Gas flow dynamics in the conduit of Strombolian explosions inferred from seismo-acoustic observations at Aso volcano, Japan

Kyoka Ishii,Akihiko Yokoo,Shin Yoshikawa,Tsuneomi Kagiyama,Takahiro Ohkura,Hiroyuki Inoue

doi:10.1186/s40623-019-0992-z

Abstract

Strombolian explosions are one of the most studied eruptive styles and are characterized by intermittent explosions. The mechanism of a Strombolian explosion is modeled as a large gas pocket (slug) migrating through the magma conduit and then bursting at the air–magma interface. These ascending and bursting processes of the slug induce characteristic seismo-acoustic signals during each explosion: very-long-period (VLP) seismic signals, eruption earthquake signals, and infrasound signals. However, at Stromboli volcano, it has been reported that the ascent velocity estimated from the time differences between observed signals is nearly an order of magnitude higher than that expected from laboratory experiments simulating slug ascent. This discrepancy between observation-based and experiment-based velocities has not yet been fully explained and strongly suggests that the conventional model of Strombolian explosions should be partially revised. In this study, we attempted to validate the model of Strombolian explosions by estimating the gas phase velocity in the conduit in the case of Aso volcano. We recorded seismo-acoustic signals accompanying Strombolian events at Aso volcano, Japan, in late April 2015 via our monitoring network, and the ascent velocity of the gas phase was determined from the difference in arrival times between the VLP signals and the infrasound signals. Our estimated velocity exceeded 100 m/s, which is much faster than the experimental value of 7.5 m/s predicted for Aso volcano. To explain this rapid ascent velocity, we propose a revised model describing the migration of the gas phase via a more complicated mechanism, such as annular flow. In this model, we assumed that the gas phase ascends in the conduit at high velocity while making a pathway leading to the magma surface, most likely due to a temporary increase in the gas flux. Our model will help to deepen the understanding of the complicated dynamics in the magma conduit during a Strombolian explosion.

Highlights

Strombolian explosions are one of the most famous eruption styles at volcanoes associated with basaltic to basaltic andesite magmatic systems
During the occurrence of ash-free Strombolian explosions at Aso volcano in April 2015, we identified characteristic seismo-acoustic signals, such as the VLP signal, eruption earthquake signal, and explosion infrasound signal (Fig. 2b)
Following Ripepe et al (2001), we can constrain two essential parameters related to the dynamics of a Strombolian explosion—the explosion depth and the ascent velocity of the gas phase—using the time differences in the arrival times of these seismo-acoustic signals

Summary

Introduction

Strombolian explosions are one of the most famous eruption styles at volcanoes associated with basaltic to basaltic andesite magmatic systems. The dynamics of Strombolian explosions have been described as the bursting of a large gas bubble (called a “slug”) that has risen in a magma conduit from depth This idea was proposed based on the results of both field observations and analog experiments (Blackburn et al 1976; Jaupart and Vergniolle 1989) and remains widely accepted today. James et al (2013) proposed a model combining both models because neither of the two idealized conduit geometries (RSD: a long, uniform vertical conduit; CF: a flat, horizontal magma chamber roof ) represents a natural system In this combined model, cascading foam collapse events effectively convert a steady gas flux into an unsteady output flux. At Etna, it is suggested that gas bubbles might accumulate at a given depth (Allard et al 2005) and that the CF model is more suitable (Vergniolle and Ripepe 2008)

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