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Paper No. 9
Presentation Time: 3:00 PM
QUANTITATIVE ASSESSMENT OF MODELS FOR THE EMPLACEMENT OF THE COLUMBIA RIVER BASALT LAVA FLOWS
We use published models to provide quantitative assessments of 3 different modes of emplacement suggested for flood basalt lava flows, including those of the Columbia River Basalt Group (CRBG). The first is the rapid, turbulent emplacement model proposed by Shaw and Swanson (1970). We find that this model can indeed produce lava flows of the dimensions seen in the CRBG. However, flow rates must be 1,000,000 m3/s or higher and the bulk of the lava must be erupted within days. While this model is physically plausible, we do not find field evidence consistent with such rapid emplacement (e.g., thermal and mechanical erosion of the substrate and a thin, disrupted upper crust). We find a model with slower, insulated, emplacement as inflated pahoehoe (Self et al., 1996) is also physically viable. It requires eruption rates of 100-1000 m3/s, eruption durations of years, and lava traveling from the source to the advancing flow front in a matter of days to weeks. The same modeling demonstrates that the CRBG lava flows could not have been produced with current Kilauea eruption rates (1-10 m3/s). The majority of the flows in the CRBG show inflation features consistent with the slower emplacement model. We also examine an intermediate case, as suggested by Ho and Cashman (1997) and Reidel (1998). We find that long lava flows can be emplaced at rates of 10,000-100,000 m3/s under a thick disrupted crust. Eruption durations would be on the order of months. About 20% of the lava in the CRBG have "rubbly pahoehoe" crusts that fit this emplacement model (Self et al. 1998). Several flow fields show classic pahoehoe features in some localities and rubbly pahoehoe tops in others, suggesting that both modes of emplacement may have been involved in some eruptions.
Ho, A. M., and K. V. Cashman, (1997) Geology, 25: 403-406.
Reidel, S. P., (1998) J. Geophys. Res., 103: 27,393-27,410.
Self, S., Th. Thordarson, L. Keszthelyi, G. P. L. Walker, K. Hon, M. T. Murphy, P. Long, and S. Finnemore, (1996) Geophys. Res. Lett., 23: 2689-2692.
Self, S., L. Keszthelyi, and Th. Thordarson, (1998) Annu. Rev. Earth. Planet. Sci., 26: 81-110.
Shaw, H., and D. Swanson, (1970) in Proceedings of the Second Columbia River Basalt Symposium, edited by E. Gilmour and D. Stradling, pp. 271-299, East. Wash. State College Press, Cheney.