Pre-Plinian Perplexity: Constraining the Transition from the Blast to Plinian Phases of the 18 May 1980 Mount St. Helens Eruption

The Mount St. Helens 18 May 1980 eruption was a formative event for a generation of volcanologists. The eruption’s massive size, as well as its location in the Pacific Northwest, have made it the subject of countless field studies. These studies have focused on discrete eruptive phases such as the Blast and Plinian stages, but relatively little has been done to study and constrain the transitional phases. The transition period in between the Blast and Plinian phases, which occurred between 0830 and 0920 on the morning of May 18, 1980, has yet to undergo a detailed review.

My project seeks to improve our understanding of specific events in this transition period, specifically through analyzing a lesser-known pyroclastic density current (PDC) deposit from this stage. I combine fieldwork with ongoing laboratory work at Boise State to develop a comprehensive picture of how and why this PDC occurred, and why it appears (based on eyewitness reports from the 1980 eruption) to have been unusually cool and pumice-rich. I am investigating the events that led to the creation of this PDC, as well as the impact that its genesis may have had on the main, Plinian eruption which followed a few minutes later. This research may help us to better understand how stratovolcanoes shift from blast to Plinian eruptions. It may also allow us to better understand how volcanic hazards evolve during major eruptions.