An investigation of Vulcanian eruption dynamics using laboratory analogue experiments and scaling analysis

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/B ^1/4), 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 = 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 Soufrière Hills volcano, Montserrat. Results compare favourably 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.