HR: 11:20h
AN: V12B-05    [Abstracts]
TI: Subglacial Lava Domes: Indicators of Paleo-Ice Thickness
AU: * Kelman, M
EM: Melanie.Kelman@gov.bc.ca
AF: British Columbia Ministry of Energy, Mines, and Petroleum Resources (Geological Survey), 5th Floor, 1810 Blanshard St., Victoria, B.C V8W 9N3 Canada
AU: Russell, K
EM: krussell@eos.ubc.ca
AF: University of British Columbia Department of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, B.C V6T 1Z4 Canada
AU: Hickson, C J
EM: chickson@gsc.nrcan.gc.ca
AF: Natural Resources Canada, 605 Robson St., Suite 101, 16th Floor, Vancouver, B.C V6B 5J3 Canada
AB: Southwestern British Columbia's Garibaldi Volcanic Belt (GVB) contains numerous subglacial lava domes. Subglacial lava domes (SLD) are steep-sided lava masses with rounded or irregular upper surfaces. They are dominated by fine-scale columnar jointing and display flow shapes and cooling joint orientations inconsistent with apparent paleotopography. Because SLD eruptions do not penetrate the ice surface, the ice thicknesses concomitant with eruption are not obvious. We attempted to determine paleo-ice thicknesses for six andesitic SLD, formed during the Late Wisconsinan (Fraser) Glaciation, in the Mount Cayley volcanic field. The maximum possible ice thicknesses that could have overlain the SLD vents at the commencement of eruption are indicated by the local maximum elevation reached by the Cordilleran ice sheet. Since all SLD are at high elevations, the maximum thickness of ice above each vent was 900 m or less. Minimum possible ice thicknesses are estimated by calculating the amount of heat released by cooling lava and assigning a realistic fraction of this heat to melting ice directly above the vents. Since eruptions did not breach the ice surface, all six SLD must have had at least 100 m of overlying ice prior to eruption. The heat transfer calculations, combined with the ice thickness maximum estimates, imply that some vents would have had less than 150 m of overlying ice at the cessation of eruption. There is outcrop-scale evidence to support this inference, in the form of edifice morphologies and joint characteristics that indicate eruption into fractured ice; this implies that ice was thin (<150 m thick) during later stages of eruption. The almost total absence of features recording eruption into water (i.e. pillows, hyaloclastite) at SLD provides additional indirect evidence about ice thickness and eruptive environment, because it indicates that meltwater drainage was continuous and efficient. Thin ice makes the formation of drainage pathways easier, while thin ice coupled with steep bedrock topography creates hydraulic gradients away from vents. Using this combination of regional paleo-ice thickness information, heat transfer calculations, and field observations, our optimal values for the initial ice thicknesses during these SLD eruptions range from 100-650 m.
DE: 1645 Solid Earth (1225)
DE: 1827 Glaciology (0736, 0776, 1863)
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005