WATER-TABLE POSITION IN STRATOVOLCANOES OF THE CASCADE RANGE: IMPLICATIONS FOR VOLCANIC HAZARDS

Water-table position has implications for several classes of volcanic hazards. For instance, above the water table there cannot be elevated fluid pressures to enhance the potential for slope failure, except perhaps very locally within ‘perched’ water bodies. Phreatic eruptions from volcanoes with deep water tables are not likely to occur in response to magma-groundwater interaction, but might be triggered instead by shallow intrusion of magma and localized recharge by melted snow and ice. Similarly, water to generate lahars would be unlikely to come from below the water-table, but might be supplied by snow and ice, and shallow groundwater perched on low-permeability zones. Numerical simulation results (Hurwitz et al., 2003) and sparse field observations suggest that water tables within Cascade Range stratovolcanoes may be relatively deep, as seems to be the case for many Hawaiian volcanoes despite high recharge rates. Model results suggest that the permeability structure within and beneath the volcanic edifice is the dominant control on water-table elevation and the distribution of pressures, temperatures, and fluid phases at depth. A volcanic edifice that is fully saturated by cold groundwater under near-hydrostatic conditions will tend to suppress hydrothermal upflow. Thus the presence of hydrothermal discharge or pervasive alteration high on the edifice supports the supposition that topography-driven downflow is somehow restricted. The necessary restriction may be related to permeability heterogeneity and/or the availability of precipitation recharge. As the edifice evolves over geologic time, there may be a positive feedback between hydrothermal alteration and water-table elevation, as alteration tends to decrease the permeability of the rock (as well as its mechanical strength). A significant unknown is the hydrogeologic role of the extensive ice caps that cover the summits and uppermost flanks of stratovolcanoes. Perhaps ice and associated permafrost restrict recharge in the summit region, as most glacial ice melts below the firn line well below the summit. Hurwitz, S., et al., 2003, J. Geophys. Res., 108, 10.1029/2003JB002565.