Morphology and Petrography of Volcanic Ashes

Research Article| July 01, 1972 Morphology and Petrography of Volcanic Ashes GRANT HEIKEN GRANT HEIKEN Geology Branch, NASA Manned Spacecraft Center, Houston, Texas 77058 Search for other works by this author on: GSW Google Scholar Author and Article Information GRANT HEIKEN Geology Branch, NASA Manned Spacecraft Center, Houston, Texas 77058 Publisher: Geological Society of America Received: 08 Aug 1971 Revision Received: 28 Jan 1972 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1972, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1972) 83 (7): 1961–1988. https://doi.org/10.1130/0016-7606(1972)83[1961:MAPOVA]2.0.CO;2 Article history Received: 08 Aug 1971 Revision Received: 28 Jan 1972 First Online: 02 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 GRANT HEIKEN; Morphology and Petrography of Volcanic Ashes. GSA Bulletin 1972;; 83 (7): 1961–1988. doi: https://doi.org/10.1130/0016-7606(1972)83[1961:MAPOVA]2.0.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 Volcanic ash samples collected from a variety of recent eruptions were studied using petrography, chemical analyses, and scanning electron microscopy to characterize each ash type and to relate ash morphology to magma composition and the type of eruption.The ashes are best placed into two broad genetic categories: magmatic and phreatomagmatic. Ashes from magmatic eruptions are formed when expanding gases in the magma form a froth that loses its coherence as it approaches the ground surface. During phreatomagmatic eruptions, the magma is chilled on contact with ground or surface waters, resulting in violent steam eruptions. Within these two genetic categories, ashes from different magma types can be characterized. The “pigeonhole” classification used here is for convenience; there are eruptions which are driven by both phreatic and magmatic gases.The morphology of ash particles from magmatic eruptions of high-viscosity magma is governed primarily by vesicle density and shape. The vitric ash particles are generally angular, vesicular pumiceous fragments or thin vesicle-wall fragments. The morphology of lithic fragments is dependent on the texture and mechanical properties of the rock units broken up during the eruption; most of the samples studied contain equant, angular to subrounded lithic fragments.Ash particles from eruptions of low-viscosity magmas are mostly droplets; droplet shape is in part controlled by surface tension, acceleration of the droplets after they leave the vent, and air friction. Shapes range from perfect spheres to a variety of twisted, elongate droplets with smooth, fluidal surfaces.The morphology of ash particles from phreatomagmatic eruptions is controlled by stresses within the chilled magma which result in fragmentation of the glass to form small blocky or pyramidal glass ash particles. Vesicle density and shape play only a minor role in determining the morphology of these ash particles. 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.