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Crater Peak (Mt. Spurr), Alaska: Eruptions of 1992 |
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.
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