Abstract
Many explosive volcanic eruptions produce underexpanded starting gas-particle jets. The dynamics of the accompanying pyroclast ejection can be affected by several parameters, including magma texture, gas overpressure, erupted volume and geometry. With respect to the latter, volcanic craters and vents are often highly asymmetrical. Here, we experimentally evaluate the effect of vent asymmetry on gas expansion behaviour and gas jet dynamics directly above the vent. The vent geometries chosen for this study are based on field observations. The novel element of the vent geometry investigated herein is an inclined exit plane (5, 15, 30° slant angle) in combination with cylindrical and diverging inner geometries. In a vertical setup, these modifications yield both laterally variable spreading angles as well as a diversion of the jets, where inner geometry (cylindrical/diverging) controls the direction of the inclination. Both the spreading angle and the inclination of the jet are highly sensitive to reservoir (conduit) pressure and slant angle. Increasing starting reservoir pressure and slant angle yield (1) a maximum spreading angle (up to 62°) and (2) a maximum jet inclination for cylindrical vents (up to 13°). Our experiments thus constrain geometric contributions to the mechanisms controlling eruption jet dynamics with implications for the generation of asymmetrical distributions of proximal hazards around volcanic vents.
Highlights
Explosive volcanic eruptions are among the most energetic displays of Earth’s internal forces
We focused on the Reynolds number (Re) and the Mach number (M) for our vent geometries to describe the fluid flow dynamics
We observed a strong influence of pressure ratio, slant angle and inner geometry on the dynamics of gas jets (Figs. 5 and 6)
Summary
Explosive volcanic eruptions are among the most energetic displays of Earth’s internal forces. They pose continual threats to life and infrastructure. Such eruptions are fuelled by gas overpressure, which derives from volatile oversaturation of magma and its resultant degassing, sometimes combined with external volatiles such as vaporised meteoric water (Mayer et al 2015). The overpressure driving melt vesiculation can be released explosively if it exceeds the tensile strength of magma, leading to failure and fragmentation (Alidibirov and Dingwell 1996b; Dingwell and Webb 1990).
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