PYROCLASTIC SURGES AND FLOWS FROM THE 8-10 MAY 1997 EXPLOSIVE ERUPTION OF BEZYMIANNY VOLCANO, KAMCHATKA, RUSSIA

Abstract

The 8–10 May 1997 eruption of Bezymiannyvolcano began with extrusion of a crystallized plug fromthe vent in the upper part of the dome. Progressive gravi-tational collapses of the plug caused decompression ofhighly crystalline magma in the upper conduit, leading at 13:12 local time on 9 May to a powerful, vertical Vulcanian explosion. The dense pyroclastic mixture col-lapsed in boil-overstyle to generate a pyroclastic surgewhich was focused toward the southeast by the steep-walled, 1956 horseshoe-shaped crater. This surge, with atemperature <200 °C, covered an elliptical area >30 km2with deposits as much as 30 cm thick and extending7 km from the vent. The surge deposits comprised mas-sive to vaguely laminated, gravelly sand (Md –1.2 to3.7φ; sorting 1.2 to 3φ) of poorly vesiculated andesite(mean density 1.82 g cm–3; vesicularity 30 vol%; SiO2content ~58.0 wt%). The deposits, with a volume of5–15×106m3, became finer grained and better sortedwith distance; the maximal diameter of juvenile clastsdecreased from 46 to 4 cm. The transport and depositionof the surge over a snowy landscape generated extensivelahars which traveled >30 km. Immediately followingthe surge, semi-vesiculated block-and-ash flows wereemplaced as far as 4.7 km from the vent. Over time thejuvenile lava in clasts of these flows became progres-sively less crystallized, apparently more silicic (59.0 to59.9 wt% SiO2) and more vesiculated (density 1.64 to1.12 g cm–3; vesicularity 37 to 57 vol%). At this stagethe eruption showed transitional behavior, with mass di-vided between collapsing fountain and buoyant column.The youngest pumice-and-ash flows were accompaniedby a sustained sub-Plinian eruption column ~14 km high,from which platy fallout clasts were deposited (~59.7%SiO2; density 1.09 g cm–3; vesicularity 58 vol%). Theexplosive activity lasted about 37 min and produced a to-tal of ~0.026 km3dense rock equivalent of magma, withan average discharge of ~1.2×104m3s–1. A lava flow~200 m long terminated the eruption. The evolutionarysuccession of different eruptive styles during the explo-sive eruption was caused by vertical gradients in crystal-lization and volatile content of the conduit magma,which produced significant changes in the properties ofthe erupting mixture.