WHAT DRIVES CHANGES IN ERUPTION STYLE AT INTERMEDIATE ARC VOLCANOES? EVIDENCE FROM THE HOLOCENE RECORD AT AUGUSTINE VOLCANO, ALASKA

Augustine Volcano, a frequently-active intermediate arc volcano in Alaska, has produced both small and large eruptions in the Holocene. Augustine’s modern eruptions (post-1741CE) have been comparatively small, dominated by ash to lapilli in proximal fall deposits <10 cm thick. In contrast, larger eruptions (Tephra B at 390 ybp, Tephra M at 750 ybp, and Tephra C at 1,100 ybp) produced proximal (<5 km from the summit) fall deposits from 20-150 cm thick, including pumice clasts that exceed 15 cm in diameter, consistent with higher explosivity than the modern eruptions. Tephras B, M, and C are an important focus of study because this more explosive eruption style is absent from the modern record. Analysis of eruptive products from these tephras can illuminate differences between the smaller modern eruptions and these larger eruptions in the recent geologic past.

We collected samples of the top, middle, and bottom of Tephras B, M, and C in summer 2022. By collecting detailed stratigraphy for these older eruptions, we aimed to create an eruption record analogous to observations during modern eruptions.

Here we present detailed geochemical, textural, and component analysis on samples from these three tephras and compare them to eruptive products from Augustine’s modern eruptions. We find that although pumice compositions are broadly consistent with components identified by Larsen et al. (2010) in the 2006 eruption, including HSA (high-silica andesite, >62 wt% SiO₂), LSA (low-silica andesite, <59 wt% SiO₂), and banded clasts of intermediate composition, there are notable differences in component proportions and textural indicators of ascent (including vesicularity and microlite number density) between Augustine’s most recent lower explosivity eruptions and larger eruptions in the past 1,100 years. These results indicate that significant changes in eruptive style at Augustine can accompany subtle changes in conditions of ascent and composition. Understanding these changes is integral to better anticipating factors that may lead to changes in the style of future eruptive behavior at Augustine and at other intermediate arc volcanoes.