A newspaper account from August 15, 1910, describes an eruption of Shishaldin: "The steamer Corwin has arrived from Nome [to Seward] with the report that Mount Shishaldin, the highest volcanic peak in the Unimak Islands, is again in eruption. Officers of the Corwin say that when they passed the island a few days ago the volcano was more active than during the former eruption.
"A great column of fire shot high into the air and volumes of smoke poured from the crater. The snow which at the time of the previous eruption had not melted far from the crater has entirely disappeared from the sides of the mountain. The Corwin was covered with volcanic ashes."

An Associated Press article of February 5, 1932, reported: "Shishaldin is reported in the most violent and spectacular eruption for many years, with huge quantities of lava pouring down its uninhabited slopes and smoke and ash scattered over the Bering sea by a strong wind.
"'The volcano presented a beautiful display of fireworks after dark, day before yesterday [February 3],' a radio message sent out by Joseph Nasenius".
From Jaggar (1932): "On February 10, 1932, a radio from John Gardner in False Pass informed me that Shishaldin erupted February 1, 1932, the outbreak lasting for several days with glowing material flowing down the mountain. A newspaper dispatch of February 4 called this eruption 'the most violent and spectacular seen in the past century' for this volcano. The report from Squaw Harbor described 'streams of lava flowing down the sides,' but this glowing material is quite as likely to be trains of red-hot bowlders [sic] for these Aleutian cones. A strong wind carried the ashes many miles northward over the Bering Sea. A dispatch of February 16 said that Shishaldin had again renewed its activity with boomings coming form the crater [sic] at two-minute intervals, the volcano hurling hot rocks thousands of feet into the air." An Associated Press article published on the same day stated that the eruption could be heard from a distance of 50 miles.
From Finch (1932): "While passing to the south of Shishaldin during the night of May 5-6, 1932, the writer observed a brilliant glow over the summit and for a short distance down the north-west slope.
"There were light falls of basaltic pumice from time to time after the outbreak of February 1, 1932, until, at least, the latter part of May. Shishaldin was grey most of the time during the spring of 1932. The white of each new snowfall was usually darkened within a day or two by a film of ash."
Finch (1932) continues: "The pressure within the volcano was never great enough during 1932 to produce a major explosion. Fragments of pumice a little larger than a walnut were picked up at a distance of 25 miles to the north-east. Their distribution to this distance was caused by a strong south-west wind. Most of the material ejected quietly overwelled the crater edge and rolled down the north slope. As a large part of this material was red hot, it continued to glow for a considerable distance below the summit. It was noticed that the summit vent would shift slightly in position and at times there was no visible crater. At the time of Father Hubbard's visit to the summit no crater was found. The small crater would become plugged when the pressure was sufficient to lift the clinkers to the crater throat but insufficient to eject them.
"The highest point we reached, 6,500 feet, was on a line up the northwest shoulder of the mountain. From that elevation at 10:30 a.m. on May 21 mild explosions were noticed and pumice fragments were scattered over the north slope. Several times, sounds that closely resembled the noise produced by rocks falling into a deep narrow crater were heard coming from within the mountain.
"As Shishaldin's flanks are always covered with snow and ice, the hot cinders erupted produced mud flows. During such mud flows, the source of Clinker Creek may be said to be the summit of the mountain, as most of the material of the mud flow course shown in figure 2 finds its way into Clinker Creek drainage. When the volume of the mud flow was especially large, part of the material overflowed into the next creek to the east."

From Neal and others (1996): "AVO received a flurry of calls on September 4, 27, and October 4, 1993 relaying pilot reports of vigorous steaming and on one occasion (September 4) 'possible ash' to 3 km. This was not confirmed by satellite imagery or ground observations of ashfall. On October 26, an experienced USFWS observer noted ash on the upper 300 m of the cone as well as a gray coloration to the plume emanating from the summit crater. More pilot reports of steaming were received on October 28. On October 29, USFWS personnel in Cold Bay reported the steam plume was perhaps 'darker and more vigorous' than usual; NWS also relayed a pilot report of steam to 20,000 ft. Again, no satellite or ground detection of distant ashfall were noted.

From Neal and McGimsey (1997): "Following the late 1995 eruption (McGimsey and Neal, 1996), AVO staff examined satellite images of Shishaldin on a daily basis and tracked the decay of a hot spot at the summit of the volcano. AVO also reviewed USCG footage of the volcano taken during routine patrol missions; the footage showed no unequivocal signs of eruptive activity but commonly captured a vigorous steam plume emanating from the summit crater. Photographs by AVO personnel on May 16, 1996, however, clearly display fresh ash on the upper flanks and crater rim [see figures 7a, 7b, in original text]; no specific eruptive event had been documented to account for this ash, and its origin is unknown. Based on its fresh and snow-free appearance, it was likely deposited not long before the photos were taken.

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).

From Neal and others (2004): "Following anomalous, post-eruption seismicity of late December 1999, a series of seismic events caused by small explosions were identified on January 15, 2000. No associated plumes or thermal anomalies appeared on satellite images. Re-analysis of seismic data for the previous several months revealed that similar small 'explosion' seismic signals had been occurring intermittently since September 1999, several months after the end of the main phase of the 1999 eruptive activity (Nye and others, 2002). The January events were low-level and difficult to discern during the frequently inclement weather when background noise was high. The explosions prompted AVO to mention the change in the weekly update, however, the level of concern color code remained GREEN (table 4.) On January 28 and 29, vigorous steam plumes were observed rising up to about 3,000 ft (1,000 m) over Shishaldin. By early February, the frequency and amplitude of the overall seismicity, including the explosion events, had increased (up to 200 per day) and were accompanied by reports of vigorous steam plumes. The activity was interpreted to be small phreatic explosions within the cone's deep central crater. Noting that a similar pattern had developed in February 1999 prior to the onset of strombolian eruptive activity, AVO upgraded the color code to YELLOW on February 3. Fortuitously, the number of seismic events abruptly decreased the next day. The color code was lowered to GREEN on February 18. A small steam plume was observed in satellite images on February 22 that extended 15-20 km (9-12 mi) east of the volcano, and small, low-frequency seismic events continued through the spring.
