Vesiculation of May 18, 1980, Mount St. Helens magma

Abstract

Research Article| May 01, 1994 Vesiculation of May 18, 1980, Mount St. Helens magma Caroline Klug; Caroline Klug 1Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403 Search for other works by this author on: GSW Google Scholar Katharine V. Cashman Katharine V. Cashman 1Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403 Search for other works by this author on: GSW Google Scholar Geology (1994) 22 (5): 468–472. https://doi.org/10.1130/0091-7613(1994)022<0468:VOMMSH>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Caroline Klug, Katharine V. Cashman; Vesiculation of May 18, 1980, Mount St. Helens magma. Geology 1994;; 22 (5): 468–472. doi: https://doi.org/10.1130/0091-7613(1994)022<0468:VOMMSH>2.3.CO;2 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 The May 18, 1980, eruption of Mount St. Helens, Washington, produced both white and gray pumice, similar in composition but varying in phenocryst, microlite, and vesicle content. The white pumice has fewer phenocrysts, no microlites, and higher vesicularity, and is thus less dense than the gray. In addition, vesicles in the white pumice are larger and more interconnected than those in the gray. Both white and gray have effective (crystal-free) vesicularities (85.7% and 72.2%, respectively), close to the traditionally accepted "fragmentation vesicularity" of 75%-77%. Given that the two pumice types were erupted concurrently for several phases of the eruption and therefore probably had similar eruption paths, we suggest that differences in vesicularity and vesicle size distributions result from the presence or absence of microlites. The presence of microlites in the gray pumice not only increased the magma viscosity and effective vesicularity, but appears to have aided bubble nucleation and hindered bubble expansion and coalescence. Thus, magmas with microlites may fragment at a lower bulk vesicularity than those without microlites. Fragmented microlite-bearing clasts are also likely to expand less after fragmentation and therefore more closely preserve the bubble distribution and structure at the time of magma fragmentation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.