Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

Abstract

We compare eruptive dynamics, effects and deposits of the Bezymianny 1956 (BZ), Mount St Helens 1980 (MSH), and Soufrière Hills volcano, Montserrat 1997 (SHV) eruptions, the key events of which included powerful directed blasts. Each blast subsequently generated a high-energy stratified pyroclastic density current (PDC) with a high speed at onset. The blasts were triggered by rapid unloading of an extruding or intruding shallow magma body (lava dome and/or cryptodome) of andesitic or dacitic composition. The unloading was caused by sector failures of the volcanic edifices, with respective volumes for BZ, MSH, and SHV c. 0.5, 2.5, and 0.05 km 3 . The blasts devastated approximately elliptical areas, axial directions of which coincided with the directions of sector failures. We separate the transient directed blast phenomenon into three main parts, the burst phase, the collapse phase, and the PDC phase. In the burst phase the pressurized mixture is driven by initial kinetic energy and expands rapidly into the atmosphere, with much of the expansion having an initially lateral component. The erupted material fails to mix with sufficient air to form a buoyant column, but in the collapse phase, falls beyond the source as an inclined fountain, and thereafter generates a PDC moving parallel to the ground surface. It is possible for the burst phase to comprise an overpressured jet, which requires injection of momentum from an orifice; however some exploding sources may have different geometry and a jet is not necessarily formed. A major unresolved question is whether the preponderance of strong damage observed in the volcanic blasts should be attributed to shock waves within an overpressured jet, or alternatively to dynamic pressures and shocks within the energetic collapse and PDC phases. Internal shock structures related to unsteady flow and compressibility effects can occur in each phase. We withhold judgment about published shock models as a primary explanation for the damage sustained at MSH until modern 3D numerical modeling is accomplished, but argue that much of the damage observed in directed blasts can be reasonably interpreted to have been caused by high dynamic pressures and clast impact loading by an inclined collapsing fountain and stratified PDC. This view is reinforced by recent modeling cited for SHV. In distal and peripheral regions, solids concentration, maximum particle size, current speed, and dynamic pressure are diminished, resulting in lesser damage and enhanced influence by local topography on the PDC. Despite the different scales of the blasts (devastated areas were respectively 500, 600, and >10 km 2 for BZ, MSH, and SHV), and some complexity involving retrogressive slide blocks and clusters of explosions, their pyroclastic deposits demonstrate strong similarity. Juvenile material composes >50% of the deposits, implying for the blasts a dominantly magmatic mechanism although hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highly viscous, phen...

Pyroclastic density currents associated with the 2008-2009 eruption of Chaitén Volcano (Chile): Forest disturbances, deposits, and dynamics

“…The nature of the proximal forest disturbance caused by the north-side PDC at Chaitén Volcano, and its deposit, are similar to disturbances and deposits of PDCs documented at Lamington, Bezymianny, Arenal, Mount St. Helens, Soufrière Hills Volcano, and Merapi (Taylor, 1958;Hoblitt et al, 1981;Waitt, 1981;Kelfoun et al, 2000;Ritchie et al, 2002;Alvarado et al, 2006;Belousov et al, 2007). However, the PDCs at all but Merapi were attributed to directed volcanic blasts (Hoblitt et al, 1981;Ritchie et al, 2002;Alvarado et al, 2006;Belousov et al, 2007;Belousova et al, 2011).…”

“…However, the PDCs at all but Merapi were attributed to directed volcanic blasts (Hoblitt et al, 1981;Ritchie et al, 2002;Alvarado et al, 2006;Belousov et al, 2007;Belousova et al, 2011). Those eruptions generated violent, vertically and longitudinally stratified pyroclastic density currents triggered by rapid unloading of a shallow magma body as a result of sector failure or dome collapse.…”

Evidence for the relative depths and energies of phreatomagmatic explosions recorded in tephra rings

“…Each couplet, and ash cap if present, therefore, can be inferred to represent the deposits from a single explosion. These couplets have many similarities with deposits associated with individual lateral blasts resulting from rapid unloading of pressurized domes/cryptodomes or hydrothermal systems (e.g., Belousov et al 2007;Breard et al 2015).…”

Deposits, character and timing of recent eruptions and gravitational collapses in Tatun Volcanic Group, Northern Taiwan: Hazard-related issues

“…In most cases such collapses occur during the course of eruption, when intruding magma disturbs the edifice of the volcano, resulting in loss of gravitational stability (Siebert, 1984;Belousov et al, 1999Belousov et al, , 2007. More than 5 scars and debris avalanche deposits formed by gravitational collapses were found in the TVG, indicating that these extremely dangerous events were common in the area.…”