Interactions Between Gas Slug Ascent and Exchange Flow in the Conduit of Persistently Active Volcanoes

Abstract

Many volcanoes around the world are persistently active with continuous degassing for years or even centuries, sometimes exceeding historic records. Such long‐term stability contrasts with short‐term instability, reflected in eruptive episodes that punctuate passive degassing. These two aspects of persistent activity, long‐term stability as opposed to short‐term instability, are often conceptualized through two distinct model frameworks: Exchange‐flow in volcanic conduits is commonly invoked to explain the long‐term thermal balance and sustained passive degassing, while the ascent of large gas slugs is called upon to understand explosive eruptions. While typically considered separately, we propose here that both flow processes could occur jointly in the conduits of persistently active volcanoes and in transient connections between subvolcanic melt lenses. To understand the dynamic interplay between exchange flow and slug ascent, we link analogue laboratory experiments with direct numerical simulations. We find that the two flows superimpose without creating major disruptions when only considering the ascent of a single gas slug. However, the sequential ascent of multiple gas slugs is disruptive to the ambient exchange flow, because it may entail continual buildup of buoyant magma at depth. While our study focuses on the laboratory scale, we propose that the dependence of exchange‐flow stability on sequential slug ascent is relevant for understanding why explosive sequences are sometimes followed by effusive eruptions. Taken together, our work suggests that integrating exchange flow and slug ascent could provide a more complete understanding of persistently active volcanoes than either model framework offers in isolation.

Disrupt the upper or the lower conduit? The dual role of gas exsolution in the conduits of persistently active volcanoes

“…Even a single eruption from a single eruptive centre can exhibit surprising variability in eruptive behaviour. For example, the two-months-long summit eruption at Kīlauea, Hawai'i, in 1959 entailed 17 distinct lava fountaining phases, each approximately 1-2-days long (Richter et al 1970).…”

“…A large change in gas content and the segregated two-phase flow it entails would have dynamic consequences of its own, such as large bubbles forming and disrupting the flow balance, as discussed by Qin et al. (2021). We assume that the core viscosity is always lower than in the annulus viscosity, because the core magma is volatile-rich and warmer.…”

“…We emphasize that our model only applies in the limit of relatively small gas fractions, and hence small changes in core density, where a mixture approximation holds at least in an approximate sense. A large change in gas content and the segregated two-phase flow it entails would have dynamic consequences of its own, such as large bubbles forming and disrupting the flow balance, as discussed by Qin et al (2021). We assume that the core viscosity is always lower than in the annulus viscosity, because the core magma is volatile-rich and warmer.…”

“…One commonly invoked physical process, building on early analogue laboratory experiments by Jaupart & Vergniolle (1988), is the regime transition from bubbly to slug and churn flow in two-phase flow, as reviewed by Sparks (2003). In addition to being important in accounting for differences in explosive eruption intensity (Melnik 2000) and phreatomagmatic eruptions (Starostin, Barmin & Melnik 2005), recent laboratory experiments support the possibility that a regime transition from bubbly to slug flow could destabilize core-annular flow by disrupting the mass balance that is essential for stability (Qin et al 2021). Similarly, Fowler & Robinson (2018) showed that steady core-annular flow can only be maintained under a very specific set of conditions in the presence of regime transitions in two-phase flow.…”