[5] Mount Spurr, the highest volcano of the Aleutian Arc, is a large stratocone constructed at the center of a roughly 5-kilometer-wide (3.1 mi) horseshoe-shaped caldera that is open to the south.

The caldera was formed by a late-Pleistocene or early Holocene sector collapse and associated pyroclastic flows that destroyed an ancestral Spurr volcano.

The youngest post-caldera dome, Crater Peak (2,309 meters, 7,575 ft), formed at the breached southern end of the caldera about 3.2 kilometers (2.0 mi) south of Spurr, has been the source of about 40 identified Holocene tephra layers.

[6] On July 26, 2004, the AVO raised the "Color Concern Code" at Spurr from green to yellow due to an increasing number of earthquakes.

In the first week of August 2004, the AVO reported the presence of a collapse pit, filled with water forming a new crater lake, in the ice and snow cover on the summit.