Description: From McGimsey and others (2004): "During the summer of 1998, the volcano became seismically restless. Activity slowly escalated and culminated in a subplinian eruption on April 19, 1999 that placed an ash cloud to 45,000 ft ASL (~13,700 m). The eruption style almost immediately changed to that of vigorous strombolian fountaining (Nye and others, 2002), which characterized the activity for the following six weeks. AVO closely monitored the activity using seismic data, daily imagery from weather satellites, rare local ground and aerial observations, and an airborne thermal instrument. The eruption was deemed over in the last few days of May.
"* * * At 10:14 AM AST on January 7, 1999, a shallow, M1.4 earthquake was recorded under Shishaldin Volcano (Jolly and others, 2001). On January 9 at 5:55 PM AST, AVO received a call from National Weather Service (NWS) personnel in Cold Bay reporting that a larger than usual steam plume with possible ash at the base was rising from Shishaldin Volcano to an estimated 10,000 to 15,000 feet ASL and extending several tens of kilometers to the northeast. No anomalous seismicity was occurring and the activity was not visible on Advanced Very High Resolution Radiometer (AVHRR) satellite imagery from 0113Z and 0222Z. A follow-up report from NWS at 6:40 PM AST indicated that the steam plume was somewhat diffuse but still more voluminous than usual.
"On February 2, AVO seismologists noted a high level of tiny seismic events occurring beneath the volcano. NWS personnel in Cold Bay reported seeing a 'good plume' at Shishaldin on February 9 rising 5,000-6,000 ft. above the vent and trailing to the south. A weak thermal anomaly was also observed on this date. Steaming continued for the next couple of days and a one-pixel thermal anomaly was visible in the AVHRR 1715Z satellite image on February 12. Shishaldin was clearly restless, and AVO announced the activity in the weekly update that morning (Friday, Feb. 12) and warned that while a thermal anomaly persisted, there was a chance of a sudden, low-level ash burst. A pilot reported a steam plume rising 9,000 ft. above the vent on the morning of February 18; this was corroborated by a NWS observer in Cold Bay. AVO seismologists determined on this date that low-level but continuous seismic tremor was occurring at the volcano. This, and the persistence of a thermal anomaly prompted AVO to raise the Level of Concern Color Code from GREEN to YELLOW at 3:15 PM AST (0015 UTC) on Thursday, February 18. [See Table 5 in original text for an explanation of the Level of Concern Color Code.]
"Steam activity seemed to wane for the next several weeks. On March 4, a shallow M5.2 earthquake struck 9 miles (14 km) southwest of Shishaldin (Moran and others, 2002). NWS observers in Cold Bay reported that on March 5 the summit crater rim of the volcano was snow-free -- an indication of increased heating of the summit; no ash was present on the snow-covered upper flanks. On March 8, AVO reported that the summit vent thermal anomaly had increased over the weekend, seismic tremor was continuing, and that an ash burst could occur with little or no warning.
"During the next three weeks -- when weather permitted a view -- NWS observers in Cold Bay reported that no steam plume was present but the upper flanks were snow-free. The summit vent thermal anomaly persisted, as did low-level seismic tremor. On April 2, a pilot confirmed the snowmelt at the summit.
"After nearly two and a half months of precursory activity strong seismic tremor began on April 7 at about 8:00 AM ADT (1600 UTC). AVO raised the Level of Concern Color Code to ORANGE and warned that an explosive ash burst or lava eruption could occur over the next several hours or days. By that afternoon, although the strong seismic tremor had subsided, the Color Code was held at ORANGE and AVO began a 24-hour monitoring effort, which would last until June 18, 1999.
"On April 12, with seismicity back down to somewhat-above-background levels since April 7, AVO reduced the Level of Concern Color Code to YELLOW; the summit vent thermal anomaly persisted. Then, late on the night of April 13, seismic tremor increased significantly following a M4.5 earthquake and aftershocks that occurred west of the volcano. AVO raised the Color Code back up to ORANGE the following afternoon, April 14.
"Seismic tremor levels began to steadily increase on April 17. On this day, an AVO scientist accompanied the Alaska State Troopers in their Forward-Looking-Infrared-Radiometer (FLIR)- equipped twin-engine aircraft on a flight down the Alaska Peninsula. Although a steam cloud obscured the summit area of Shishaldin, FLIR images revealed that energetic strombolian fountaining was occurring with blocks and spatter hurled up to 600 feet (~200 m) within the vent [see figs. 9, 10 in original text]. NWS observers and a ship's crew offshore reported that snowmelt had run partway down the northwest flank.
"At 11:33 AM ADT (1933 UTC) on April 19, seismic tremor amplitudes dramatically increased and at 11:45 AM a pilot reported seeing a steam and ash plume that rose to 30,000 ft (~9,150 m). AVO raised the Color Code to RED at 12:15 PM ADT (2015 UTC) and announced that a significant eruption was in progress. By early afternoon the plume had reached 45,000 ft (~13,700 m).
Satellite data suggests that the plume reached a maximum height of about 56,000 ft. (17,000 m) (Dave Schneider, oral communication). Ash was dispersed southward at higher altitudes and northward at lower altitudes [See fig. 11 in original text].
