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Reconstructing eruption intensity at Augustine Volcano, Alaska: tephra stratigraphy and componentry of the 750 ybp Tephra M eruption

Ryan D\'Errico1 , Alison Koleszar1, Kristina Walowski2, Samuel Ash1, Rylie Berwanger1, Jessie Farrell1, Saisha Brody2, Matthew Loewen3

  • Affiliations: 1Department of Earth and Environmental Geosciences, Colgate University, Hamilton NY, USA; 2Geology Department, Western Washington University, Bellingham WA, USA; 3USGS-Alaska Volcano Observatory, Anchorage AK, USA

  • Presentation type: Poster

  • Presentation time: Friday 16:30 - 18:00, Room Poster Hall

  • Poster Board Number: 161

  • Programme No: 3.4.36

  • Theme 3 > Session 4

Abstract

Augustine Volcano is one of the most frequently active and highest threat volcanoes in Alaska (Ewert et al., 2018). Eruptions since 1812 have been smaller and less explosive than those from 390-2100 ybp (Waitt and Beget, 2009). We quantify changes in grain size (Pmax, Lmax, Md) and componentry through a 135 cm thick deposit of Tephra M (750 ybp), the thickest in the late Holocene, to assess changes in explosivity within a single eruptive sequence and compare to modern eruptions at Augustine.          Tephra M comprises eight sublayers that vary by grain size and/or the abundance of lithic fragments. The basal sublayer is 10 cm of fine lapilli with >20% lithic fragments, consistent with dome destruction and conduit opening. The deposit coarsens to coarse lapilli in the next two sublayers (75 cm), signaling an increase in eruption intensity. This is overlain by 10 cm of ash-coated coarse lapilli (>55 % ash). The uppermost four sublayers (40 cm) of the deposit are ash-poor coarse lapilli, and record cycles of coarsening that we interpret as pulses in eruptive intensity. Using correlations between grain size and plume height (e.g., Eychenne and Engwell, 2024), the coarsest layers of Tephra M suggest eruptive columns reached 20-30 km high.           Unlike modern eruptions, we lack direct observations of the eruption of Tephra M, one of the most explosive late Holocene eruptions of Augustine. We reconstruct the eruptive dynamics of Tephra M to compare with modern, lower-explosivity eruptions and highlight key differences in Augustine's variable eruptive behavior.