Rapid Coalescence of Bubbles Driven by Buoyancy Force: Implication for Slug Formation in Basaltic Eruptions

In basaltic eruptions, bubbles move freely and collide within a volcanic conduit, leading to frequent bubble coalescence. Understanding the dynamics of buoyancy-driven coalescence of bubbles is crucial for predicting the explosivity of basaltic eruptions. We examine the evolution of the bubble volume distribution while considering buoyancy-driven coalescence and expansion due to decompression. We find that, at lower decompression rates, the bubble volume distribution $n\left(v,t\right)$ rapidly evolves into a power-law distribution with an exponent of approximately $-2$ as $n\left(v,t\right)\propto v^{-2}$. This suggests that, in basaltic magma, the repeated coalescence of bubbles rapidly forms large bubbles within 45 min to 3 days. We then examine the occurrence of eruption styles, specifically Strombolian or Hawaiian, under the assumption that the bursts of slugs, produced from bubble coalescence within the conduit, trigger Strombolian eruptions. Consequently, we identify a critical condition for the transition between eruption styles in terms of the ascent velocity of magma. This critical ascent velocity is consistent with the observed transitions between Strombolian and Hawaiian eruptions at Izu-Oshima and Kilauea.