VOLCANIC DISRUPTION OF SUMMIT ICE AT MOUNT SPURR VOLCANO, ALASKA, 2004-2007

An increase in heat flow from Mount Spurr Volcano in Alaska in 2004 produced an ice cauldron and lake in the previously snow- and ice-filled summit crater of this 3374-m-high stratovolcano. Coincident increases in volcanic gas output and seismicity beneath the volcano suggest that a magmatic intrusion in mid-2004 prompted melting of summit ice. Formation of the initial lake and ice cauldron occurred in two episodes of collapse of the ice cap, each accompanied by expulsion of debris flows along preexisting englacial and possibly subglacial pathways east and southeast of the summit. A third debris-flow generating event was captured on a web camera sequence of images in May of 2005. From July 2004 through August 2005, the lake grew in diameter as additional snow and ice from the ~80-m-high walls of the cauldron were undercut, toppled into the lake, and melted. The roughly circular cauldron was eventually contained mostly by bedrock, stabilizing at its maximum diameter of ~300 m and covering ~70,000 m² by late summer 2005.

Lake level estimated from analysis of oblique aerial photography has changed slowly over the period of observation with notable drops following the May 2005 debris flow and in the late spring of 2006. We infer that overall stability of the lake level is maintained by slow leakage of water over a spillway or through an ice-tunnel in the southeast rim. Morphologic changes in the glacier surface and warping of a buried tephra layer visible in the cauldron wall also suggest leakage of lake water down the southeast flank of the summit cone. The fate of this water is uncertain, however, high anion content and conductivity from a single water analysis at the front of the Kidazgeni Glacier 10 km downstream suggest possible long distance transport of hydrothermally modified water within or beneath the ice.

Seismicity and gas output gradually waned through late 2005 and into 2006. With the onset of fall and winter in 2006, snow and ice began to accumulate atop the lake surface from a combination of freeze-up of the lake, mass wasting from the enclosing walls, and new precipitation. Historical photography indicates that prior geothermal heating events may have occurred at the Mount Spurr summit resulting in minimal disruption of the snow and ice cover. The present ice-cauldron is the most dramatic modification of the summit on record.