HR: 1340h
AN: V33A-1457    [Abstracts]
TI: Emplacement of Large-volume rhyolite lavas in the Eastern Snake River Plains: The Reynolds Creek flow
AU: * Semple, A
EM: amsemple@buffalo.edu
AF: University of Buffalo (SUNY), Dept of Geology, 876 NSC, Buffalo, NY 14260 United States
AU: Gregg, T
EM: tgregg@geology.buffalo.edu
AF: University of Buffalo (SUNY), Dept of Geology, 876 NSC, Buffalo, NY 14260 United States
AU: Bonnichsen, B
EM: billb@uidaho.edu
AF: University of Idaho, Moscow, Idaho Geological Survey, Moscow, ID 83844 United States
AU: Godchaux, M
EM: mgodchau@MtHolyoke.edu
AF: University of Idaho, Moscow, Idaho Geological Survey, Moscow, ID 83844 United States
AB: Although less well known than their smaller counterparts, large-volume (>4 km3) evolved lavas (SiO2 > 60%) are common and contribute significantly to the volume of continental crust. However, little is known about the eruption and emplacement of evolved flows as they have not yet been observed while active. In the western Snake River Plains (SRP), Idaho, there are a number (~10-15) of rhyolite flows each with volumes of ~10-200 km3; we are investigating these to understand the eruption and emplacement of large-volume evolved flows. Here, we present the preliminary results of our investigation of the Reynolds Creek Rhyolite in the western SRP. This flow is useful for investigation because it is well preserved and accessible. Previous work revealed that the Reynolds Creek flow is smaller than most of the rhyolites of the SRP at 2.5-3.5 km3 and was probably not more than twice this volume at its greatest. It is a high-silica rhyolite (SiO2 = 75%) which erupted from a fissure ~0.4 km long and flowed mostly to the NE. In total, it is about 9 km long, smaller in the proximal region (0.2 km), widening distally (4 km in the NE) and ranges from 50-150 m thick. Results from Jeffrey's equation suggest that the Reynolds Creek flow may have had a flow velocity of up to 9 m/day, indicating emplacement in about 2-3 years. Graetz number calculations yield an effusion rate of ~40 m3s-1, suggesting an eruption duration of 1.5-3 years. These calculations all suggest that the volume of this flow was emplaced in months to years, rather than decades. These eruption duration calculations are comparable to that estimated for the 15 km3 Badlands flow (also in the Eastern Snake River Plains), at 6 - 16 years; and the Ben Lomond Flow, New Zealand, calculated at 1.5 - 6 years. In contrast, the Mount St Helens dacite has an estimated effusion rate of 1.4-40 m3s-1 for single lobes and the Santiaguito dacite, Guatemala, is estimated at 0.6-1.9 m3s-1; this may be due to periods of repose still included in the estimates. Yield strength estimates for the Reynolds Creek flow, based on flow thickness and underlying slope, range from 104-105 Pa which are comparable to the 8 x 105 Pa estimated for the large-volume (26 km3) Chao Dacite, Chile, suggesting similar rheologies and emplacement behaviors. The Mount St Helens dacite also falls within the same yield strength rate on the order of 1-2 x 105 Pa. Continued investigations will reveal whether the large-volume evolved flows have fundamentally different emplacement styles than the more familiar, small-volume domes.
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting