The 1956 eruption of Bezymianny volcano (Kamchatka). Part II—Magma dynamics and timescales from crystal records

Abstract

Laterally directed blasts are explosive events following a major sector collapse of a volcano, with the potential for devastating areas of several hundred km2, due to powerful dilute and turbulent pyroclastic density currents. The catastrophic flank collapse on 30 March 1956 of Bezymianny (Kamchatka, Russia) was the climactic phase of the first historical magmatic eruption of this volcano, after 1000 years of dormancy. Magma stored in a cryptodome was depressurized by a sector collapse, generating a laterally directed blast immediately followed by pumiceous concentrated pyroclastic density currents. By combining petrological data from Bezymianny plumbing system and temporal constraints from orthopyroxene, magnetite, and amphibole chronometers, we tracked magmatic processes over twelve years prior to the eruption, followed by magma ascent to a shallow reservoir and a heating process at least three months before the eruption. Magma was last stored in a cryptodome at least two months before the climactic phase of the eruption. Evidencing magma dynamics of a few months to a few years before major flank collapses and laterally directed blasts thus represents valuable information for volcanic risk mitigation (as it also occurred at Mt St. Helens).