Glacio-Hydrologic Studies of The Intracaldera Icefield of Mt. Veniaminof, Alaska

We conducted a 3-dimensional, ground-based radio echo-sounding and GPS survey of the intracaldera icefield of Mt. Veniaminof, a large active stratovolcano in SW Alaska. The study is part of the Alaska Volcano Observatory (USGS) volcano hazards program. The primary goals of the survey are to determine the topography of the caldera floor, calculate ice volume, and to develop subglacial melt water flow paths. A principal motivation is to characterize potential subglacial flow paths of melt water formed by lava flows from small eruptions in 1983 and 1993. The flow paths of recent melt events are used to determine the possible flood and lahar dangers for nearby villages and fisheries. We collected over 100 km of 8 MHz radar profiles and covered nearly 2/3 of the 10 km x 10 km caldera, avoiding crevasses, steep slopes, and exposed rock. A GPS base station and roving GPS on the skidoo-towed radar sled collected geolocation data for the radar traces and used to develop the ice surface topographic map. The caldera bedrock surface consists of two major basins to the north and south divided by a bedrock ridge that connects the active cinder cone to a broad but buried dormant cone. Other than the cinder cone ridge much of the subglacial topography is muted by the overlying ice (up to 425 m thick), particularly in the southern basin. Cone Glacier drains most of the ice from the northern basin with the remainder flowing to the NE through Crab Glacier. Ice from the southern basin flows over the much lower southern caldera rim as a nearly unbroken sheet. Low surface slopes and the buried caldera rim result in low ice flow rates within the southern basin. Subglacial water flow is controlled by ice surface slopes and subglacial topography. Hydraulic potential contours show that melt water from the 1983 and '93 eruptions should flow over the SE caldera wall and drain into the Pacific Ocean. However, the radar data shows evidence of cold (sub-zero) ice within much of the south basin and little or no evidence of water present at the bed. Thus, while it is possible that the melt water flowed subglacially at the ice-bed interface, we propose that the melt water drained through fractured or porous subglacial material in a manner that cannot be predicted from simple subglacial hydraulic potential calculations.