Pyroclastic surges and flows from the 8-10 May 1997 explosive eruption of Bezymianny volcano, Kamchatka, Russia

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 gravitational 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 collapsed 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 <200 degreesC, covered an elliptical area >30 km(2) with deposits as much as 30 cm thick and extending 7 km from the vent. The surge deposits comprised massive to vaguely laminated, gravelly sand (Md -1.2 to 3.7phi; sorting 1.2 to 3phi) of poorly vesiculated andesite (mean density 1.82 g cm(-3); vesicularity 30 vol%; SiO2 content similar to58,0 wt%). The deposits, with a volume of 5-15x10(6) m(3), 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 progressively less crystallized, apparently more silicic (59.0 to 59.9 Wt% SiO2) 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 divided between collapsing fountain and buoyant column. The youngest pumice-and-ash flows were accompanied by a sustained sub-Plinian eruption column similar to14 km high, from which platy fallout clasts were deposited (similar to59.7% SiO2; density 1.09 g cm(-3); vesicularity 58 vol%). The explosive activity lasted about 37 min and produced a total of similar to0.026 km(3) dense rock equivalent of magma, with an average discharge of -1.2x10(4) m(3) s(-1). A lava flow similar to200 m long terminated the eruption. The evolutionary succession of different eruptive styles during the explosive eruption was caused by vertical gradients in crystallization and volatile content of the conduit magma, which produced significant changes in the properties of the erupting mixture.