Seismicity at Spurr during 16-23 September remained above background levels, but the overall rate continued to gradually decline. Preliminary data from a gas-sensing flight earlier in the week showed a substantial reduction in gas emissions compared to previous measurements taken in May 2005. The declining seismicity, reduced gas emission, and the changing summit-lake color (thought to reflect lower levels of acidity), all suggested a reduced level of volcanic activity. Minor steaming continued from the summit "melt pit" and occasionally from Crater Peak. 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)