Elevated levels of seismicity continued at Spurr during 30 July to 6 August, with about 10-20 earthquakes occurring daily beneath the summit. This level of activity had remained relatively constant for the last several weeks. AVO noted that although this represents a notable increase above background seismicity, there were no indications that an eruption was imminent.

Aerial observations of Spurr made by AVO staff during the report week indicated the presence of a circular collapse pit in the snow/ice cover, approximately 50 x 75 meters in diameter on the NE flank of the summit dome at an elevation of approximately 3,110 m a.s.l. The collapse pit appeared to contain standing water of indeterminable depth. Arc-shaped scarps and small translational slumps in the snow/ice cover around the summit dome also were observed. AVO noted that these features and the collapse pit may indicate an increase in heat flux through the summit dome that could be related to the intrusion of magma at depth. Photographs of Spurr's summit taken on 20 June 2004 did not show a collapse pit, but did indicate an arc-shaped scarp in the area where the present collapse pit has formed. Spurr remained at Concern Color Code Yellow.

Geological Summary. The summit of Mount Spurr is a large lava dome constructed at the center of a roughly 5-km-wide horseshoe-shaped caldera open to the south. The volcano lies 130 km W of Anchorage and NE of Chakachamna Lake. The caldera was formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an ancestral edifice. The debris avalanche traveled more than 25 km SE, and the resulting deposit contains blocks as large as 100 m in diameter. Several ice-carved post-caldera cones or lava domes lie in the center of the caldera. The youngest vent, Crater Peak, formed at the breached southern end of the caldera and has been the source of about 40 identified Holocene tephra layers. Eruptions from Crater Peak in 1953 and 1992 deposited ash on the city of Anchorage.

Source: US Geological Survey Alaska Volcano Observatory (AVO)