"Satellite analysis detected a few weak thermal anomalies in the summit crater through the spring and summer of 2000. On occasion, steam plumes extended up to 15 km (9 mi) from the summit. In early May, a significant increase in the number of small, low-frequency earthquakes was recorded, however no attendant change in thermal character or visual observations was noted, and
seismicity declined by the end of summer. AVO mentioned both the seismicity and thermal anomalies in weekly updates for a period of one month, but remained at Level of Concern Color Code GREEN for the remainder of the year."

From McGimsey and others (2014): "Shishaldin Volcano is one of the most active volcanoes of the Aleutian arc. Minor unrest and eruptive activity have been documented for all but a couple of the past 20 years. In 2008, Shishaldin maintained the pattern of producing steam plumes, occasionally with minor phreatic ash emissions (Neal and others, 2011). Thermal anomalies (TA) were observed on September 5 and December 18, 2008. This activity increased in early 2009 with the occurrence of a significant 2-pixel thermal anomaly and a slight increase in seismicity on January 5. AVO issued a VAN the following day upgrading the Aviation Color Code/Volcano Alert Level to YELLOW/ADVISORY. That day, pilots and ground observers reported a constant steam plume rising about 1,000 ft (about 300 m) above the summit and trailing of 16-25 km (10-15 mi) to the southeast. Satellite views on January 8 showed a steam-filled crater and no ash on the snow-covered flanks, and a pilot's photograph taken January 11 shows a pulsing steam plume. A few days later, on January 13, AVO seismologists identified low-amplitude, minor tremor in the seismicity at Shishaldin, which continued for several weeks and then apparently faded out.
"On January 17, the on-island Shishaldin Web camera, which had been out of service since October 22, 2008, was made operational again, although images were not recorded until February 20. Over the next couple of weeks, thermal anomalies were few, seismicity remained low, and although the reports of steaming continued, steam emissions from Shishaldin are considered normal. The return to background conditions prompted AVO to downgrade the Color Code/Alert Level to GREEN/NORMAL on February 11, 2009. A February 28 Web camera image shows a modest steam plume rising from the summit. Image quality problems with the Web camera in March and April led to the camera being taken off-line on April 30.
"Over the next 7 weeks, occasional thermal anomalies were observed along with continuous low-level tremor, which was not considered unusual activity for Shishaldin. Then, on April 7, a PenAir pilot reported Shishaldin streaming more vigorously than in the previous 16 months of observing during his weekly flights by the volcano. A thermal anomaly was reported in satellite imagery that day as well. Activity continued during the next couple of weeks, and on April 20, thermal activity at the summit spiked with the recording of multiple thermal anomalies having saturated pixels, indicative of high ground temperatures (more than 300C; about 600F). This level of thermal activity was last observed at Shishaldin during the run-up to the 1999 eruption (J. Dehn, AVO UAFGI, 2009 internal log entry 31659). A pilot reported steaming on May 5, and an observer on a different flight that day reported also seeing dark colored, linear features on the northern side of the summit. These would later be interpreted to be minor streams of 'dirty' water; no significant deposits were produced.
"Throughout June, thermal anomalies were detected on about one-third of the days, with a particular strong anomaly noted on June 9; no unusual seismicity was detected. A clustering of thermal anomalies appeared to coincide with favorable vertical view angles. On the night of June 25, an ASTER thermal infrared satellite image showed a thermal anomaly and a 22-km-long (14 mi) steam plume extending east-northeast from Shishaldin. Clouds blanketed most of Unimak Island, but the top of Shishaldin was visible above the cloud deck. An observer in Cold Bay, Alaska, called on June 29 to report increased steaming at Shishaldin during the previous couple of days. During the first full week of July, thermal anomalies increased in strength, with a return of saturated pixels (high ground temperatures). Based on this trend, as well as the persistence of the thermal anomaly, AVO elevated the Color Code/Alert Level to
YELLOW/ADVISORY on July 10, 2009. Neither seismicity nor deformation had changed appreciably, and satellite data showed no significant sulfur dioxide gas emissions.
"Airborne emissions were detected in the daily analysis of satellite imagery on July 13, which was a day of rare, cloud free conditions, and a pilot also reported a steam plume rising 2,000 ft (600 m) above Shishaldin and moving to the northwest. Then, on July 15, the Ozone Monitoring Instrument (OMI) satellite imagery appeared to show a small plume-like cloud rich in SO2 originating at Shishaldin; a PUFF simulation using current winds supported the emission source as being Shishaldin.
"During the remainder of July and the first half of August, weather permitting, views of Shishaldin showed steaming from the summit. Thermal anomalies were observed in satellite images, particularly in August. A Plate Boundary Observatory (PBO) crew working on Unimak Island replaced the AVO Web camera for Shishaldin, and it began recording images on August 10; the Web camera would go off-line again on October 11, 2009.
"Following the report of a thermal anomaly on August 16, no more anomalies were detected through the remainder of 2009 except for a weak thermal anomaly on November 2. In mid-September, seemingly anomalous air waves were detected on pressure-sensors located on Shishaldin (station SSLN_BDF), which could be indicative of minor explosions. Retrospective analysis of pressure-sensor data for the previous 2 months revealed that these air waves are common phenomenon and correlated to episodic gas bursts, as documented in 2003-04 (Petersen and McNutt, 2007).
