Styles of explosive activity during the 2021 Cumbra Vieja eruption, as illuminated by high-frequency imaging and acoustic sensing

A variety of eruptive styles concurred to define the explosive activity of the 2021 Cumbre Vieja eruption (La Palma, Canary Islands, Spain). These styles include, as broadly defined, lava fountaining, Strombolian explosions, rapid Strombolian, spattering, ash-rich jets, and ash venting, and occurred both alternately and simultaneously at the multiple vents that hosted the activity during the more-than-three-months-long eruption. In order to capture the defining features and the underlying processes of these styles and of their transitions, we deployed FAMoUS (Fast, MUltiparametric Setup) during two field campaigns, between 22 September-1 October and between 5-9 November 2021. FAMoUS includes one high-speed camera (frame rate 250 to 500 frames per second (FPS) and 0.021-0.147 m/pixel resolution at the vent), one thermal camera (up to 50 FPS and 0.2-0.8 m/pixel .ca), three high-definition cameras (25 FPS, 0.03-1.2 m/pixel ca.), and one microphone (flat response between 0.5 and 10000 Hz, sampling rate 20 kHz). Preliminary video processing results, obtained using both manual tracking and Optical Flow routines, reveal ejection velocities of pyroclasts in the 20-220 m/s range, with the highest and the lowest values of peak velocity being recorded during Strombolian explosions and ash venting, respectively. All activity styles display ejection velocity fluctuations and variably marked ejection pulses, which are more pronounced during Strombolian explosions. Lava fountains feature the highest mean ejection velocity and a variety of fluctuation patterns, with larger-amplitude and more abrupt ones when transitioning towards Strombolian explosions. The maximum settling velocity of bomb-to lapilli-sized pyroclasts in the vicinity of the vent is remarkably stable around 50 m/s. The transition between the different styles of activity is marked by changing rates of ejection pulse frequency/amplitude and relative proportions of ash and bombs, pointing to a feedback between the volume, ascent rate, and frequency of gas pockets rising in the conduit, and the changes induced by their transit through the magma residing in the uppermost termination of the conduit.