Compaction and gas loss in welded pyroclastic deposits as revealed by porosity, permeability, and electrical conductivity measurements of the Shevlin Park Tuff

Research Article| January 01, 2014 Compaction and gas loss in welded pyroclastic deposits as revealed by porosity, permeability, and electrical conductivity measurements of the Shevlin Park Tuff Heather M. Wright; Heather M. Wright † 1U.S. Geological Survey, MS 910, 345 Middlefield Road, Menlo Park, California 94025, USA †E-mail: hwright@usgs.gov Search for other works by this author on: GSW Google Scholar Katharine V. Cashman Katharine V. Cashman 2Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA3School of Earth Sciences, University of Bristol, Queens Road, Bristol BS81RJ, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Heather M. Wright † 1U.S. Geological Survey, MS 910, 345 Middlefield Road, Menlo Park, California 94025, USA Katharine V. Cashman 2Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA3School of Earth Sciences, University of Bristol, Queens Road, Bristol BS81RJ, UK †E-mail: hwright@usgs.gov Publisher: Geological Society of America Received: 05 Jan 2012 Revision Received: 11 Jun 2013 Accepted: 11 Sep 2013 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2013 Geological Society of America GSA Bulletin (2014) 126 (1-2): 234–247. https://doi.org/10.1130/B30668.1 Article history Received: 05 Jan 2012 Revision Received: 11 Jun 2013 Accepted: 11 Sep 2013 First Online: 08 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 Heather M. Wright, Katharine V. Cashman; Compaction and gas loss in welded pyroclastic deposits as revealed by porosity, permeability, and electrical conductivity measurements of the Shevlin Park Tuff. GSA Bulletin 2014;; 126 (1-2): 234–247. doi: https://doi.org/10.1130/B30668.1 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 SocietyGSA Bulletin Search Advanced Search Abstract Pyroclastic flows produced by large volcanic eruptions commonly densify after emplacement. Processes of gas escape, compaction, and welding in pyroclastic-flow deposits are controlled by the physical and thermal properties of constituent material. Through measurements of matrix porosity, permeability, and electrical conductivity, we provide a framework for understanding the evolution of pore structure during these processes. Using data from the Shevlin Park Tuff in central Oregon, United States, and from the literature, we find that over a porosity range of 0%–70%, matrix permeability varies by almost 10 orders of magnitude (from 10–20 to 10–11 m2), with over three orders of magnitude variation at any given porosity. Part of the variation at a given porosity is due to permeability anisotropy, where oriented core samples indicate higher permeabilities parallel to foliation (horizontally) than perpendicular to foliation (vertically). This suggests that pore space is flattened during compaction, creating anisotropic crack-like networks, a geometry that is supported by electrical conductivity measurements. We find that the power law equation: k1 = 1.3 × 10–21 × ϕ5.2 provides the best approximation of dominant horizontal gas loss, where k1 = permeability, and ϕ = porosity. Application of Kozeny-Carman fluid-flow approximations suggests that permeability in the Shevlin Park Tuff is controlled by crack- or disk-like pore apertures with minimum widths of 0.3 and 7.5 μm. We find that matrix permeability limits compaction over short times, but deformation is then controlled by competition among cooling, compaction, water resorption, and permeable gas escape. These competing processes control the potential for development of overpressure (and secondary explosions) and the degree of welding in the deposit, processes that are applicable to viscous densification of volcanic deposits in general. Further, the general relationships among porosity, permeability, and pore geometry are relevant for flow of any fluid through an ignimbritic host. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.