During 3-10 September, elevated levels of seismicity continued to be recorded at Spurr as has been the case for several weeks. About 90 earthquakes were recorded during the report period within 30 km of the summit, with about 13 occurring per day. Observations and photography during the week revealed that the ice pit located near the summit had enlarged substantially (currently about 150 m x 170 m), presumably as the roof of the meltwater basin continued to subside and collapse. 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)