VOLCANOLOGY

Pavlof Darkens the Alaskan Sky

    In the sparsely populated wilderness of Alaska, you might think volcanic eruptions would have relatively little impact on civilization. But as those who monitor Alaska's volcanoes know, the threat is very real, and not just to people on the ground.

by Tina Neal, U.S. Geological Survey/Alaska Volcano Observatory, Anchorage, Alaska

In September 1996, Pavlof Volcano began a vigorous strombolian eruption that continued into early 1997. Historically the most active volcano in the Aleutian volcanic arc, Pavlof has erupted about every 3–8 years since the mid-1700s, with the last activity occurring between April 1986 and May 1988. Both the 1986–1988 and the current eruption are characterized by steam and ash emissions, strombolian lava fountaining at the summit, and spatter-fed lava flows and derivative lahars that spread down the northwest and

    southeast flanks of the volcano. Although the volcano has been quiet since late December 1996, strombolian activity could resume at any time. In fact, if the volcanic conduit becomes plugged and overpressurized, Pavlof could spring back to life with a sudden burst that could send ash higher than 10 km in altitude.

    Little is known about the prehistoric life of Pavlof, a symmetrical snow- and ice-covered stratocone that is 2518 m high. It is the most active of a cluster of volcanoes that have formed within and just northeast of the Emmons Lake volcanic center, a Quaternary caldera complex. The volcano is located in Izembek National Wildlife Refuge, a remote wilderness area that is rarely visited. No structures or populations are at immediate risk from significant tephra fall or lava flows: the volcano is located 965 km southwest of Anchorage, and the nearest

    settlements are Cold Bay (60 km southwest), King Cove (48 km southwest), Sand Point (97 km east), and Nelson Lagoon (80 km northeast). Together, these four settlements have a total year-round population of about 1,800 people, and the worst that most of those towns have endured is an occasional ash fall. During the current eruption, King Cove, Sand Point, and Nelson Lagoon received light dustings of ash. (In the 1986–1988 eruption, Pavlof dumped 3 mm of ash on Cold Bay.)

    While Pavlof poses little threat to nearby inhabitants, it is a real threat to aviators. The volcano lies beneath heavily traveled North Pacific jet airplane routes which handle 10,000 passengers and millions of dollars of cargo each day. During Pavlof's 1986–1988 eruption, an 8-hour explosive event sent ash more than 15 km above sea level. A similar, unanticipated explosive eruption today would pose a significant hazard to air traffic in the region since volcanic ash clouds can seriously damage aircraft and, in the worst cases, stall jet engines.

Natural Air Traffic Control

    To address this airborne hazard, the Alaska Volcano Observatory (AVO)—a joint operation of the U.S. Geological Survey (USGS), the Geophysical Institute of the University of Alaska-Fairbanks, and the Alaska State Division of Geological and Geophysical Surveys—installed six seismic stations near Pavlof in July 1996. The new instruments, designed to complement satellite images and other observations of the volcano, were installed with funding from the Federal Aviation Administration because of the volcano's frequent eruptions and their potential impact on air travel.

    AVO first detected unusual seismic rumblings under the volcano at 9:37 a.m. Alaskan Daylight Time (ADT) on September 11, 1996.

    Intermittent volcanic tremor burst was recorded at PVV, a seismic station located 8.3 km southeast of the active vents. Researchers monitoring Pavlof are surprised that prior to the eruption only two volcano-tectonic earthquakes were recorded beneath the volcano, and very few have been detected by the Pavlof area network since the eruption began.

    The first visual clues of an eruption came on September 16, when a National Weather Service observer in Cold Bay called AVO to report that steam and incandescent ejecta had been spotted above the volcano at 8:30 a.m. Later analysis of seismic data and of satellite images from the National Oceanic and Atmospheric Administration's advanced very high resolution radiometers (AVHRR) indicated that the eruption likely began on September 12.

