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Crater Peak (Mount 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 damage and 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. |
View of Crater Peak and Mt. Spurr before eruption | |
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View of the 3,374-m summit of Mt. Spurr (background) and 2,309-m Crater Peak (foreground), the site of eruptions in 1953 and 1992. Crater Peak is a satellitic vent perched on the southern rim of a caldera which formed 10,000-20,000 years ago, truncating an ancestral stratovolcano. Mt. Spurr is an ice-covered, silicic-andesite dome complex that has not erupted in historic times. Crater Peak, which has been 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 1992, it last erupted in 1953. The volcano lies 124 km due west of Anchorage, the principal population center in the state of Alaska. Photo credit: R.G. McGimsey, U.S. Geological Survey, 10/9/91 |
Warm lake within the Crater Peak basin prior to eruption | |
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Within the Crater Peak basin, a warm lake, approximately 500 m across, developed some time after the 1953 eruption. On June 11, the lake had a temperature of 59 degrees C and a pH of 2.5. A darkening of the lake color and upwelling zones, the largest about 5 m across is visible to the right in this view, indicated a possible addition of sulfur and an increased heat flux below the lake. Geysers of superheated water were observed streaming through talus on the north shore of the lake. An increased number of earthquakes had been detected below the volcano since August, 1991. On June 17, scientists from the Alaska Volcano Observatory (AVO) issued a notice about the increased likelihood of eruptive activity. Photo credit: R.G. McGimsey, U.S. Geological Survey, 6/11/92 |
Steam rising from the Crater Peak vent | |
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An overflight on June 26 revealed that the lake in Crater Peak had disappeared. After four hours of energetic seismicity, Crater Peak erupted explosively at 0704 AST on June 27, 1992. The eruption occurred under heavy cloud cover and was not photographed. Tephra from the 15-km high eruption column darkened the snow and ice of the surrounding mountains. Ashfall was reported as far away as Manley Hot Springs, 425 km north of the volcano. In this view, looking NNW towards Crater Peak, the tephra-mantled summit of Spurr is visible behind Crater Peak and to the right. A steam-condensate plume rises from the Crater Peak vent. During the eruption, avalanches of hot debris flowed down the south flank of Crater Peak and mixed with snow to form debris flows, or lahars, which reached the Chakachatna River, not visible in this view. Photo credit: T.P. Miller, U.S. Geological Survey, 7/3/92 |
Damage to seismometer on Crater Peak by the June 27 eruption | |
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A seismometer had been installed on the rim of Crater Peak, in August of 1991. Signals from this instruments allowed AVO to track the build up of seismicity prior to the eruption. Several hours into the eruption, the seismometer stopped transmitting. This twisted steel pipe with 1/4 inch thick walls and a melted, blasted piece of corrugated, galvanized steel was all that remained of the geophone, batteries, culvert, and antenna. Photo credit: U.S. Geological Survey, 7/6/92 |
The ash column from the August 18 eruption | |
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The volcano quieted down after the June 27 eruption, and it appeared as if, similar to the 1953 eruption, this phase of activity had ended. However, on August 18, 1992, a pilot reported ash over the volcano. At 1641 AST, without significant seismic precursors, Crater Peak again erupted, suddenly and explosively, sending pulverized rock to more than 14 km altitude. Winds blowing east over the volcano carried the tephra plume over upper Cook Inlet and the City of Anchorage. AVO observers on board a small aircraft documented the violent eruption column roiling over the vent. In this view, steam-condensate rises around the base of the column where hot debris has fallen on snow and ice. Photo credit: R.G. McGimsey, U.S. Geological Survey, 8/18/92 |
Ash cloud rising off south flank of Crater Peak | |
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A lighter toned ash cloud rising off the south flank of Crater Peak resulted from avalanches of hot debris, or pyroclastic flows, that had traveled down the side of the cone towards the Chakachatna River. The summit of Mt. Spurr is at left. The spreading eruption cloud at this time has reached at least 14 km altitude and formed a wide, anvil-shaped cloud. Photo credit: R.G. McGimsey, U.S. Geological Survey, 8/18/92 |
South flank of Crater Peak showing bomb impact craters | |
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Each eruption violently ejected large bombs along a narrow ballistic trajectory from the vent. This oblique view of the south flank of Crater Peak shows the fall-colored tundra and a covering of pyroclastic flow deposits dotted with numerous impact craters formed when these bombs slammed into the ground. These ballistic showers probably occurred during explosions of superheated water that gained access to the hot rock within the eruptive conduit below Crater Peak. Blocks 1 m across were thrown as much as 3 km from the vent. Photo credit: C.A. Neal, U.S. Geological Survey, 9/3/92 |
New magma within the pyroclastic flow deposits | |
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Juvenile (new magma) material within the pyroclastic flow deposits consists of these brown, cauliflower-shaped cobble and boulder-sized clasts. The chemical composition of most of the newly erupted material is that of basaltic andesite with a SiO2 content of about 57%, a rock type typical for Crater Peak. Under the microscope, these rocks are only slightly vesicular and contain the minerals plagioclase, pyroxene, and hornblende in a micro-crystalline matrix with very little glass. Also ejected in smaller quantities were fragments, up to several meters across, of old volcanic material from the edifice of Crater Peak, smaller pieces of deeper-seated plutonic and metamorphic rock and partially melted and remobilized basement rock. Photo credit: C.A. Neal, U.S. Geological Survey, 9/3/92 |
Inflated gneiss ejected in 1992 | |
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A spectacular product of this eruption includes this example of inflated gneiss which was ejected in small quantities during each of the three eruptions in 1992. This material appeared in various states of vesicularity and is believed to represent remelted, frothy blobs of basement rock beneath Crater Peak. This material is found as separate ejecta clasts and also as inclusions in the 1992 andesite, indicating that the new magma involved in this eruption heated, partially melted, and mingled with this remobilized material prior to eruption. Photo credit: T.P. Miller, U.S. Geological Survey, 10/23/92 |
Successive pyroclastic avalanches | |
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Successive pyroclastic avalanches formed overlapping, lobate tongues of coarse debris that coursed down the upper flanks of Crater Peak and funneled into pre-existing drainage. The farthest traveled pyroclastic flows moved about 3 km from the crater rim, descending more than 1000 m, as shown in this oblique view of the south flank of Crater Peak. Photo credit: C.A. Neal, U.S. Geological Survey, 9/3/92 |
Geologist near one of bomb craters on south flank of Crater Peak | |
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A geologist stands near one of the bomb craters on the south flank of Crater Peak. By measuring the orientations of some of the craters, it appears that many of the impacting bombs were spinning like a curve ball when they hit the ground. Photo credit: C.A. Gardner, U.S. Geological Survey, 9/26/92 |
Fallout from tephra cloud deposited on the ground | |
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Fallout of coarse material from the spreading tephra-cloud resulted in a narrow zone of deposition on the ground. This is a Photo of the deposit from the August 18 eruption. Sixteen cm of lapilli and coarse sand representing about 3.5 hours of fallout accumulated 8 km downwind of the vent. The different colors within the tephra-lighter at the base and darker at the top-may represent some pre-eruption chemical and textural heterogeneties within the magma stored beneath Crater Peak. Photo credit: R.G. McGimsey, U.S. Geological Survey, 9/9/92 |
The inside of Crater Peak after the August eruption | |
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Following the August eruption, the inside of Crater Peak was mantled with pryoclastic debris and coarse talus from rock avalanches from the steep crater walls. The actual orifice or vent for the eruptions is located at the back (NW) corner of the crater, here steaming profusely. Other "rootless" fumaroles emit steam through the thick blanket of debris on the crater rim and where banked against the crater walls. Photo credit: T.P. Miller, U.S. Geological Survey, 8/20/92 |
Fallout in Anchorage from the August 18 eruption cloud | |
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By the time the August 18 eruption cloud passed over Anchorage, 124 km distant, material raining out consisted of coarse to fine sandy ash which fell as thickly as 3 mm in parts of the city. The period of fallout lasted about 3.5 hours. Residents were able to observe the oncoming ash-cloud which appeared as a dark storm cloud moving steadily overhead. Photo credit: R.G. McGimsey, U.S. Geological Survey, 8/19/92 |
Ash on the ground in Anchorage becomes resuspended | |
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Ash on the ground in Anchorage became re-suspended in the air with the slightest breeze. Vehicular traffic on the roads also contributed to unhealthy particulate levels in the air over the city. Residents resorted to particle masks and had to take precautions to protect sensitive electronic equipment, car engines, etc. People with respiratory sensitivities were warned to stay indoors and keep physical activity to a minimum. Photo credit: R. Emanuel, 8/19/92 |
Clean-up at the Anchorage International Airport | |
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The Anchorage International Airport was closed for 20 hours due to the ash blanketing runways. Clean-up took several days and was best accomplished by wetting down the ash followed by grading and sweeping trucks. Around the city, residents hosed down homes, driveways, and cars. Photo credit: R. Emanuel, 8/19/92 |
September eruption of Crater Peak | |
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Crater Peak erupted for the final time in 1992 during the night of September 16-17. Like the first two eruptions, this event was brief and explosive, sending a tephra cloud to 15 km in altitude over the volcano. Hot debris cascaded onto the surface of the Kidazgeni Glacier, adjacent Crater Peak, causing extensive melting and scouring of the ice, shown here. Down slope, these avalanches of hot debris became increasingly water-rich and formed lahars which rushed down a steep-walled canyon and temporarily dammed the Chakachatna River. The tephra cloud from this eruption traveled ENE just missing Anchorage but heavily impacted the communities in the Matanuska-Susitna and Cooper River basins. Photo credit: C.A. Gardner, U.S. Geological Survey, 9/23/92 |
Lahars from the September eruption | |
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The lahars of the September eruption were cold and dark when photographed only hours after emplacement. They formed these steep-sided lobate tongues of poorly sorted debris. Photo credit: R.G. McGimsey, U.S. Geological Survey, 9/17/92 |
The Crater Peak vent after the September eruption | |
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Following the September 16-17 eruption, the interior of Crater Peak was once again mantled by many meters of fragmental debris varying in size from very fine ash to large blocks meters across. The vent, here marked by the column of steam, appeared to have migrated somewhat further to the NW against the wall of the crater. Photo credit: C.A. Neal, U.S. Geological Survey, 9/24/92 |
Plume of gas from Crater Peak as seen from Anchorage | |
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In early October, 1992, a period of intense tremor-like seismicity sent AVO to Level of Concern Color Code RED, the highest level of concern. Although no eruption ensued, this robust plume of magmatic gas and steam was startlingly visible from Anchorage. The activity was later interpreted to be related to vigorous degassing of the hydrothermal system beneath Crater Peak. In early November, an earthquake swarm probably related to shallow intrusion of magma was recorded at shallow levels beneath Crater Peak. Although seismicity slowly declined, AVO scientists continued to measure detectable amounts of sulfur dioxide and carbon dioxide over Crater Peak into the early part of 1993. This probably reflects the continued degassing of unerupted magma stored beneath the vent. Photo credit: M.P. Doukas, U.S. Geological Survey, 10/2/92 |