"The persistent absence of thermal anomalies, decrease in steam emissions, and seismicity considered to be within background levels, prompted AVO to downgrade the Aviation Color Code/Volcano Alert Level for Shishaldin to GREEN/NORMAL on October 19, 2009. The volcano remained quiet for the remainder of 2009."

From Cameron and others, 2017: "In early 2014, Shishaldin Volcano began a low-level eruption that produced intermittent lava within the crater, low-level steam plumes, and occasional dustings of ash and ballistics on its upper flanks. This activity persisted from January 30 through the remainder of 2014. During the eruption, AVO monitored Shishaldin using the seismic array on Unimak Island, infrasound arrays at Akutan (135 km [84 mi] to the southwest) and Dillingham (582 km [362 mi] to the northeast), satellite imagery, a Web camera on adjacent Isanotski volcano looking northwest to Shishaldin, mariner observations, and pilot reports.
"Web and satellite imagery of Shishaldin Volcano showed persistent, low-level steam plumes beginning on January 17, although such activity is not unusual at Shishaldin. On January 30, satellite data showed increased surface temperatures in the summit crater, and AVO upgraded theAviation Color Code and Volcanic Alert Level to YELLOW/ADVISORY. Activity increased in early February, manifested by intermittent seismic tremor and airwave explosion signals recorded on distant infrasound instruments. On February 7, satellite data showed an ice-rich cloud at altitudes as high as 7,600 m (25,000 ft) ASL coming from Shishaldin, and AVO increased the daily watch schedule. Although this activity may have indicated a low-level eruption present in the summit crater, the eruption began no later than March 25, when satellite data indicated temperatures in the summit crater consistent with lava extrusion, and seismic and infrasound data recorded small explosions. On March 28, citing the inferred presence of lava in the summit crater, AVO upgraded the Aviation Color Code and Volcano Alert Level to ORANGE/WATCH. On March 30, 2014, a passing mariner photographed a darkened area at the crater rim, likely from a minor ash emission.
"Throughout April, seismic and infrasound stations continued to record intermittent explosion signals, and steaming often was visible in clear Web camera views. On April 26, AVO received a PIREP of a steam plume at Shishaldin at an altitude as high as 3,800 m (12,500 ft) ASL. Probable ash darkened the upper flanks of the summit crater in Web camera images taken on May 5. On May 13, AVO again observed elevated surface temperatures at the crater consistent with lava extrusion in the crater. One month later on June 14, a NASA Earth Observing-1 Advanced Land Imager (EO-1 ALI) image showed a dusting of ash on the snow around the summit crater. A mariner report on June 28 also described a slight dusting of ash on the snow, and satellite data from July 1, 17, and 27 showed fresh deposits of ash on the flanks of the edifice, suggesting persistent, low-level eruptive activity within the summit crater, consistent with heightened seismicity and elevated surface temperatures.
"AVO personnel conducting fieldwork photographed incandescence within the summit crater on August 10, 2014. Activity at Shishaldin remained remarkably consistent throughout August and September-elevated surface temperatures observed in satellite imagery, evidence of explosions from infrasound and seismic data, intermittent tremor, and occasional steam plumes viewed by pilots or in the Web camera.
"On October 1, satellite data again indicated temperatures within the summit crater consistent with extrusion of new lava. However, these elevated temperatures decreased by mid-October. Temperatures increased again on October 25, consistent with the reappearance of lava within the summit crater. This was accompanied by an increase in seismic tremor and explosions detected by infrasound. In response, AVO issued a Volcanic Activity Notice (VAN) on October 28 but the Aviation Color Code and Volcanic Alert Level remained at ORANGE/WATCH. After an apparent lull in eruptive activity between November 3 and November 20, the number and size of earthquake events at Shishaldin increased sharply, and AVO increased the watch schedule. At 10:00 UTC (1 a.m.
AKST) on November 24, seismic activity again increased sharply, prompting AVO to issue another VAN at 10:49 UTC (1:49a.m. AKST) warning of a greater risk of ash emission outside the crater. For the next 2 days, surface temperatures increased and there was an increase in the number of seismic events and explosions detected by infrasound. A robust steam plume was observed in satellite imagery, but no significant ash deposition occurred outside the crater. This slightly elevated activity lasted 2 days, after which the usual low-level eruptive activity continued at Shishaldin into 2015."
In late January 2015, strongly elevated temperatures were observed in satellite images, consistent with active lava within the crater. A wispy, low-leve ash emission was observed in webcam images on February 2, 2015.
Throughout February and March, 2015, clear satellite views often show elevated surface temperatures at the crater, seismicity remained above background, and low-level steam emissions were frequently seen in webcam images. It is likely that low-level eruptive activity continued within the summit crater. On April 16, 2015, several pilot reports of an ash plume at Shishaldin resulted in the issuance of an Ash Advisory SIGMET. No ash was visible in satellite and web camera images. Throughout April, and until at least the time of this writing (May 15, 2015), Shishaldin continued to show evidence of low-level eruptive activity, including elevated seismicity, elevated surface temperatures within the summit crater, and a visible steam plume. On May 8, observers from a cruise ship in the area reported a steam/ash plume from Shishaldin, heading east. On the morning of May 15, a robust steam plume was detected in web camera views, possibly containing small amounts of ash.
Low-level eruption continued at Shishaldin throughout May, 2015. On June 18, 2015, pilot reports and satellite imagery indicated a weak ash plume rising a few hundred feet from the summit crater; weak ash emissions continued on June 19, 2015. Elevated seismicity, with intermittently observed elevated surface temperatures continued at Shishaldin throughout June, July, August, September, and October.
