Insights into magmatic processes at Shishaldin Volcano based on satellite and airborne volcanic gas measurements and magma degassing models

Taryn M. Lopez , Allan Lerner, Bruno Franco, Matthew Loewen, Peter Kelly, Christoph Kern, Laura Clor, Cynthia Werner, Lieven Clarisse, Daniel Rasmussen, Valerie Wasser, Skye Kushner, Pablo Saunders-Shultz, Israel Brewster, Nicolas Theys, Pavel E. Izbekov

Shishaldin Volcano is a remote and frequently active basaltic volcano with eruptions in 2019–2020 and 2023 that exhibited notable variations in monitoring signals and explosivity. Here we combine complementary ultraviolet (TROPOMI) and infrared (IASI) satellite SO2 observations with airborne gas measurements, petrologic constraints, and degassing models to investigate magma degassing processes and quantify SO2 emissions during these recent eruptions. Cumulative SO2 emissions differed by an order of magnitude between the 2019–2020 and 2023 eruptions (~40 vs ~240 kt), and SO2 emission rates spanned a wide range (<100 to ~85,000 t/d) over the 5-year study period, suggesting highly variable magma supply rates. SO2 plumes are almost exclusively observed from satellites during or following volcanic unrest and eruption, suggesting substantial SO2 degassing from magma at relatively shallow depths, consistent with degassing models. Airborne CO2/SO2 gas ratios of 1-2 measured during 2019–2020 and following the 2023 eruptions correspond to relatively shallow (<2-4 km) magma degassing depths, while a ratio of ~11 measured between these eruptions corresponds with magma degassing at >12 km depths. Collectively, these observations indicate that while exsolved volatiles can separate from their host magma, consistent with open-system degassing in the upper crust, most SO2 is released during eruption. We speculate that magma ascent rates control Shishaldin’s degassing behavior and eruption style, with lower magma fluxes enabling open-system degassing and quiescent to weakly-explosive eruptions, and higher fluxes producing closed-system degassing and highly-explosive eruptions. These results demonstrate that satellite SO2 observations can successfully track magma supply to the shallow crust and if acquired at sufficient temporal resolution SO2 observations may provide advanced warning of impending paroxysms.