Valve-like dynamics of gas flow through a packed crystal mush and cyclic strombolian explosions

Anna Barth,Andrew Woods,Marie Edmonds

doi:10.1038/s41598-018-37013-8

Abstract

Strombolian volcanic explosions are commonly attributed to the rise and burst of conduit-filling gas slugs. The magmas associated with strombolian activity, however, are typically not only volatile-rich but also highly crystalline, with mush regions in the shallow plumbing system, where an exsolved volatile phase may also be abundant. Through analogue experiments, we explore a new mechanism to form gas slugs and strombolian explosions. A steady flux of gas is supplied to the base of a particle-rich liquid layer, generating a localised gas intrusion, which initially grows through plastic deformation. Once the pressure in the intrusion overcomes the effective tensile strength of the particle pack, a localised channel opens, allowing gas to propagate upwards. As the pressure in the intrusion falls, the gas pocket collapses. The continued supply of gas leads to the formation of a new intrusion, and the cycle repeats. With higher gas fluxes, continuous channelised gas flow occurs. Highly crystalline shallow portions of basaltic conduits may act as a flow valve, transforming a steady gas flux into a series of discrete gas slugs which cause explosions.

Highlights

The periodicity of typical strombolian eruptions has been attributed to the rise and bursting of large, conduit-filling, over-pressurized gas slugs[1,2,3,4]
At Stromboli volcano, for example, the CO2-rich gas that plays such an important role in the formation of the gas slugs which drive strombolian explosions is thought to be derived from >4 km depth (Burton et al, 2007) and the larger, less frequent paroxysms may be driven by hot, CO2-rich magma at depths of a few kilometers which may stall in sills, allowing efficient gas segregation and rise, perhaps triggering explosive activity[6,25]
The relatively small volume of magma erupted during typical strombolian eruptions, combined with the high mass flux of emitted volcanic gases, requires that degassed, highly crystalline magmas are stored in the plumbing systems of such volcanoes

Summary

Introduction

The periodicity of typical strombolian eruptions has been attributed to the rise and bursting of large, conduit-filling, over-pressurized gas slugs[1,2,3,4]. We explore the buoyancy-driven flow of gas through a packed bed of particles in a Hele-Shaw cell, as a model for the gas flux through a crystal-rich layer in a magmatic system, which may be a mush[40] in a region of a shallow magma reservoir.

Results

Conclusion