Research Article| October 01, 1994 Volcanic mixed avalanches: A distinct eruption-triggered mass-flow process at snow-clad volcanoes THOMAS C. PIERSON; THOMAS C. PIERSON 1U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98661 Search for other works by this author on: GSW Google Scholar RICHARD J. JANDA RICHARD J. JANDA 1U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98661 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1994) 106 (10): 1351–1358. https://doi.org/10.1130/0016-7606(1994)106<1351:VMAADE>2.3.CO;2 Article history first online: 01 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 THOMAS C. PIERSON, RICHARD J. JANDA; Volcanic mixed avalanches: A distinct eruption-triggered mass-flow process at snow-clad volcanoes. GSA Bulletin 1994;; 106 (10): 1351–1358. doi: https://doi.org/10.1130/0016-7606(1994)106<1351:VMAADE>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 SocietyGSA Bulletin Search Advanced Search Abstract A generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at Mount St. Helens, Nevado del Ruiz, and Redoubt Volcano between 1982 and 1989. These "mixed avalanches" traveled as far as 14 km at velocities up to ∼27 m/s, involved as much as 107 m3 of rock and ice, and left unmelted deposits of single flow units as thick as 5 m. During flow downslope, heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures, apparently giving these mixed avalanches mobilities equal to or greater than those of "dry" debris avalanches of similar volume. The avalanches studied for this report began as snow avalanches, triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris. Erosion by the avalanches incorporated additional snow, fragments of glacier ice, and other rock debris.Mixed-avalanche deposits were massive, very poorly sorted, and ungraded to inversely graded prior to melting. Although rock debris composed as much as 70 wt% of the frozen samples, the bulk volumetric content of rock debris was only 9% to 36%. Snow and ice composed between 36% and 72% of the frozen samples, with void space making up the remainder of the volume. Deformation and consolidation after melting of the ice content reduced the deposits to loose, porous layers that were only a fraction of their original thickness and that later gave little hint of their origin. After melting and desiccation, the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record. 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.