Explosive caldera-forming eruptions and debris-filled vents: Gargle dynamics

Abstract

Large explosive volcanic eruptions are commonly associated with caldera subsidence and ignimbrites deposited by pyroclastic currents. Volumes and thicknesses of intracaldera and outflow ignimbrites at 76 explosive calderas around the world indicate that subsidence is commonly simultaneous with eruption, such that large proportions of the pyroclastic currents are trapped within the developing basins. As a result, much of an eruption must penetrate its own deposits, a process that also occurs in large, debris-filled vent structures even in the absence of caldera formation and that has been termed “gargling eruption.” Numerical modeling of the resulting dynamics shows that the interaction of preexisting deposits (fill) with an erupting (juvenile) mixture causes a dense sheath of fill material to be lifted along the margins of the erupting jet. This can cause an eruption that would otherwise produce a buoyant plume and fallout deposits to instead form pyroclastic currents as the dense sheath drives pulsing jet behavior. Increasing thickness of fill amplifies the time variation in jet height. Increasing the fill grain size relative to that of the juvenile particles can result in a much higher jet due to poorer mixing between the dense sheath and the dilute jet core. In all cases, material collapses along the entire height of the dense sheath rather than from the top of a simple fountain. These gargle dynamics provide strong backing for processes that have been inferred to result in intraplinian ignimbrites and simultaneous deposition from high and low-energy pyroclastic currents.