Several Aleutian Arc volcanoes periodically threaten cities and towns in the Anchorage area of southern Alaska, USA. The Aleutian Volcanic Arc consists of dozens of volcanoes, all formed by subduction of the Pacific Plate beneath the North American Plate. Mt. Spurr, in the northeastern Aleutian Volcanic Arc, is a 10-20,000 year old andesitic, subduction zone stratovolcano that formed after the collapse of a Late Pleistocene volcano (“ancestral Spurr Volcano”). Spurr Volcano had ash eruptions in 1953 and 1992.
Porphyritic andesite volcanic ash - from 5 AM to 6 AM on 9 July 1953, Mt. Spurr had an explosive ash eruption from the Crater Peak Vent, a satellite eruptive center. Crater Peak Vent is a basaltic-andesitic cone located just south of Mt. Spurr's summit.
The 1953 ash cloud reached 70,000’ high and blanketed the city of Anchorage with an approximately quarter-inch thick (~0.25”) layer of ash, which fell from about noon to late afternoon on 9 July 1953.
Hornblende andesite volcanic ash (field of view ~5.1 cm across) - on 18 August 1992, Mt. Spurr, had a 3.5-hour long, subplinian ash eruption. Ash coated the city of Anchorage. Like the 1953 eruption, the 1992 eruption was also from the Crater Peak Vent.
For additional information on the 1953 and 1992 eruptions, see:
Wilcox (1953) - Preliminary Report of the Eruption of Mount Spurr Volcano, Alaska, July 9, 1953, and the Ashfall in the Anchorage Area. 25 pp.
Keith (1995) - The 1992 eruptions of Crater Peak Vent, Mount Spurr Volcano, Alaska. U.S. Geological Survey Bulletin 2139. 223 pp.