Analysis of magma flux and eruption intensity during the 2021 explosive activity at La Soufrière, St Vincent, West Indies

Abstract

Abstract Real-time Seismic Amplitude Measurement signals and eruption cloud height measurements were used to estimate peak intensities of 40 explosive events during the 8–22 April 2021 activity of La Soufrière volcano. We estimated magma supply rates and erupted volumes in each explosion, characterized uncertainty by stochastic modelling and identified four eruptive stages. Stage 1 included an intense period of 9.5 hours with 11 explosive events with peak eruption intensity between 2000 and 4000 m 3 s −1 and magma supply rate reaching 828 m 3 s −1 . Twelve high-intensity explosions ( c. 4000 m 3 s −1 ) occurred in Stage 2 with average magma supply rate of 251 m 3 s −1 . Stage 3 involved both declining intensity and magma supply rate and lengthening repose periods between explosions. Stage 4 involved three much weaker explosions. The total erupted volume of magma is estimated at 38.5 × 10 6 m 3 (90% credible interval: [22.0 .. 61.9] × 10 6 m 3 ) consistent with independent estimates from analysis of tephra deposits and volcano subsidence sourced at c. 6 km depth. The 150-fold increase in magma supply rate, from the preceding effusive phase to Stage 1 of the explosive phase, is attributed to replacement of very high-viscosity degassed magma occupying the shallow conduit system with new, lower-viscosity, volatile-rich magma from the magma chamber.