Shallow magma dynamics at open-vent volcanoes tracked by coupled thermal and SO2 observations

Open-vent volcanic activity is typically sustained by ascent and degassing of shallow magma, in which
the rate of magma supply to the upper feeding system largely exceeds the rate of magma eruption.
Such unbalance between supplied (input) and erupted (output) magma rates is thought to result from
steady, degassing-driven, convective magma overturning in a shallow conduit/feeding dyke. Here, we
characterize shallow magma circulation at Stromboli volcano by combining independent observations of
heat (Volcanic Radiative Power; via satellite images) and gas (SO2 , via UV camera) output in a temporal
interval (from August 1, 2018 to April 30, 2020) encompassing the summer 2019 effusive eruption and
two paroxysmal explosions (on July 3 and August 28, 2019). We show that, during the phase of ordinary
strombolian explosive activity that preceded the 2019 effusive eruption, the average magma input rate
(0.1-0.2 m3 /s) exceeds the magma eruption rate (0.001-0.01 m3 /s) by ∼2 orders of magnitude. Conversely,
magma input and output rates converge to an average of ∼0.4 m3 /s during the summer 2019 summit
effusion, implying an overall suppression of magma recycling back into the feeding system, and hence of
excess degassing. We find that, during the effusive eruption, the peak in SO2 emissions lags behind the
thermal emission peak by ∼27 days, suggesting that magma output, feeding the lava flow field, initially
dominates over magma input in the conduit. We propose that this conduit mass unloading, produced by
this initial phase of the effusive eruption, leads to an overall decompression (of up to 30 Pa/s) of the
shallow plumbing system, ultimately causing ascent of less-dense, volatile-rich magma batch(es) from
depth, enhanced explosive activity, and elevated SO2 fluxes culminating into a paroxysmal explosion on
August 28. Our results demonstrate that combined analysis of thermal and SO2 flux time-series paves the
way to improved understanding of shallow magmatic system dynamics at open-vent volcanoes, and of
the transition from explosive to effusive activity regimes.