HR: 10:50h
AN: V12B-03 INVITED     [Abstracts]
TI: Magma degassing, lava fracturing and subglacial-subaerial transitions: a comparison of three palaeo-ice thickness indicators at volcanoes
AU: * Tuffen, H
EM: h.tuffen@lancaster.ac.uk
AF: Lancaster University, Department of Environmental Science, Lancaster University, Lancaster, LA1 4YQ United Kingdom
AB: The volatile contents of volcanic glasses, fracture patterns of lavas and sedimentary structures of fragmental deposits may record a wealth of palaeo-environmental information at volcanoes clad by ice or snow. However, these indicators must be interpreted with caution, as each may be either very approximate or of only limited applicability. In this presentation I give a brief but critical overview of the usefulness of each indicator. Ideally, dissolved volatile contents in glasses record quenching pressures and can therefore be used to estimate water depths or ice thicknesses. The drawbacks are that degassing may be non-equilibrium, especially if the magma is rhyolitic, deposits may be remobilised post-quenching and the pressure of quenching may be considerably less than the ice overburden pressure. The water contents of basaltic and rhyolitic glasses from subglacial eruptions in Iceland are used to illustrate the potential and the problems of this technique. Distinctive fracture patterns may develop in lava flows that encounter ice, snow and meltwater, due to differences in the cooling environment and stress field. Unfortunately for palaeo-climatologists, fracture patterns may also be affected by other factors such as magma composition, crystallinity, slope and effusion rate. This technique may therefore only be used with confidence in certain cases, where extra constraints such as eyewitness reports or glacial striae patterns are available. Finally, the transition from subaqueous to subaerial facies at tuyas has long been considered to provide a rough estimate of palaeo-ice thicknesses, although this transition may occur at a considerably lower elevation than the ice surface. Simple analytical models are used to show that the difference in elevation may vary considerably, not only due to differing firn thicknesses but also due to the relative rates of ice flow and melting. It is concluded that the most successful palaeo-climatic reconstructions will combine as many different indicators as possible, and that the most useful data may come from studies of stratovolcanoes with a prolonged history of activity during glacial and non-glacial periods.
DE: 8404 Volcanoclastic deposits
DE: 8408 Volcano/climate interactions (1605, 3309)
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005