A revised narrative of the May 18, 1980 Plinian eruption of Mount St. Helens: Changes in the conduit and magma supply

Abstract

This compilation re-visits the May 18, 1980 Plinian eruption sequence of Mount St. Helens and correlates new and existing observations and measurements for a revised narrative of when and how eruption behavior changes occurred. It presents new observations of vesicle textures in white pumice and analyses of lithic materials that are correlated with observed eruption behavior. These and the compositional and seismic research by others provide constraints to a proposed process of how the conduit system was modified during the Plinian sequence. The eruption sequence is sub-divided into 3 periods interpreted to represent changes in magma supply and transport as a result of conduit changes. Based on observed pumice textures and composition, the microlite-free, white pumice in the earliest Plinian tephra is interpreted to have been emplaced in a poorly-interconnected, conduit system on or about May 16. The Morning period began with the landslide and lateral blast and initial conduit decompression. The early Plinian column then discharged conduit-resident, white pumice through an effective conduit of ≥20 m radius that produced small, irregular tephra clasts with abundant, small tubular vesicles. Small pyroclastic flows observed about 1010 h started the Transition period and the discharge of small batches of magma from the magma reservoir. Increasingly larger pumice clasts and large, coalesced vesicle textures are attributed to the increased ascent path from the magma reservoir and increased, average ascent rates that increased the rates and extent of bubble growth and coalescence. The eruption intensity that occurred at 1215 h is attributed to an increased magma supply through a modestly, improved conduit. The Afternoon period started at 1300 h as the vertical, eruption column dissipated. Episodic pyroclastic flows from collapsed fountains increased in volume until climax, as a preferred path in the complex, lower conduit was established. Lithic fragments comprise about 40% of deposits of the Pumice Plain and about 70% of the glass was lost as very-fine ash during emplacement. It is proposed that syn-eruptive conduit modification incrementally consolidated adjacent dikes and fractures, improved local interconnections among conduit segments, and removed the lithic debris, so that the conduit configuration was simplified and the effective conduit radius enlarged to ≥43 m during the Climactic phase. The Climactic phase after 1500 h resulted from full conduit interconnection, and magma supply and transport from deeper parts of the reservoir.