On November 20, 2015, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW/ADVISORY, because no elevated surface temperatures had been observed since October 19, 2015. Slightly elevated seismicity continued at Shishaldin, and robust steam plumes were often observed.
On March 10, 2016, AVO lowered the Aviation Color Code and Volcano Alert Level to GREEN/NORMAL, stating "There has been an steady decrease in detected thermal activity at Shishaldin over the past several months. No anomalous activity has been observed in several clear satellite images of Shishaldin since moderately elevated surface temperatures were detected on January 13, 2016. Airwaves associated with low-level explosive degassing have not been detected in infrasound data since Feb. 7. Low-amplitude seismic tremor consistent with an open, degassing system system continues to be seen in seismic data and is considered to be within the bounds of background activity for Shishaldin."

From Dixon and others, 2020: "Shishaldin Volcano did not erupt in 2017, but volcanic unrest in December resulted in an elevated Aviation Color Code and Volcano Alert Level of YELLOW and ADVISORY, respectively, for much of the month. On April 20, AVO received a pilot report of an ash rich plume. Web-camera images confirmed the plume was composed of gas with no evidence of ash. Gas plumes are common at Shishaldin Volcano, and occasional reports of gas plumes and increased low-frequency seismic events were reported in AVO’s internal logs for most of 2017.
"In the last week of October, AVO noted elevated surface temperatures at the summit of Shishaldin Volcano. Throughout November, AVO observed increased low-frequency seismicity likely to be associated with small explosions. At the end of November, these explosions were being recorded on the infrasound array at Sand Point, Alaska, 230 km (140 mi) east of the volcano. In response to multiple signs of unrest, AVO elevated the Aviation Color Code to YELLOW and Volcano Alert Level to ADVISORY on December 6. Observations of degassing and energetic small explosions continued through December."
From Cameron and others, 2023: "The overall activity at Shishaldin Volcano declined throughout January 2018, and on February 7, 2018, this continued decline triggered AVO to lower the Aviation Color Code and Volcano Alert Level of the volcano to GREEN and NORMAL. No other notable unrest took place at the volcano for the remainder of 2018."

From Orr and others, 2023: "Shishaldin Volcano erupted from July 2019 through the end of the year, with Strombolian explosions, lava flows and lahars on the volcano’s flanks, and sporadic ash clouds. The eruption was the most significant at Shishaldin Volcano since 1999, when an eruption produced Strombolian explosions, lahars, and a subplinian ash cloud that reached 45,000 ft (13,700 m) ASL (Nye and others, 2002; Stelling and others, 2002; McGimsey and others, 2004). Prior to 2019, the most recent eruption to send lava flows down the volcano’s flanks took place in 1955 (Anchorage Daily News, 1955). A questionable news report from 1976 (Andersen, 1976) described lava flows at Shishaldin Volcano that should probably be attributed instead to Pavlof Volcano, which was erupting at that time. Although Shishaldin Volcano erupted from September through October 1975, no lava flows were reported. Thus, the 2019 flows were likely the first on the flanks of Shishaldin Volcano in 64 years and represent a departure from the typical style of its historically observed eruptions. Eruptions at Shishaldin Volcano more commonly consist of Strombolian explosions and lava fountaining within the summit crater.
"The initial 2019 eruptive activity of Shishaldin Volcano began in July, continued into September, and featured the growth of a small spatter cone in the summit crater. The lava column then withdrew in mid-September, causing the crater floor to collapse and pausing the eruption for approximately one month. Activity resumed in mid-October with a new, rapidly growing spatter cone within the summit crater, while small lava flows spilled out of the crater and ran ~2 km [1.2 mi] down the volcano’s north flank. These flows melted into the snow and ice, producing small lahars that followed drainages north to the Bering Sea. Several collapse events from the summit spatter cone in November and December left lobate flowage deposits on Shishaldin Volcano’s north flank and produced small ash plumes that drifted downwind. Finally, a collapse event on December 12 produced a larger ash plume, which reached an altitude as high as 23,000 ft (7,000 m) ASL, generated three detected lightning strokes, and deposited ash on the southeast flank of the volcano.
"The following paragraphs describe each phase of the 2019 eruption in greater detail…
"Eruption Buildup (July 1-July 23)
"Satellite imagery indicated elevated surface temperatures at Shishaldin Volcano starting July 1, and the brightness temperatures continued increasing for the next two weeks. Tremor and LP earthquakes were also detected during the same period and may have started occurring as early as mid-June. On July 10, field crews noted that the summit plume was unusually vigorous, although no sulfur dioxide (SO2) was detected in satellite data that day.
"On July 12, an overflight by a crew associated with the Plate Boundary Observatory recorded visible incandescence within the summit crater (K. Austin, University NAVSTAR Consortium [UNAVCO], written commun., 2019). This report, along with increasing surface temperatures detected in satellite data and increased seismic activity, prompted AVO to raise the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on July 13. Elevated surface temperatures and an increasing amplitude of seismic tremor continued from July 12 to 23.
"Cone Eruption (July 24-September 19)
"On July 23, AVO field crews photographed several new volcanic features at the summit of Shishaldin Volcano: a small cone within the summit crater, active lava flowing around the base of this cone, and minor tephra deposits on the inside walls of the crater. The confirmation of active lava at the surface triggered AVO to raise the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH on July 24. Clear, high-resolution satellite images documented the spatter cone as it continued to grow and showed signs of activity through mid-September. These images also showed occasional light ash deposits on the upper flanks of the volcano, but no lava or significant amounts of ash appeared outside the summit crater.
