As explained in a post here earlier today (http://tinyurl.com/cbrpsjt) volcanic ash is a significant hazard, with a record of affecting air transport across Alsaska on a number of instances. Perhaps the most dramatic incident was played out in December 1989, not long after the AVO was established. A KLM passenger flight passed through an ash cloud spewed 9000 m into the air by Mount Redoubt. All engines failed, and the plane dropped 3000 m in four minutes before they could be restarted, the flight landing safely at Anchorage. It is just this sort of event that AVO now seeks to plan against.
Seismic monitoring of the most potentially dangerous volcanoes is one of their principal tools. Often the first indication of an impending eruption is an increase in earthquake activity. Generally, a network of six to eight seismometers are positioned around a volcano. The AVO successfully made advance warnings a number of times during the 1989-90 eruptions of Redoubt Volcano, as well as for two of the three eruptions of Mount Spurr in 1992.
The volcanoes of Alaska are associated with the north-eastward subduction of the Pacific oceanic plate beneath the North American continental plate. As hydrous minerals within the subducted oceanic crust follow their path into the mantle beneath North America the increased temperature and pressure dehydrates them. The water and other volatiles released from the subducting slab percolates into the mantle wedge above, lowering the melting point of the rocks, resulting in partial melt and volcanism at the surface. Here it results in volcanoes that form the the Aleutian Island chain extending west-southwest from the Alaska mainland. These volcanoes are strung out on the continental plate side of the subduction trench, set back from it, a phenomenon reflected all around the Pacific ring of fire.
Image: Augustine during eruption, January 18th 2006. A vigorous steam plume travels many miles to the east at an approximate altitude of 8500 ft. The cloud deck is at approximately 4000 ft. (Credit: Jill Shipman, image courtesy of the AVO/UAF-GI)