Tephra compositions and textures from historical and Holocene eruptions of Shishaldin Volcano, Alaska

Abstract:

Shishaldin Volcano is one of the largest and most frequently active volcanoes in the Aleutian arc. Its last eruption in 2019–20 produced minor tephra deposits distributed as far away as Cold Bay, AK (~100 km NE). Observations from passing airplanes, distant communities, web cameras, satellites, and a local seismic network indicated both Strombolian and Hawaiian-style fountaining as well as at least one minor collapse of the summit cone. Limited samples from this eruption were collected during a 1-day field visit in December, 2019, and during ashfall events in False Pass (~40 km ENE) and Cold Bay on January 7 and 19, 2020. Tephra from all three events have diverse groundmass textures suggesting both primary and recycled origins for erupted clasts. Tephra produced by fountaining have highly fluidal grains whereas the tephra associated with a cone collapse produced more angular grains. These samples contrast with the tephra generated in a subplinian VEI 3 eruption in 1999. While glass compositions are largely similar in both 1999 and 2019–20, the groundmass of the 1999 tephra is highly microlitic with interlocking plagioclase laths suggestive of rapid decompression and crystal growth. Here we examine the composition and textures of observed eruptions with known eruption styles and compare them to a suite of older Shishaldin tephra deposited during the Holocene—collected during fieldwork in 2018—in order to better characterize eruptive history and explore what drives contrasting eruption styles in this frequently active mafic arc volcano. Preliminary observations of the Holocene tephra record suggest tephra grains similar to the explosive 1999 subplinian eruption, as well as lower-explosivity fountaining, occur repeatedly and often are mixed in the same deposit.