Conduit flow experiments help constraining the regime of explosive eruptions
Pierfrancesco DellinoFabio DioguardiBernd ZimanowskiRalf BüttnerDaniela MeleL. La VolpeRoberto SulpizioDomenico M. DoronzoIngo SonderRosanna BonasiaSonia CalvariEnrica Marotta
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Abstract:
It is currently impractical to measure what happens in a volcano during an explosive eruption, and up to now much of our knowledge depends on theoretical models. Here we show, by means of large‐scale experiments, that the regime of explosive events can be constrained on the basis of the characteristics of magma at the point of fragmentation and conduit geometry. Our model, whose results are consistent with the literature, is a simple tool for defining the conditions at conduit exit that control the most hazardous volcanic regimes. Besides the well‐known convective plume regime, which generates pyroclastic fallout, and the vertically collapsing column regime, which leads to pyroclastic flows, we introduce an additional regime of radially expanding columns, which form when the eruptive gas‐particle mixture exits from the vent at overpressure with respect to atmosphere. As a consequence of the radial expansion, a dilute collapse occurs, which favors the formation of density currents resembling natural base surges. We conclude that a quantitative knowledge of magma fragmentation, i.e., particle size, fragmentation energy, and fragmentation speed, is critical for determining the eruption regime.Keywords:
Electrical conduit
Overpressure
Inflow
Fragmentation
Pyroclastic fall
Peléan eruption
Volcanic hazards
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"Pyroclastic fallout" is the process of fallout of the particles, which is one of the most common processes in volcanology and is generally associated with all types of explosive eruptions. This chapter shows how the study and monitoring of pyroclastic fallout products play a key role in volcanic risk assessment. The pyroclastic fallout process is, in its simplest formulation, the sedimentation of pyroclasts through the atmosphere and their deposition on the Earth's surface. For fallout deposits, the subdivision into proximal, medial or distal deposits depends on the size of the eruption considered. During eruptive crises a sampling of the eruptive products is generally carried out in the hours following the beginning of each eruption. Geochemical and petrographic analysis of pyroclasts can constrain the initial conditions from the magma chamber to the surface via the conduit. Total grain size distribution represents the theoretical eruptive mixture injected into the atmosphere during volcanic explosive eruptions.
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Silicic tuffs infilling an ancient submarine caldera, at Mineral King in California, show microscopic fabrics indicative of welding of glass shards and pumice at temperatures >500 degrees C. The occurrence indicates that subaqueous explosive eruption and emplacement of pyroclastic materials can occur without substantial admixture of the ambient water, which would cause chilling. Intracaldera progressive aggradation of pumice and ash from a thick, fast-moving pyroclastic flow occurred during a short-lived explosive eruption of approximately 26 cubic kilometers of magma in water >/=150 meters deep. The thickness, high velocity, and abundant fine material of the erupted gas-solids mixture prevented substantial incorporation of ambient water into the flow. Stripping of pyroclasts from upper surfaces of subaqueous pyroclastic flows in general, both above the vent and along any flow path, may be the main process giving rise to buoyant-convective subaqueous eruption columns and attendant fallout deposits.
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