Origin of Basaltic Subplinian Eruption at Shishaldin Volcano (Alaska): A Vigorously Degassing Magma Reservoir Hosting Small Bubbles

Abstract The 1999 basaltic eruption of Shishaldin volcano (Alaska) displayed a transition between Subplinian and Strombolian activity. Strombolian bubbles indicate the presence of a periodically unstable foam at the top of magma reservoir. In contrast, a long foam, whose rupture led to the eruptive column, was also able to collect in the conduit. Laboratory experiments show that long foams can be produced in a conduit by the spreading of a stable foam accumulated at the top of the reservoir. The existence of a Taylor bubble at the onset of the Subplinian phase, also reproduced by my laboratory experiments, suggests that the foam in the reservoir was just at the transition between stable and unstable. This constrains the bubble diameter prior to the Subplinian phase to be 0.034–0.038 mm when using the foam dimensionless analysis and the underlying gas flux (0.52–0.80 m 3 /s). The increase in bubble diameter and potentially gas flux prior to the Strombolian activity, 0.81–1.4 m 3 /s, is sufficient to explain the foam in transition to be unstable. The radius of the magma reservoir is small, 200–210 m, as expected. The bubble diameter is the smallest of those estimated for classical basaltic eruptions (Etna, Kı̄lauea, Erta 'Ale), while the gas flux is among the largest. A dilute suspension of small and isolated bubbles cannot explain the large gas flux at Shishaldin. This implies numerous bubbles with a gas volume fraction ≥0.63−2%, a regime for which the bubbles form bubble clusters. The diameter of these bubble clusters, 3.0–5.4 mm, is sufficient to explain large gas fluxes.