An empirical scaling of shear-induced outgassing during magma ascent: Intermittent magma ascent causes effective outgassing

Outgassing, which changes the distribution of volcanic gases in magmas, is one of the most important processes to determine the eruption styles. Shear deformation of ascending bubbly magmas at the vicinity of the volcanic conduit wall has been considered as an efficient mechanism of outgassing. On the other hand, seismological observations of volcanic eruptions reveal the gas bursting associated with long-period (LP) earthquakes and tremors, suggesting the existence of a large void space in the conduit. However both, the quantitative features of shear-induced outgassing and a mechanism to make a large void space, have still remain unknown. Here I perform a series of model experiments in which shear localization of syrup foam causes outgassing by making large bubbles or a crack-like void space, likely a gas bursting source. There is a critical strain, γ, above which outgassing occurs depending on the Capillary number, Ca, γ>1 for Ca<1 and γ>Ca−1 for Ca≥1. The width of the region in which outgassing occurs is described as a function of γ0.48Ca0.24. Outgassing occurs efficiently at the very beginning of the deformation, suggesting that intermittent magma ascent causes effective outgassing such that the eruption style becomes effusive. This hypothesis is consistent with the fact that cyclic activity has been observed during effusive dome eruptions.

332Highlights► I perform a model experiment of ascending bubbly magma in a volcanic conduit. ► Shear-induced outgassing occurs by making a large void space. ► There is a critical strain for outgassing, γ>1 for Ca<1 and γ>Ca−1 for Ca≥1. ► The width of the outgassing region is described as γ0.48Ca0.24. ► This scaling suggests that intermittent magma ascent can help to avoid explosive eruption.