HR: 1330h
AN: V42B-0340    [PDF]
TI: Dacite Dome Growth and Disruption During the 1912 Novarupta Eruption: The Missing Episode
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: * Adams, N K
EM: nanadams@soest.hawaii.edu
AF: SOEST, 1680 East-West Rd., University of Hawaii-Manoa, Honolulu, HI 96822 United States
AU: Hildreth, W
AF: Volcano-Hazards Program, USGS, MS 910 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: The eruption of Novarupta on June 6-8, 1912 was the largest eruption of the 20th century, producing c. 11 km$^{3}$ of ignimbrite and c. 17 km$^{3}$ of Plinian fall deposits in $\sim$60 hrs. Three episodes of Plinian activity were followed by dome extrusion ultimately responsible for the rhyolitic dome currently occupying the Novarupta vent New mapping shows a dacite block bed overlies the fine ash deposited at the close of Episode III and underlies the Episode V rhyolite dome. In places the blocks also underlie thin deposits of fluvially and slope-reworked Episode III pumice implying a short time break after the close of the main explosive phase of the 1912 eruption. This dacite block bed covers an elliptical area of at least 5 km$^{2}$ and extends to at least 2 km south of Novarupta. The blocks appear to have mostly followed ballistic trajectories; the largest clasts are up to 8 m$^{3}$ in volume or 10$^{4}$ kg in mass. We have mapped the size, lithology, and distribution of 700 blocks, including reconstruction of broken clasts. The blocks are of four types: 1) pumiceous dacite often with breadcrust exteriors, 2) dense glassy dacite that may be flow-banded and/or breadcrusted, 3) monolithic slightly to moderately welded dacite breccias, and 4) moderately to densely welded vitrophyres. The vitrophyres represent wall rock fragments derived from the vent-filling tuffs of principally Episode I; the dacite breccias are juvenile in origin. Point counting of 200 blocks suggests that 54% of the deposit is pumiceous dacite, 25% dense dacite, 12% juvenile breccia and 9% vitrophyres. The block bed seemingly formed in a series of Vulcanian explosions that followed the close of Plinian volcanism and disrupted a body of dacitic magma which had undergone a very heterogeneous pattern of degassing and, to a much lesser extent, wall rock.. Block morphology and textures strongly suggest that this dacitic magma was first erupted as a relatively gas-rich lava dome (Episode IV) but incomplete outgassing led to explosive disruption of this precursor to the present day (rhyolitic) dome (Episode V).
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting