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Crater Peak (Mt. Spurr), Alaska: Eruptions of 1992
browse graphic Alaska has a number of active and potentially active volcanoes. More than one-half of the population of Alaska lives within 300 km of an active volcano. In the last 100 years there have been two eruptions at Mr. Spurr, three at Redoubt Volcano, and four eruptions at the Augustine Volcano. The 1989-1990 eruption of Redoubt Volcano resulted in 160 million dollars of damageand loss. This set follows the story of Crater Peak activity from June through October, 1992, and discusses precursors, the eruptions, and effects on the environment. Mt. Spurr, located 124 km due west of Anchorage, Alaska, is an ice-covered, silicic-andesite dome complex that has not erupted in historic times. Crater Peak is a satellite vent perched on the southern rim of a Mt. Spurr caldera that formed 10,000-20,000 years ago. Crater Peak, active for at least the last 5,000 years, is a basaltic-andesite stratocone with a summit crater approximately 800 m across at its rim. Prior to the 1992 eruptions, it last erupted in 1953. There were vigorous fumarole fields and a smallwarm lake in the summit crater. Increased seismicity began at Crater Peak in August of 1991. By June of 1992 scientists noted an increase in the temperature and acidity of asummit lake. A flight over the summit on June 26 revealed that the summit lake had drained away. Seismicity increased and Crater Peak erupted explosively on 0704 AST on June 27. The eruption column reached 15 km and ash fall was reported 425 km to the north. Avalanches of hot debris flowed down the south flank and mixed with snow to form debris flows (lahars) that traveled up to six km from the crater. Following the June 27 eruption the volcano's seismicity returned to pre-August 1991 levels. Almost two months later on August 18, a commercial flight over the volcano discovered an ash plume emanating from Crater Peak. Then at 1641 AST, almost without warning, Crater Peak explosively erupted, sending a plume of ash to more than 14 km in altitude. Large volcanic bombs were thrown 750 m above the vent. Lithic blocks of up to one meter diameter were thrown as far as 3.8 km southeast of Crater Peak. Pyroclastic (hot debris) flows traveled as much as three kilometers from the crater rim. The volume of the ash plume was about twice the volume produced by the June eruption. During the night of September 16-17, Crater Peak again erupted. As in the first two eruptions a 15-km tephra cloud and pyroclastic flows also accompanied this eruption. A variety of eruption products including frothy, glassy material and large brown to gray andesitic bombs were produced by all three eruptions. Fragmental debris varying in size from very fine ash to large blocks several meters across littered the slopes of the volcano. The August 18 eruption of Crater Peak produced troublesome ash falls in Anchorage, 124 km distant. Residents had to wear particle masks and take special precautions to protect car engines and electronic equipment. The International Airport at Anchorage was closed for 20 hours by the ash fall. The ash fall from the September 16-17 eruption heavily impacted the communities in the Matanuska-Susitna and Copper River basins. There have been periods of intense seismicity since the eruption in August of 1992.