Instability of Gas Flow Through a Crystal Mush and the Cyclic Explosions at Stromboli Volcano

Abstract:

Eruption dynamics are often interpreted with reference to two-phase flow experiments. However, many volcanic eruptions involve three phases (solid, liquid, gas), and the crystals can have a dramatic effect on the physical properties of the magma. Strombolian explosions display striking periodicity, with a frequency of ∼ 10-20 minutes. These explosions have been attributed to the rise and bursting of large, conduit-filling gas bubbles at the surface. However, recent petrological studies have indicated that the magma in the upper region of Stromboli’s plumbing system is highly crystalline, with crystallinities above the threshold (∼ 40-60 vol%) at which magma rheology becomes non-Newtonian, and develops a yield stress.

Using analogue experiments, we explore the mechanism by which gas, issuing from a steady source, interacts with a concentrated suspension of solid particles in a viscous liquid. Using video analysis of the gas migration through the mush, we identify two degassing regimes. At low gas flux, gas pockets grow and become pressurised until they overcome the yield stress of the mush, subsequently rising through a series of opening and closing ‘fractures’. For higher fluxes a channel forms intermittently, allowing a constant stream of gas to be emitted from a point at the top of the mush. The results of these experiments suggest a new process which may regulate the periodic gas explosions at Stromboli.