"Bursts of seismic tremor, thought to be caused by Strombolian explosions, were first detected on July 25 and occurred intermittently through August. This eruptive style was confirmed on August 16 by a passing observation plane operated by the National Oceanic and Atmospheric Administration Alaska Fisheries Science Center, which recorded visible and infrared video of the volcano. Seismic tremor, recorded as real-time seismic amplitude measurements (RSAM), also steadily increased through August, peaked around September 6, and then decreased markedly after September 14. Minor SO2 emissions were detected on August 27-28 and September 2 in sensitive ultraviolet (UV) satellite images (from the TROPOspheric Monitoring Instrument [TROPOMI] on the Copernicus Sentinel-5 Precursor satellite), but not by less-sensitive infrared (IR) satellite sensors (Infrared Atmospheric Sounding Interferometer [IASI] instruments onboard the Meteorological Operational satellite series). The last visual confirmation of eruptive activity at the summit during this period was a Landsat 8 satellite image taken on September 9.
"Clear, high-resolution satellite images showed that the spatter cone continued growing with signs of activity through mid-September, although it remained confined within the summit crater. Besides the occasional dusting of light ash on the upper flanks of the volcano, no lava or significant amounts of ash were deposited outside the crater.
"Pause (September 19-October 13)
"On September 19, the spatter cone, which had grown since July, collapsed into the crater. The event was recognized during a retrospective analysis of borehole tiltmeter data from stations installed on the flanks of Shishaldin Volcano by the UNAVCO Plate Boundary Observatory. This collapse was the largest-amplitude tilt signal recorded during the eruption and is interpreted to reflect the drainage of magma from the conduit.
Although cloudy conditions blocked satellite views at the time of the collapse event, clear satellite images taken on September 23 showed reduced mid-IR signatures, indicating lower surface temperatures and a lack of significant eruptive activity. More satellite images taken on September 26 confirmed the crater floor had collapsed and that no evidence of ongoing eruptive activity remained. As a result, on September 26, AVO downgraded the Aviation Color Code and Volcano Alert Level to YELLOW and ADIVSORY. The lack of eruptive activity and the collapse of the cone were again confirmed in a clear, high-resolution satellite image taken on October 3.
"Renewed Eruption; North Flank Lava Flows and Lahars; Cone Collapses (October 13-End of Year)
"On October 13, satellite imagery showed an increase in surface temperatures at Shishaldin Volcano, signaling renewed eruptive activity. More satellite observations from October 17 confirmed the growth of a new spatter cone within the summit crater. In response, AVO changed the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH later that day. Activity at the volcano escalated rapidly, as indicated by the detection of Strombolian explosions in infrasound data, observations of incandescence in webcam images, the detection of SO2 emissions in satellite data, and an increase in seismic tremor. Infrasound signals were first recorded on October 18 and took place at 15-30-second intervals by October 21. The first observation of incandescence at Shishaldin Volcano during this period was made from a webcam on the southwest flank of Isanotski Volcano during the night of October 19-20. On October 21, satellite TROPOMI sensors detected SO2 emissions, and AVO recorded a spike of seismic tremor at the volcano. This first tremor spike, as well as subsequent ones, was characterized by RSAM values that increased slowly over several days and sharply decreased over several hours, resulting in a 'shark fin' pattern.
"On October 24, during another seismic tremor peak, a clear satellite image captured an active, 800-meter-long lava flow traveling down the northwest flank of Shishaldin Volcano. The flow melted snow and ice, generating a lahar that had traveled ~3 km [~2 mi] down drainages to the north. In addition, minor ash deposits were seen on snow 8 km [5 mi] southeast of the summit in the image. The same day, an anonymous pilot of a passing airplane reported to AVO the presence of clouds over the volcano that looked like 'smoke rings.' The regional infrasound array at Sand Point, Alaska, detected clear explosions associated with this activity.
"Cyclic increases in seismic tremor, presumably from Strombolian-type explosions, were accompanied by ash and gas emissions and continued to take place through the end of 2019. Observers on a passing U.S. Fish and Wildlife Service flight and AVO field crews on Unimak Island confirmed this Strombolian-type explosive behavior on November 11 and December 20, respectively. At times of increasing tremor amplitude and when viewing conditions permitted, active lava flows on the north flank of the volcano were seen in satellite and webcam views and by observers in the City of Cold Bay. Infrared satellite sensors also detected an increase in radiative power at the volcano, reflecting the increased effusive activity.
"In partnership with UNAVCO, AVO scientists experimented with recording high-rate tilt data (1 sample per second) using the tiltmeter at station AV36, located on the western margin of Shishaldin Volcano. The instrument detected several episodes of explosive activity at the summit while recording at this sampling rate; during each episode, the data showed an hours-long increase in amplitude culminating in several hours of high-amplitude activity bursts. Ground motions during these events were generally tangential to the edifice. These data show that open-system volcanoes like Shishaldin Volcano generate appreciable ground deformation over timescales and at amplitudes that can be recorded by borehole tiltmeters.
"During the summer and early fall of 2019, only UV satellite instruments, such as TROPOMI, detected SO2 at Shishaldin Volcano as a result of their higher sensitivity than IR sensors. Detections from these instruments stopped in November, however, as the available UV light decreased. In contrast, IR SO2 sensors, such as IASI sensors, although less sensitive, do not lose effectiveness in the winter. IASI sensors began detecting SO2 from Shishaldin Volcano on October 28, and these detections continued in November and December. Considering the lower sensitivity of satellite IR to SO2, the IASI detections indicate that gas emissions were higher at the end of the year than earlier in the eruption.
"After each tremor and emission spike, activity quickly decreased and clear satellite images showed a pause in lava effusion. Synthetic Aperture Radar (SAR) images from the TerraSAR-X and TanDEM-X satellites, provided during the eruption by S. Plank (German Aerospace Center), indicated that the summit spatter cone experienced partial collapses during many of these episodes. Collapse events were specifically noted on November 11, November 23, December 5, and December 12. Lobate flowage deposits appeared downslope from the cone after each event.
