Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

Water-saturated, hydrothermally altered rocks reduce the strength of volcanic edifices and increase the potential for sector collapses and far-traveled mass flows of unconsolidated debris. Iliamna Volcano is an andesitic stratovolcano located on the western side of the Cook Inlet, similar to 225 km southwest of Anchorage and is a source of repeated avalanches. The widespread snow and ice cover on Iliamna Volcano make surface alteration difficult to identify. However, intense hydrothermal alteration significantly reduces both the electrical resistivity and magnetization of volcanic rock and can therefore be identified with airborne geophysical measurements. We use airborne electromagnetic and magnetic data to map snow and ice thickness and identify underlying alteration zones at Iliamna Volcano, Alaska. Resistivities were calculated to an average depth of >300 m, and a 3-D susceptibility model extends from the surface to the base of the volcano, about 3,000 m below the summit. Geophysical models image low resistivity (<30 ohm-m) and low susceptibilities near the summit of Iliamna and below its older vent complex, with the low susceptibilities indicating alteration up to similar to 800 m in thickness. Thin conductors (similar to 50-100 m thick) on the edifice slopes coincide with recorded locations of repeated debris avalanches over the past similar to 60 years and are attributed to saturated zones at high elevation. Three-dimensional slope stability models based upon the geophysically constrained alteration distribution suggest the edifice of Iliamna is unstable and could lead to collapse scars similar to 400 m deep near the current and former vent complexes.

Automated summary

Water-saturated, hydrothermally altered rocks weaken volcanic edifices, leading to increased risk of sector collapses and mass flows of unconsolidated debris. Airborne geophysical measurements can be used to identify hydrothermally altered areas at Iliamna Volcano and suggest its instability.