HR: 11:05h
AN: V31G-04    [PDF]
TI: Episodes II and III of the 1912 Novarupta Eruption : Understanding Pauses and Shifts in Intensity Using Pyroclast Textures
AU: * Adams, N K
EM: nanadams@soest.hawaii.edu
AF: SOEST, 1680 East-West Rd., University of Hawaii-Manoa, Honolulu, HI 96822 United States
AU: Houghton, B F
EM: bhought@soest.hawaii.edu
AF: SOEST, 1680 East-West Rd., University of Hawaii-Manoa, Honolulu, HI 96822 United States
AU: Hildreth, W
AF: USGS, MS 910 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: The eruption of Novarupta in June 1912 produced 11 km$^{3}$ of ignimbrite and 17 km$^{3}$ of Plinian fall deposits in 60 hrs. The fall deposits are divided into eight layers (A through H) formed during three episodes (I through III) of sustained eruptive activity interrupted by two short breaks of up to several hours in duration. The initial fallout belonging to Episode I was rhyolitic, but as the eruption progressed (Episodes II and III), erupted material became dominantly dacitic. The Plinian activity ended with the emplacement of a dacite dome, subsequently destroyed by Vulcanian activity which created the post-H block bed. Emplacement of the rhyolitic dome seen today marked the final stage of the eruption. Density data show pumices from Episode II have a very similar narrow range of vesicularity, but during Episode III the density values increase and the range between high and low values steadily widens. Selected clasts from each layer were used to obtain quantitative evidence for textures indicative of syneruptive degassing. The clasts have very similar vesicle volume distributions in which coalescence signatures are superimposed on prolonged nucleation and free growth of bubbles. Clasts from Layers F and C, both representing the start of an episode, are the most similar whereas a comparison between Layers D and G deposited during the close of Episodes II and III reveals dissimilarities. Layer D has thinner glass walls resembling Layers C and F, or episode initiation, but its bubble population is more coarsely skewed. For both medial and proximal fall deposits of Episode III, Layer G has the lowest vesicularity and thicker glass walls, and the shapes of the larger bubbles are complex. The contrast between clasts erupted at the start of an episode (Layers C and F) and the end of Plinian activity (Layer G) reflects a shift in the degassing process from coupled magma ascent-gas exsolution to open-system degassing necessary for effusive dome growth. While the transition to effusive activity is evident in the pyroclasts, external changes involving the conduit more likely effected the break in explosive activity at the end of Episode II.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting