Cover photograph: An aerial view, looking westward, over the western half of the 18 × 12 kilometer caldera of Fisher volcano, with the snow-covered Westdahl group of volcanoes (Westdahl, Faris Peak, Pogromni, and Pogromni's Sister, left to right) SUMMARY OF HAZARDS AT FISHER VOLCANOFisher volcano, containing the largest Holocene caldera in the Aleutian volcanic arc, is an active volcano near the center of Unimak Island, about 120 kilometers southwest of Cold Bay and about 175 kilometers northeast of Dutch Harbor. The volcano is composed of numerous small volcanic centers around and within a large, oval caldera 12 by 18 kilometers in diameter and 500 to 1,000 meters deep that formed during a catastrophic eruption about 9,400 years ago. Since then, more than 30 separate vents inside and outside the caldera have erupted; the most recent eruption occurred in 1826 (Grewingk, 1850; Miller and others, 1998). These eruptions have produced lava flows and widespread tephra (volcanic ash) deposits, and have occasionally been accompanied by large floods and lahars. The history of Fisher volcano includes repeated explosive and non-explosive eruptions of varying levels of intensity, and indicates that this pattern will continue in the future. Understanding the variety of potential hazards associated with different types of volcanic processes typical of Fisher volcano can help minimize the adverse effect of future eruptions on humans and infrastructure. Volcanic ash cloudsThe generation of clouds of pulverized rock debris, pumice, volcanic ash, and gas that are blasted high into the atmosphere and then drift away from the volcano with the wind is the most prominent hazard associated with future explosive eruptions of Fisher volcano. Ash-rich clouds formed during large eruptions can reach heights of 20 kilometers or more above the volcano. Prevailing winds near Fisher Caldera are from the west, which would carry ash clouds to the east and northeast of the volcano, although dispersal in other directions is possible. The geologic history of Fisher volcano bears evidence of several large eruptions, although most recent eruptions of Fisher have been smaller. All aircraft should avoid flying near ash clouds, as contact with ash particles of any size can damage aircraft windows and instruments, clog air vents, and disable jet engines. Volcanic ash fallout SUGGESTIONS FOR READING THIS REPORT Readers who want a brief overview of the hazards at Fisher volcano are encouraged to read the summary and consult sheet 1 and the illustrations. Individual sections of this report provide a slightly more comprehensive overview of the various hazards at Fisher volcano. A glossary of geologic terms is included and additional information about Fisher volcano may be obtained by consulting the references cited at the end of this report.