    Once the eruption was confirmed, the

    AVO issued its level of concern code "orange," its second-highest level of warning to the public. For Pavlof, this designation meant that the volcano was in eruption and that ash clouds could possibly reach 7 km above sea level. Government agencies, the media, and air carriers were quickly notified by phone, fax, and e-mail. AVO put staff to work 24-hours-a-day to monitor the eruption, and updates were issued on a daily basis. These updates and other information on the eruption also were posted on AVO's web site (http://www.avo.alaska.edu/ volcanoes/pavl/pavl.html.)

Flow of News and Lava

    As is true of many remote eruptions in Alaska, the AVO relied heavily on reports from pilots and observers on the ground to characterize and track the details of the Pavlof eruption. Nearby residents kept logs of activity that confirmed an intermittent, pulsing, low-level eruption from near the summit of the volcano. Early in the eruption, local pilots reported incandescent ballistic fragments the size of pickup trucks, and minor ash clouds or steam plumes alternately rose as high as 2 km above the volcano.

    Photographs and video gathered on scientific flights in late September revealed that two separate vents were emitting lava fountains. One was on the east edge of an 190-m-diameter crater located on the northwest summit of the volcano.

    The second, more active fountain was perched on the west edge of this crater, 100–150 m below the summit. The two vents were only 100 m apart but rarely spewed lava at exactly the same time. A small spatter cone formed at the west vent, and spatter and lahars extended about 500 meters down the flank. A lava flow plunged down the steep flank, melting a narrow channel through seasonal snow and ice.

    By September 29, the lava flow had reached the base of the cone, about 4 km from its source, and was widening into a fan. Dark lahar deposits extended beyond the toe of this lava flow and across the gently sloping ground to the northwest of the volcano, coming within 50–70 m of AVO's seismic station PV6. By late October, a second lava flow issued from the east vent, and when Alaska State Troopers videotaped the flow on December 2, it was the more active of the two and had nearly reached the base of the cone.

Look! Up in the Sky!

    During the first 2 weeks of the eruption, occasional clouds of ash were detected in AVHRR satellite images. However, ash clouds of significant density did not reach altitudes that would affect the major North Pacific airplane routes.

    During the third week, both pilot reports and satellite image analyses documented larger but still diffuse ash clouds trailing as far as 175 km downwind, though they rarely reached above 6 km altitude. By October 3, the FAA issued an international notice cautioning pilots of the possibility of ash below 7 km near Pavlof.

    Periods of nearly continuous lava fountaining accompanied by higher levels of seismicity and production of more energetic ash plumes began on October 15. In response, the FAA increased the altitude of restricted air space to approximately 8 km.

    AVO briefly declared its highest level of concern (code "red") on October 18 and again on November 4, cautioning pilots that ash clouds might reach above 7.5 km, high enough to enter the North Pacific airplane routes.

    Pavlof erupted sporadically for the rest of December. Seismicity decreased abruptly on December 4, and ash was not visible above the cloud cover that obscured the summit for several days.

    Accordingly, AVO reduced the level of concern color code to "yellow" on December 6. On December 9, bursts occurred with increasing frequency, and on December 10, seismicity increased significantly and there was one report of ash. Late on December 10, AVO reinstated the highest level of concern (red), but downgraded this to orange two days later and to yellow on December 15 as seismicity continued to decline. Another episode of heightened seismicity prompted a brief return to red on December 27. From December 29 through mid-March 1997, AVO has remained at level of concern color code yellow.

    The last reliable observation of ash emission occurred on January 3, 1997, although pilots and observers in Cold Bay reported possible ash in the steam plume over the volcano on a few occasions through February 6.

    Volcanic tremor recorded by AVO seismometers remains slightly above the detection threshold. AVO scientists consider this to mean that a shallow magma body below the volcano continues to degas. The summit of the volcano remains bare as new lava flows and spatter melt the new snowfall.

    Such quiet periods between episodes of violent activity are common in eruptions at Pavlof and can last from days to months. The FAA maintained its restriction on flights near Pavlof until January 27, and AVO continues to monitor Pavlov closely and will issue prompt notice should seismic activity increase. Field studies of the new lava flows and flood deposits are planned for this summer when AVO crews visit the area to conduct routine maintenance of the seismic stations.

    Source: Eos, December 17, 1996, p. 519.

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