Dynamics of Mount Nyiragongo lava lake inferred from thermal imaging and infrasound array

Abstract

Lava lakes provide a direct observation window into processes which usually remain hidden, such as magma convection and outgassing dynamics. We here report a coupled analysis of thermal infrared footage and infrasound array recordings at Mount Nyiragongo (D. R. Congo), and derive a conceptual model of the lava lake’s convective system and outgassing mechanism. We suggest that surface flow results from a horizontal pressure gradient at the surface of the lake, driving the crust from high-pressure regions where hot upwelling magma impinges the surface, to low-pressure regions where cold downwelling magma pulls away from the surface. The ascending current of this convection cell carries gas pockets, which once at the surface, are dragged across the lake into downwelling sinks. Such sinks are characterized by persistent chaotic bubble bursting (spattering), whose intensity and position are tracked from infrasound array analysis. Fluctuations of these are observed, but have not been correlated with oscillations of the lava lake level, nor with the variations of surface velocities, both recorded from infrared footage. We also report the activity of a new eruptive vent, which opened early 2016 near an inner circular fracture of the crater’s third terrace. We show that the vent’s activity was intermittent, alternating between explosive strombolian activity and effusive activity. The latter produced lava flows which spread on the crater’s last terrace before cascading into the active lava lake. Although no significant change in the lake behavior was witnessed while the new eruptive vent was active, increased attention should be addressed as this new activity could reflect over-pressurization of the shallow magmatic system. The variety of phenomena captured by this study complements and expands observations reported at other low-viscosity lava lakes, chiefly Kilauea (Hawai’i) and Erta Ale (Ethiopia). Despite Nyiragongo’s more vigorous convective regime (where multiple convective cells can operate simultaneously), we suggest that the mechanisms controlling the surface motion and outgassing are similar at all three systems, pointing to generic processes governing the dynamics of low-viscosity lava lakes.