"The largest of these collapse events, which took place on December 12 at 16:10 UTC, was detected in seismic and infrasound data, webcam photos, and satellite imagery. Photographs of the volcano after the event showed an ash cloud reaching an altitude of about 25,000 ft (7,600 m) ASL. Three lightning strokes were also detected from this cloud. Unlike other collapse events, the December 12 event was followed by elevated tremor and continued lava effusion, the latter of which was visible in satellite images and in photographs taken from the City of Cold Bay. This event was associated with the largest ashfall of 2019, although only a minor amount of ash was deposited on the southeast flank of Shishaldin Volcano.
"A field crew visited Shishaldin Volcano on December 20, 2019, and although the lava flows were inactive during the visit, the vent itself was producing regular Strombolian explosions. The crews sampled the December 12 ash deposit, later analysis of which determined the tephra to be a mix of lithic, tachylite, and sideromelane grains. The sideromelane grains were basaltic, with glass composed of ~52 weight percent SiO2 and minerology consisting of plagioclase, olivine, and magnetite, although only plagioclase and olivine existed as larger (greater than 0.1 millimeter [0.004 inch]) phenocryst phases. The high proportion of tachylite and lithic grains in the tephra supports a cone-collapse origin for the deposit - the composition indicates a high proportion of the material was mobilized from previously deposited and cooled grains.
"The next active lava effusion periods were noted on December 21 and December 26 (after the December field visit). Cloudy conditions generally obscured activity at Shishaldin Volcano during the last few days of the year, but eruptive activity continued into January 2020.
"Although the 2019 eruption deposited only minor amounts of ash on the flanks of Shishaldin Volcano, the lava flows from the event extended 1-2 km [0.6-1.2 mi] down its north flank. Associated lahar deposits traveled even farther, reaching as far north as the Bering Sea. The lava flows of 2019 were the first historically well-documented ones at Shishaldin Volcano and likely represented the first lava flow activity outside its summit crater in more than 60 years."
From Orr and others, 2024: "Shishaldin Volcano erupted from July 2019 to March 2020. The 2019 activity was documented in Orr and others (2023) and the 2020 activity is documented herein…
"Ash-Rich Paroxysm Sequence (January 1–20)
"Eruptive activity at the start of 2020 followed a similar pattern to that established in November 2019: seismic tremor and lava flow activity generally increased over a period of several hours before abruptly shutting down in days-long pauses. Unlike activity in the prior weeks, however, three periods of escalating activity in January culminated in increased ash emissions, prompting AVO to issue a Volcanic Activity Notice (VAN) each time.
"Shishaldin Volcano began 2020 with an Aviation Color Code and Volcano Alert Level of ORANGE and WATCH. Elevated surface temperatures visible in satellite data on January 2 suggested that weak eruptive activity, confined to the vent, was occurring. Seismicity began to increase on January 3, indicating increasing eruptive activity. This was confirmed by a passing pilot, who reported a clear view of lava fountaining and a robust, steam-rich plume that probably contained some ash from the fountaining. At the same time (starting around 19:00 UTC [10:00 AKST]), satellite views and additional PIREPs recorded ash-poor plumes from the volcano that may have reached as high as ~24,000 ft (~7,300 m) ASL. The fountaining at Shishaldin Volcano was associated with increasing seismic tremor and the emplacement of lava flows mostly concentrated on the volcano’s northwest flank. AVO issued a VAN at 20:38 UTC (11:38 AKST) but did not change the Aviation Color Code or Volcano Alert Level.
"Seismic tremor decreased sharply at 20:48 UTC (11:48 AKST), and at about the same time, a PIREP indicated that the plume height had risen to ~27,000 ft (~8,200 m) ASL. Volcanic lightning was detected at 21:07 UTC (12:09 AKST), suggesting that the concentration of ash in the plume had increased. These ash emissions did not last long; the concentration was decreasing by 21:30 UTC (12:30 AKST). A WorldView-2 satellite image acquired at 22:22 UTC (13:22 AKST) revealed the state of the volcano: the lava flows active earlier in the day had stalled and were cooling, new lobate pyroclastic flow deposits had been emplaced on the west and south flanks, and an ash-rich plume was drifting southeastward. The new pyroclastic flow deposits were the first to affect the south flank of the volcano during this eruption; prior deposits were restricted to the north flank. Deposition on the west and south flanks of the volcano required overtopping the topographic high point of the summit crater, suggesting they were deposits from the collapse of an ash column as opposed to debris from a tephra cone collapse. Eruptive activity associated with the January 3 event was not observed directly afterward. Lava flow activity at Shishaldin Volcano increased again after January 3, and by January 6, incandescent flows were visible from the City of Cold Bay. The frequent detection of infrasound signals suggested the occurrence of Strombolian explosive activity. Seismic tremor, already elevated, began increasing further on the morning of January 7, and starting around 16:00 UTC (7:00 AKST) that morning, a plume reaching an altitude of ~20,000 ft (~6,100 m) ASL was detected in satellite data. Strong mid-infrared satellite signatures accompanied the plume, suggesting ongoing lava effusion and vigorous lava fountaining, similar to the behavior seen on January 3. A VAN noting this increased activity was issued at 18:39 UTC (9:39 AKST), although the Aviation Color Code and Volcano Alert Level were not changed. At ~20:00 UTC (~11:00 AKST), the seismic tremor started to decline, followed by the detection of volcanic lighting at 20:25 UTC (11:25 AKST). Satellite images acquired shortly afterward indicated that the plume had become more ash-rich and now reached an altitude as high as ~27,000 ft (~8,200 m) ASL, although tremor remained low. These observations prompted AVO to increase the Aviation Color Code and Volcano Alert Level to RED and WARNING at 21:33 UTC (12:33 AKST). Light ashfall from this event was reported in the City of Cold Bay.
