Analysis of magma rheology from lava spreading and explosive activity during the 2020–21 eruption of La Soufrière, St Vincent, with implications for eruption dynamics

Abstract

We apply dynamical models to estimate the rheological properties of magma and lava during the 2021–22 eruption of La Soufrière Volcano, St Vincent. Analysis of the emplacement of a lava coulée gives vis-cosities in the range 0.94 × 1010 Pa s to 5.97 × 1010 Pa s. A static Bingham model gives a yield strength of 4.1 × 105 Pa. A dynamical model of conduit flow during the explosive phase of the eruption gives a viscosity range from 1.2 × 107 to 2.3 × 109 Pa s. A petrological model of magma viscosity in the source region falls in the range 102 to 103 Pa s. The very high viscosity of the lava is attributed to the presence of a remnant degassed and partially crystallized magma from previous eruptions that occupied the shallow conduit–vent system prior to onset of the 2021 explosive eruptions. Gas-rich magma pushed this degassed remnant magma out over a three-month period at a steady rate of about 1.2 m3 s−1 and the explosive phase began when volatile-rich and much lower viscosity magma reached the surface.