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 anddeposits of the Bezymianny 1956 (BZ), Mount St Helens1980 (MSH), and Soufrière Hills volcano, Montserrat 1997(SHV) eruptions, the key events of which includedpowerful directed blasts. Each blast subsequently generateda high-energy stratified pyroclastic density current (PDC)with a high speed at onset. The blasts were triggered byrapid unloading of an extruding or intruding shallowmagma body (lava dome and/or cryptodome) of andesiticor dacitic composition. The unloading was caused by sectorfailures of the volcanic edifices, with respective volumesfor BZ, MSH, and SHV c. 0.5, 2.5, and 0.05 km3. Theblasts devastated approximately elliptical areas, axialdirections of which coincided with the directions of sectorfailures. We separate the transient directed blast phenome-non into three main parts, the burst phase, the collapsephase, and the PDC phase. In the burst phase thepressurized mixture is driven by initial kinetic energy andexpands rapidly into the atmosphere, with much of theexpansion having an initially lateral component. Theerupted material fails to mix with sufficient air to form abuoyant column, but in the collapse phase, falls beyond thesource as an inclined fountain, and thereafter generates aPDC moving parallel to the ground surface. It is possiblefor the burst phase to comprise an overpressured jet, whichrequires injection of momentum from an orifice; howeversome exploding sources may have different geometry and ajet is not necessarily formed. A major unresolved questionis whether the preponderance of strong damage observed inthe volcanic blasts should be attributed to shock waveswithin an overpressured jet, or alternatively to dynamicpressures and shocks within the energetic collapse and PDCphases. Internal shock structures related to unsteady flowand compressibility effects can occur in each phase. Wewithhold judgment about published shock models as aprimary explanation for the damage sustained at MSH untilmodern 3D numerical modeling is accomplished, but arguethat much of the damage observed in directed blasts can bereasonably interpreted to have been caused by highdynamic pressures and clast impact loading by an inclinedcollapsing fountain and stratified PDC. This view isreinforced by recent modeling cited for SHV. In distal andperipheral regions, solids concentration, maximum particlesize, current speed, and dynamic pressure are diminished,resulting in lesser damage and enhanced influence by localtopography on the PDC. Despite the different scales of theblasts (devastated areas were respectively 500, 600, and>10 km2for BZ, MSH, and SHV), and some complexityinvolving retrogressive slide blocks and clusters of explo-sions, their pyroclastic deposits demonstrate strong similar-ity. Juvenile material composes >50% of the deposits,implying for the blasts a dominantly magmatic mechanismalthough hydrothermal explosions also occurred. The character of the magma fragmented by explosions (highlyviscous, phenocryst-rich, variable microlite content) deter-mined the bimodal distributions of juvenile clast densityand vesicularity. Thickness of the deposits fluctuates inproximal areas but in general decreases with distance fromthe crater, and laterally from the axial region. The proximalstratigraphy of the blast deposits comprises four layersnamed A, B, C, D from bottom to top. Layer A isrepresented by very poorly sorted debris with admixturesof vegetation and soil, with a strongly erosive groundcontact; its appearance varies at different sites due todifferent ground conditions at the time of the blasts. Thelayer reflects intense turbulent boundary shear betweenthe basal part of the energetic head of the PDC and thesubstrate. Layer B exhibits relatively well-sorted fines-depleted debris with some charred plant fragments; itsdeposition occurred by rapid suspension sedimentation inrapidly waning, high-concentration conditions. Layer C ismainly a poorly sorted massive layer enriched by fines withits uppermost part laminated, created by rapid sedimenta-tion under moderate-concentration, weakly tractive con-ditions, with the uppermost laminated part reflecting adilute depositional regime with grain-by-grain tractiondeposition. By analogy to laboratory experiments, mixingat the flow head of the PDC created a turbulent dilute wakeabove the body of a gravity current, with layer B depositedby the flow body and layer C by the wake. The uppermostlayer D of fines and accretionary lapilli is an ash falloutdeposit of the finest particles from the high-rising buoyantthermal plume derived from the sediment-depleted pyro-clastic density current. The strong similarity among theseeruptions and their deposits suggests that these casesrepresent similar source, transport and depositionalphenomena.