"The plume appeared to have detached from the vent by 22:00 UTC (13:00 AKST), indicating that ash emission had slowed or stopped. The Aviation Color Code and Volcano Alert Level were subsequently lowered to ORANGE and WATCH on January 8 at 04:17 UTC (January 7 at 19:17 AKST). Synthetic aperture radar images acquired later that day showed that the crater had deepened, and the cone had subsided or collapsed. Like the January 3 event, the January 7 paroxysm was followed by a period of quiescence. The only activity detections at Shishaldin Volcano over the next week were infrasound signals consistent with Strombolian activity on January 10. On January 14, a clear WorldView-2 satellite image showed no volcanic activity within the summit crater or on the flanks.
"Eruptive activity increased again on January 18; lava flows were visible on the volcano flanks and seismic tremor intensified. At 17:18 UTC (08:18 AKST), a pilot reported visible lava but no ash emissions. Observers in the Cities of Cold Bay and King Cove, Alaska, documented the incandescent lava flow during clear weather that evening. Overnight webcam images from the south flank of Isanotski Volcano also showed lava fountaining, and by January 19 at 9:30 UTC (00:30 AKST), satellite images showed an ash-poor gas plume rising as high as ~18,000 ft (~5,500 m) ASL. The level of activity continued to increase, and as a result, the Aviation Color Code and Volcano Alert Level were increased to RED and WARNING at 17:28 UTC (08:28 AKST). By this point, a continuous, 150-kilometer-long plume was visible in satellite images. PIREPs at 18:15 UTC (09:15 AKST) described ongoing lava flow activity and measured that the plume had reached an altitude of ~25,000 ft (~7,600 m) ASL. Over the following hours, seismic tremor continued to increase and trace ashfall was reported in the City of False Pass, Alaska, 38 km northeast of Shishaldin Volcano. A WorldView-2 image captured activity at the vent during this period. Another PIREP at 21:42 UTC (12:42 AKST) indicated that the plume had climbed to ~30,000 ft (~9,100 m) ASL.
"The seismic tremor dropped precipitously just after 00:00 UTC on January 20 (January 19 at 15:00 AKST). Satellite data acquired about an hour later showed that the plume had transitioned to a more ash-rich composition, a change confirmed by PIREPs. Ash emissions continued for the next several hours. Then, shortly before 05:00 UTC (20:00 AKST), ash emissions stopped and the plume detached from the vent. With the cessation of eruptive activity at the vent, AVO lowered the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH at 09:37 UTC (00:37 AKST).
"Prolonged Eruption Hiatus (January 20–March 11)
"Satellite data after the January 20 paroxysm showed new ash deposition—the ash-poor lava fountaining phase produced trace deposits extending northeastward toward False Pass, whereas the later ash-rich phase produced more substantial deposits extending southeastward. Satellite radar imagery also showed that the volcano crater was larger and deeper after the event. The last detected infrasound and seismic event of note during this period was on January 24, and a WorldView-3 satellite image from January 25 showed no visible eruptive activity. Some discreet seismic events and infrasound signals were detected occasionally later on, but these did not build to a clear eruptive signal like that which followed the eruption events earlier in January. On February 7 at 01:20 UTC (February 6 at 16:20 AKST), the Aviation Color Code and Volcano Alert Level were lowered to YELLOW and ADVISORY. No other significant activity was detected at the volcano in February, and clear satellite images showed quiet conditions consisting of minor steaming at the vent and cooling lava flow deposits on the flanks.
"Final Renewed Eruption (March 11–31)
"After weeks of quiescence, eruptive activity resumed in March 2020, although it was contained within the summit crater. The first indication of renewed activity appeared in a WorldView-2 satellite image from March 11 that showed a small area of recent ash deposits near the summit crater. The inside of the crater was mostly obscured by steam in the image, but it was generally similar to its appearance in other high-resolution satellite images from February. In the following days, mid-infrared satellite images began to show increased surface temperatures at the volcano. A WorldView-3 image from March 14 showed a saturated short-wave infrared signature at the summit, indicating that lava was erupting again within the summit crater. In response, AVO increased the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH on March 15 at 6:31 UTC (22:31 AKDT on March 14).
"During the following few weeks, seismicity was elevated and small explosions (probably from Strombolian activity) were detected occasionally in infrasound data. Satellite radar images suggested renewed cone growth, although all eruptive activity was confined within the summit crater. Another WorldView-3 image from March 22 showed activity similar to that on March 14.
"Seismicity declined thereafter through the end of the month, and an April 1 satellite image showed only a steam plume and no evidence of a heat source, suggesting the eruption had ended. On April 2, a clear satellite image with an unobscured view into the summit crater confirmed that no eruptive activity was occurring. Due to an absence of activity, AVO lowered the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY on April 16 at 19:44 UTC (11:44 AKDT).
"Aftermath (April 1–End of Year)
"Low-level unrest continued at Shishaldin Volcano for months after its 2019–2020 eruption. Elevated surface temperatures continued appearing in satellite images, and frequent satellite detections of SO2 were made in late April and May. These SO2 detections also coincided with the increasing ultraviolet radiation of long summer days, which raises the sensitivity of the TROPOspheric Monitoring Instrument (TROPOMI), a satellite instrument used for these detections. Other remote sensing observations made during this period of low-level unrest indicated that magma was still stored shallowly within the conduit, enabling magma degassing, high temperatures, and minor collapse events within the summit crater.
