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 Bezymianny volcano began with extrusion of a crystallized plug from the vent in the upper part of the dome. Progressive gravi- tational collapses of the plug caused decompression of highly 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-over style to generate a pyroclastic surge which was focused toward the southeast by the steep- walled, 1956 horseshoe-shaped crater. This surge, with a temperature 30 km 2 with deposits as much as 30 cm thick and extending 7 km from the vent. The surge deposits comprised mas- sive to vaguely laminated, gravelly sand (Md -1.2 to 3.7φ; sorting 1.2 to 3φ) of poorly vesiculated andesite (mean density 1.82 g cm -3 ; vesicularity 30 vol%; SiO2 content ~58.0 wt%). The deposits, with a volume of 5-15×106 m3, became finer grained and better sorted with distance; the maximal diameter of juvenile clasts decreased from 46 to 4 cm. The transport and deposition of the surge over a snowy landscape generated extensive lahars which traveled >30 km. Immediately following the surge, semi-vesiculated block-and-ash flows were emplaced as far as 4.7 km from the vent. Over time the juvenile lava in clasts of these flows became progres- sively less crystallized, apparently more silicic (59.0 to 59.9 wt% SiO 2 ) and more vesiculated (density 1.64 to 1.12 g cm-3; vesicularity 37 to 57 vol%). At this stage the eruption showed transitional behavior, with mass di- vided between collapsing fountain and buoyant column. The youngest pumice-and-ash flows were accompanied by a sustained sub-Plinian eruption column ~14 km high, from which platy fallout clasts were deposited (~59.7% SiO 2 ; density 1.09 g cm-3; vesicularity 58 vol%). The explosive activity lasted about 37 min and produced a to- tal of ~0.026 km 3 dense rock equivalent of magma, with an average discharge of ~1.2×104 m3 s-1. A lava flow ~200 m long terminated the eruption. The evolutionary succession 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 of the erupting mixture.