"The eruption lasted about 7 hours, and by 11 PM on April 19, seismic tremor had substantially decreased although strombolian eruptive activity likely continued based on seismicity. A further abrupt and significant decrease of seismicity in the early morning hours of April 20 indicated that explosive activity had subsided, prompting AVO to lower the Color Code to ORANGE. A thermal anomaly persisted in the summit crater. However, about 4 PM ADT (0000 UTC) on April 20, seismic tremor began to increase again and strengthened about 11 PM ADT (0700 UTC). By midmorning on April 21, seismicity was back up to levels similar to that in the hours prior to the explosive eruption on April 19 prompting AVO to raise the Color Code to RED at 11:15 AM ADT (1915 UTC) and issue a warning that a moderately strong strombolian eruption was likely occurring and that a significant explosive event could occur at any time. Satellite imagery on this day revealed no major ash cloud but a very large thermal anomaly was visible through the night; lava fountaining to a few hundred feet above the summit was observed along with occasional steam and ash clouds under 15,000 ft. (~4,600 m).
"An explosive eruption was not forthcoming and on Thursday morning, April 22, although seismicity continued to fluctuate, the overall level had decreased from that of the previous morning. AVO lowered the Color Code to ORANGE at 10 AM ADT (1800 UTC) April 22. AVO personnel flying with the Alaska State Troopers late that afternoon observed low-level strombolian activity. Seismicity soon began to increase and by 9 PM ADT (0500 UTC), based on rapid increase of tremor levels, an explosive eruption began prompting AVO to again elevate the Color Code to RED (9:50 PM ADT, 0550 UTC, April 22). The tremor signals that occurred during the eruptions of Shishaldin Volcano on April 19 and 23 were the strongest ever recorded in the Aleutian Arc by AVO in its 11-year history (Thompson and others, 2002).
"About 4 hours later, seismic tremor rapidly diminished heralding an end to this eruptive event. Later that morning AVO reduced the Color Code to ORANGE (7:50 AM ADT, 1550 UTC, April 23). The size of the summit thermal anomaly indicated the continuance of low-level strombolian activity, although no ash clouds were visible on satellite images. Seismicity continued to be relatively low throughout the day. An AVO observer aboard the State Troopers plane documented a short-lived, mildly explosive ash burst that rose to about 15,000 ft ASL (~4,600 m) [See cover of original document]. Also observed was that the flanks were mantled with ash, mudflows, and probable spatter-fed lava flows from earlier eruptive activity [See figure 12 in original text].
"For the next two days, occasional strombolian eruptions occurred—based on continuation of relatively low seismicity and persistence of the summit thermal anomaly—and on the morning of April 26, a fishing vessel located 40 miles east of the volcano reported light ashfall. Satellite data confirmed a narrow plume extending for more than 100 mi (161 km) to the northeast and a weak summit thermal anomaly. By April 28, seismicity had declined to levels that indicated that eruptive activity was not likely occurring, no ash clouds had been observed in the past 2 days, and the summit thermal anomaly, which had persisted since early February, was not present on clear satellite images. AVO responded by lowering the Color Code to YELLOW but warning that eruptive activity could resume with little or no warning.
"Nothing much changed until the night of May 12 when a weak thermal anomaly appeared on a GOES satellite image and weak seismic tremor was recorded. The following morning, May 13, the crew of a NWS boat at the north end of False Pass observed a small steam and ash burst (~10:25 AM ADT, 1825 UTC). A PIREP at 11:15 AM ADT (1955 UTC) confirmed a small plume that rose to about 1,000 ft (300 m) above the summit. A weak thermal anomaly and lowlevel seismic tremor continued for the next 10 days. Then, late in the evening of May 24 (11:11 PM ADT, 0711 UTC), a PIREP indicated that a plume was present to about 20,000 ft. ASL (6,100 m) above the volcano. Satellite data at 6:59 AM ADT (1459 UTC) on the following morning revealed a narrow, ash-rich steam plume extending 100 miles (161 km) south from Shishaldin at an altitude of about 15,000 ft. ASL (4,600 m). Low to moderate levels of seismicity continued. AVO raised the Color Code to (9:30 AM ADT, 1730 UTC, May 25) and announced that low-level steam-and-ash eruptions and ash bursts were occurring at the volcano.
"For the next several days, narrow, ash-rich plumes were observed via satellite emanating from the volcano and seismicity indicated that short-lived, low-level steam and ash explosions were occurring. A small thermal anomaly persisted. Satellite images on May 28 revealed no evidence of ash plumes or a thermal anomaly and the seismicity declined. An AVO field crew working on the north flank of the volcano reported that only white steam was rising from the summit crater. The lowered level of seismicity and the absence of ash plumes and a thermal anomaly prompted AVO to decrease the Color Code to YELLOW on June 1. This effectively marked the end of the 1999 Shishaldin eruption. The seismicity remained just slightly above normal background levels for the next several weeks and no plumes or thermal anomalies were present. Activity at the volcano had returned to 'normal', that is, a nearly continuous low-level steam plume, non-tremor micro-seismicity associated with minor phreatic activity. AVO reduced the Level of Concern Color Code to GREEN on Friday, June 18 (the 14th Color Code change of the eruption, see Table 3 in original text), and ended its 24-hour-a-day surveillance of the volcano. The AVO weekly update of volcano activity in Alaska dropped Shishaldin as a feature on Friday, June 25, 1999.
"In addition to local minor ash dustings, the eruption produced minor mudflows down the flanks [See figure 12 in original text], a significant ash deposit on the south flank, and a lahar deposit down the north flank [See figure 13 in original text]. The lava produced in this eruption of Shishaldin is evolved basalt of about 49% SiO2 (Nye and others, 2002)."
Stelling and others (2002) estimate a total tephra volume of 4.7x10^7 cubic meters, with a Dense Rock Equivalent (assuming solid rock density of 2,600 kg m^-3) of 1.4x10^7 cubic meters.
For detailed chronologic tables of the events in this eruption, see Nye and others (2002) or McGimsey and others (2004).