WATER-TABLE ELEVATION WITHIN VOLCANIC EDIFICES ALONG THE CASCADE RANGE: INSIGHTS FROM NUMERICAL SIMULATIONS

Water table elevation within a volcanic edifice has significant implications for volcano hazards, geothermal energy, and epithermal mineralization. We have extended the HYDROTHERM numerical simulator to allow for a free-surface (water-table) upper-boundary condition and a wide range of recharge rates, heat-input rates, and thermodynamic conditions representative of volcano-hydrothermal systems along the Cascade Range. Simulation results suggest that the permeability structure of the volcanic edifice and underlying material is the dominant control on water-table elevation and the distribution of pressures, temperatures, and fluid phases at depth. Several conditions facilitate the ascent of a hydrothermal plume into a volcanic edifice: a sufficient source of heat and magmatic volatiles at depth, strong buoyancy forces, and a relatively weak gravity-driven flow system. Further, the rising plume must be connected to a deep heat source through a pathway with a time-averaged effective permeability of at least 1x10^-16m², which is probably maintained by frequent seismicity. Gravity-driven flow may be retarded by low permeability in the edifice and/or the lack of precipitation recharge; in the latter case, the water table may be relatively deep. Simulation results were compared with observations from the Quaternary stratovolcanoes along the Cascade Range of the western USA to infer hydrothermal processes within the edifices. Both the simulation results and limited observational data allow for the possibility that the water table beneath the stratovolcanoes along the Cascade Range is relatively deep.