Magmatic precursors to the 18 May 1980 eruption of Mount St. Helens, USA

Abstract

Research Article| February 01, 2004 Magmatic precursors to the 18 May 1980 eruption of Mount St. Helens, USA K.V. Cashman; K.V. Cashman 1Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA Search for other works by this author on: GSW Google Scholar R.P. Hoblitt R.P. Hoblitt 2U.S. Geological Survey, Hawaiian Volcano Observatory, P.O. Box 51, Hawaii Volcanoes National Park, Hawaii 96718, USA Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (2): 141–144. https://doi.org/10.1130/G20078.1 Article history received: 30 Jul 2003 rev-recd: 16 Oct 2003 accepted: 17 Oct 2003 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation K.V. Cashman, R.P. Hoblitt; Magmatic precursors to the 18 May 1980 eruption of Mount St. Helens, USA. Geology 2004;; 32 (2): 141–144. doi: https://doi.org/10.1130/G20078.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Perhaps the most difficult task facing volcanologists today is that of distinguishing between low-level volcanic restlessness and activity that presages a full-scale eruption. We illustrate these difficulties by reexamining the sequence of events that led to the 18 May 1980 eruption of Mount St. Helens, activity that is often presented as a classic example of early phreatic explosions leading to a magmatic eruption. This interpretation is problematic, however, given the known ascent of juvenile magma to within ∼250 m of the surface prior to the climactic event. Reexamination of ash samples from several precursory events shows that small amounts of juvenile material were erupted as early as 28 March 1980, just two weeks after the first seismic signals of reawakening, and that the juvenile content of the ash had probably increased by early May. The magmatic component of these eruptions was not recognized at the time because of the high crystallinity of the juvenile material—a signature of extensive degassing-induced crystallization during magma ascent. Identification of juvenile material in the precursory ash from Mount St. Helens has important implications for initiation of eruptive activity. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.