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Pumice and Ash

Camille Pastore1, Kathy Cashman1, Adam Kent2, Sophia Wang2

  • Affiliations: 1 Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA 2 Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA

  • Presentation type: Poster

  • Presentation time: Monday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 10

  • Programme No: 6.1.16

  • Theme 6 > Session 1

Abstract

Juvenile pyroclasts produced by explosive eruptions vary in size, density and internal textures (bubble and crystal population). Petrologists analyze lapilli-sized clasts to assess magma storage conditions and volcanologists use lapilli to measure both density and the vesicle population. The ash fraction, however, is often ignored, despite representing the bulk of many deposits. Here we demonstrate that the ash fraction provides a complementary part of the story, including insight into both pre-eruptive magma staging within the conduit and syn-eruptive fragmentation. Here we compare the morphologies and internal textures of pyroclasts that range in size from lapilli (8 mm) to ash particles (125 mm) from pre-climactic and climactic deposits of the ~7.7 ka Mount Mazama eruption (Crater Lake, Oregon). Preliminary results show that ash fractions have lower vesicularities than measured in pumice lapilli. Additionally, the ash fraction of precursor deposits appears to have lower vesicularity than ash produced during the climactic phase. Precursor deposits also show a wide range in bubble shape and size as well as in groundmass microlite content. Together, these data provide evidence of pulsatory onset to the climactic eruptive activity.