HR: 14:25h
AN: V23E-04    [Abstracts]
TI: Evidence for an Unusually Energetic Basaltic Phreatomagmatic Eruption at the Table Rock Complex in South-central Oregon (USA): Using field evidence to constrain surge flow dynamics
AU: * Brand, B D
EM: bbrand@asu.edu
AF: Arizona State University School of Earth and Space Exploration, Box 871404, Tempe, AZ 85287-1404, United States
AU: Clarke, A B
EM: amanda.clarke@asu.edu
AF: Arizona State University School of Earth and Space Exploration, Box 871404, Tempe, AZ 85287-1404, United States
AB: We estimate the velocity of pyroclastic base-surges associated with the TRC2 tuff ring, one of several basaltic hydrovolcanoes located at the Table Rock Complex, Christmas Valley Basin, south-central Oregon (TRC; Pliocene-Pleistocene; first documented by Heiken, 1971, J. Geophy Res:76:5615-5626). TRC2 represents the last and most energetic eruption of the complex. Surge deposits from this eruption form a hummocky topography, which is morphologically similar to hummocks produced in storm-surge marine environments, but much larger in scale. Wavelengths range from 20-800 m perpendicular to the flow, and 20-200 m parallel to the direction of flow depending on distance from source, producing three-dimensional bowl features distributed radially around the vent. The scale of these dune forms are more than an order of magnitude larger than most measured at other hydromagmatic edifices, and instead are consistent with bedforms associated with larger scale eruptions such as the eruption of El Chichon in 1982, and the Mt St Helens lateral blast in 1980. The hummocks are composed of fine-grained, laterally continuous, centimeter to decimeter thick strata of matrix supported tuff and lapilli tuff. Individual dune hummocks truncate underlying hummocks, suggesting that the deposits are a consequence of multiple, highly erosive, dilute density currents. The most distal exposures are located 4.7 km from the vent where the hummocks are 80 m in wavelength parallel to the direction of flow and the total deposit is 4-8 m thick, suggesting the surges extended further from source but deposits have subsequently been eroded away. We used dune wavelengths and two additional features, one a ramp-up feature 4.7 km from source, and the other a 13 m high chute and pool feature 1.6 km from source, to estimate surge velocities at various distances from the vent. Results indicate initial velocities between 110-280 m s-1, which are in the range of values estimated for the Mt St Helens blast (i.e., Kieffer, 1981, Nature: 291:5816:568-570). The estimated minimum velocities at 1.6 and 4.7 km from source are 33 and 20 m s-1 respectively. These estimates are much higher than rare eye- witness measurements for base surges, which generally range from 60 m s-1 at the source, and decay to 20 m s-1 at distances of 1.5 km (i.e. Rohrer, 1965, Lawrence Radiation lab, PNE-217P:62; Moore, 1967, BullVolc.30: 337-363). Using the initial flow velocities and assuming that initial velocities resulted only from a conversion of potential energy to kinetic energy, we calculated the corresponding column collapse height. Collapse estimates range from 2500 m, corresponding to an initial velocity of 110 m s-1, and up to16000 m, corresponding to an initial velocity of 280 m s-1. As most phreatomagmatic eruption columns reach maximum heights between 1000-4000 m (i.e. Yamamoto, 2001, Geological Survey of Japan: 52:4-5:231-239; Moore, 1967), our estimates suggest that the collapse height may have been unusually high for this style of eruption. This thus expands the envelope on the energy and scale in this style of explosive volcanism.
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting