HR: 1330h
AN: V42B-0339    [PDF]
TI: Flow-Package Emplacement of the Valley of Ten Thousand Smokes Ignimbrite During the 1912 Novarupta, Alaska, Eruption
AU: * Fierstein, J
EM: jfierstn@usgs.gov
AF: U.S. Geological Survey, Volcano Hazards Team, MS 910 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Wilson, C J
EM: c.wilson@gns.cri.nz
AF: Institute of Geological \& Nuclear Sciences, PO Box 30368, Lower Hutt, 6315 New Zealand
AB: The Valley of Ten Thousand Smokes (VTTS) ignimbrite was emplaced by 9 sequential flow packages, each with distinct compositional proportions of rhyolite (R), dacite (D) and andesite (A) pumice clasts that permit us to map their boundaries and flow paths from vent to distal extents. Changing R:D:A proportions link ignimbrite formation to contemporaneous fall deposition during the first $\sim$16 hours (Episode I) of the eruption. Documentation of pumice compositional proportions in the ignimbrite was accomplished by counts on at least 100 lapilli-sized pumice clasts at multiple levels in vertical sections wherever accessible, and more widely over most of the ignimbrite surface in the VTTS. An initial 100% rhyolite flow package (time-equivalent to regional fall Layer A) was followed by packages with increasing proportions of andesite then dacite (time-equivalent to regional fall Layers B1-B3). The coeval regional fall deposits are locally intercalated with the ignimbrite and show parallel changes in R:D proportions, but lack significant amounts of andesite. Andesite is thus inferred to represent only a low-fountaining component in the eruption column, and is preferentially represented in packages filling the VTTS north of vent. Maps of the packages show that the most-extensive deposits occur midway through the sequence (B1 and early-B2 times) where flow mobility and volume were optimised; the earlier all-rhyolite flows were highly energetic but less voluminous, while later packages were less voluminous. Boundaries between flow packages are expressed as one or more of: sharp color changes corresponding to compositional variations, persistent finer-grained basal parts of flow units, compaction swales filled by later packages, erosional channels cut by the flows that fill them, lobate accumulations of material of one composition, and (particularly south of vent) intercalated fall deposit layers. Emplacement of the VTTS ignimbrite involved accumulation of successive, largely overlapping packages of material, with breaks in deposition only locally apparent, typically between the products of non-successive packages.
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting