Mount St Helens Research Paper

Mount St. Helens eruption - May 18th 1980

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INTRODUCTION

Mount St. Helens is one of fifteen volcanoes located in the Cascade Range, a mountain range that extends southward from Mount Garibaldi, British Columbia, through Washington and Oregon to Lassen Peak in Northern California.

The Cascade Range volcanoes, along with the Alaskan volcanoes, comprise the North-American section of the Pacific Ring of Fire (1), a long horseshoe shaped chain of volcanoes and tectonic fault lines that delineates the North Pacific ocean in its entirety and circumscribes a number of tectonic plates, including the Pacific, Cocos, Philippine, Nazca and Juan de Fuca plates and marks the perimeter of several continental plates

In general, volcanic activity related to subduction in oceanic and continental arcs seems to be caused by the melting of the mantle wedge above the subsiding lithosphere. A process which is induced by the release of hot fluids, essentially water (dehydration), by the descending slab into the asthenospheric wedge resulting in a decrease of the wedge's melting point often followed by its partial melting (4,5).

Prior to the 1980 eruption, Mount St. Helens had been in a dormant state since 1856, having over the past 4 millenia displayed sporadic eruptive activity, seemingly increasing in frequency, with initial eruptions being separated by dormant intervals ranging from thousands to several hundreds of years to more recent intervals of 1 or 2 centuries in length

Helens. The first major earthquake of which (on March 20th) registered at a magnitude of 4.2 on the Richter scale (8). The number of earthquakes increased over the course of the following week, amounting to hundreds of seismic events. The seismic frequency finally peaked on the 27th of March ultimately producing St. Helens' first (non-magmatic) eruption since 1856; a phreatic eruption i.e. a series of steam explosions which sent a cloud of ash ad steam roughly three kilometers into the atmosphere (4) and lay bare a 60x70 meter crater where once St. Helens' ice-cap covered peak had stood (8,9). This hydrothermal eruption had been triggered as a result of the contact between the rising magma in the conduit and the local groundwater; the heat of the magma causing the rapid phase change from surrounding water to steam