming within a few months a small cone of about 30 m high. Based on a mathematical model incorporating fluid mechanics, we calculate using computer simulations the overpressure of the magmatic system due to the accumulation of magma circulating back from the lava lake into the feeding magmatic reservoir. Combined with the fractured nature of the edifice, we estimate this that overpressure renders the possibility of dikes reaching the surface inside the crater, as currently observed, and also outside the summit crater, in which case the threat of a future major flank eruption becomes real. Overpressure exerted in the shallow magmatic reservoir by buoyancy force is sufficient to initiate a dike feeding the new spatter coneGeochemical and petrological analyses suggest that the feeding systems of the lava lake and spatter cone are of the same originThe magmatic flow model tuned to fit field observations indicates 0.9 km3of magma convected since 2002 for a reservoir volume of 10 M m3