Abstract Vulcanian eruptions are frequent, small-scale, short-lived explosive volcanic eruptions, which are thought to be produced by impulsive sources. The experiments presented here, produced by injections of mixtures of water, alcohol or salt and solid particles into fresh water, created a wide variety of turbulent flows from steady and impulsive sources. We focus on the experimental flows analogous to Vulcanian events – unsteady, finite-volume releases of buoyancy (thermals) and momentum (puffs), and short releases driven by both momentum and buoyancy. Dimensional analysis, based on two controlling source parameters, total injected momentum ( M ) and total injected buoyancy ( B ), identified a universal scaling relationship for the propagation of the flows; the non-dimensional, time-varying velocity term ( ut 1/2 ), where u is flow front vertical velocity and t is time from flow onset, varies with the time-varying, non-dimensional ratio of source parameters ( M/Bt ), such that ut 1 / 2 / B 1 / 4 = 14; k(M/Bt) 1 / 2 . The quantitative relationship successfully describes experiments and several Vulcanian eruptions for a wide range of initial conditions. The utility of the relationship is demonstrated by estimating total mass erupted and vent mass flux as a function of time, two parameters important to hazards assessment, for the well-documented 7 August 1997 Vulcanian eruption at Soufrie`re Hills volcano, Montserrat. Results compare favour-ably to independent estimates of total mass erupted (based in part on deposit studies) and eruption duration (based on seismic analysis) with the advantage that our approach requires only the determination of the eruption cloud front velocity from conventional video analysis.

Abstract Laboratory flows, self-capped by high-viscosity fluid, exhibit vertical pressure gradients similar to those postulated within conduits feeding dome-building eruptions. Overpressure and pressure cycles exhibited at laboratory scale provide insight into the mechanism of tilt cycles at volcanic scale. Experimental pressure cycles correlated with the rate of gas escape, with pressure rise being controlled by diffusion of volatile into bubbles during times when gas escape from the flow was negligible. The increase in pressure continued until margin decrepitation created preferential pathways for rapid gas escape from permeable foam, thereby reducing pressure within the flow. As pressure reduced, the gas escape pathways sealed and diffusion repressurized the system. This implies that tilt cycles, such as those exhibited by the Soufrière Hills Volcano, Montserrat, result from a diffusively pumped process that oscillates around the viscous-elastic transition within the outer regions of the flow. Phases of open- and closed-system degassing result, with gas escaping through fractures created and maintained by the flow process itself. Fluid-dynamically, this mechanism generates an oscillation between Poiseuille flow and plug flow, with a 1-D model of plug motion giving a reasonable representation of observation in both experimental and volcanic cases.