"On June 24 at 20:00 UTC (12:00 AKDT), the Aviation Color Code and Volcano Alert Level were lowered to GREEN and NORMAL, reflecting an overall decrease of activity to background levels at the volcano, although AVO continued recording evidence of additional minor collapse events using satellite radar images. Some of these events appeared to produce trace ash deposits on the upper flanks, as seen on April 29 and May 7. AVO workers carrying out annual geophysics station maintenance in August and September did not observe any eruptive activity, but helicopter gas surveys at the same time detected continued SO2 degassing at an emission rate of 100±30 metric tons per day. The surveys also measured high carbon dioxide concentrations relative to measurements from 2015 and 2019, which indicated a new deep magma input into the system.
"Final Deposits and Samples
"The 2019–2020 eruption of Shishaldin Volcano resulted in (1) many lava flows on the north flank of the volcano, (2) pyroclastic flow deposits related to cone collapse events in December and three eruption paroxysms in January, and (3) lahars that inundated drainages north of the volcano and reached the Bering Sea coast. Accurate mapping of the lava flows was difficult because of poor orthorectification in many of the available high-resolution satellite images, in turn caused by the typically oblique image viewing angles and the steep flanks of Shishaldin Volcano.
"Satellite images taken on January 14 and January 25 provided close-to-nadir images (11º and 17º off nadir, respectively) that allowed the final deposits to be mapped with reasonable accuracy. The lava flows covered less than 0.9 square kilometers (km2) and extended as far as 3 km from the vent, primarily occupying three distinct drainages, with each new flow burying the previous one. The pyroclastic flow and lahar deposits were gradational and, in many places, difficult to distinguish from each other in satellite images. Deposits on the volcano’s south flank, especially those from the January 3 paroxysm, were likely all pyroclastic flows generated from ash column collapse that overtopped the high point on the crater rim. The lahars and pyroclastic flows to the north were intermixed, forming lobate deposits on the flatter plains north of the volcano. The deposits suspected to be pyroclastic flows generally extended no more than 3 km from the summit vent, whereas lahars followed drainages and traveled farther, some reaching all the way to the coast (more than 30 km north of the summit). Because of the difficulty in distinguishing between pyroclastic and lahar deposits using satellite images alone, they are mapped as a single unit in figure 14 [in original].
"Only a few samples are available from the 2019–2020 eruption. Samples of a tephra deposit from a cone collapse event, collected by field crews on December 20, 2019, are described in Orr and others (2023). AVO field crews also sampled the toe of a lava flow on the northeast flank of the volcano in 2022; this sample is currently being studied. Community members in the City of Cold Bay collected ash from the paroxysm of January 7, 2020; others in False Pass collected ash from the early stage of the January 19 paroxysm. Both of these samples were dominated by roughly equal parts (1) juvenile, highly fluidal and glassy sideromelane grains and (2) partially devitrified tachylite grains (classifications by Loewen and others, 2021). However, the samples also contained phenocrysts of plagioclase and olivine, along with microlites of plagioclase, olivine, and magnetite. The glass composition was basaltic and was similar to samples from the 1999 eruption (Stelling and others, 2002)."

On July 11, 2023, AVO raised the Aviation Color Code to YELLOW and the Alert Level to ADVISORY for Shishaldin Volcano, citing the following information:
Strongly elevated surface temperatures at the summit of Shishaldin Volcano have been observed in satellite data over the past day. Intermittent tremor and low-frequency earthquakes over the past week have gradually become more regular and consistent in the past day. In response, the Alaska Volcano Observatory is raising the Aviation Color Code to YELLOW and the Alert Level to ADVISORY. These observations represent a departure from normal background activity at Shishaldin, but do not necessarily indicate that an eruption will occur.
On the following day, July 12, AVO raised the Aviation Color Code to ORANGE and the Alert Level to WATCH for Shishaldin, citing the following information:
Strongly elevated surface temperatures continue to be observed at the summit of Shishaldin Volcano in satellite data. Incandescence at the summit was observed in web camera images from last night and sulfur dioxide was detected in satellite data over the past day. In addition, seismic tremor amplitudes have increased over the past day. Together, these observations suggest that lava is likely present within the summit crater of Shishaldin.
On Friday, July 14, two explosions at 1:09 and 7:10 am AKDT, producing ash clouds to 25,00-40,000 feet (9-12 km) asl. Plumes drifted SE across the Pacific for at least 250 mi. Volcanic mudflows were generated on volcano’s flanks, the largest on the north side. Gas detections suggest elevated SO2 in the ash plume. Several smaller explosions with ash clouds below 15,000 ft occurred around 10:30 am. The Color Code and Volcano Alert Level remained ORANGE/WATCH.
On the evening of Saturday July 15, the eruption intensified, and AVO raised the Aviation Color Code and Volcano Alert Level to RED/WARNING at 11:57 pm AKDT, stating "The explosive eruption of Shishaldin Volcano is continuing. A continuous ash plume now extends over 80 mi (125 km) to the SSE from the volcano with an altitude of about 16,000 ft (4.9 km) above sea level. Seismicity has remained elevated for over 6 hours and frequent explosion signals are being detected at regional infrasound (pressure sensor) networks. Some explosions are sending ash plumes as high as 20,000 ft (6 km) above sea level."
Overnight, the eruptive activity declined and AVO lowered the Color Code and Volcano Alert level to ORANGE/WARNING. The drifting ash cloud was still visible in satellite imagery about 350 nautical miles ESE from the volcano. The NWS estimated the cloud as under 16,000 ft asl.
Lava continued to erupt from the summit crater on July 16 and 17.
Tuesday July 18 saw a return to explosive activity, with a rapidly growing ash cloud observed starting at 7 am local time and drifting SSE. AVO raised the Aviation Color Code and Volcano Alert Level to RED/WARNING in response for approximately three and a half hours.
On Wednesday, July 19, seismic stations and webcams south of the volcano were brought back online, increasing the monitoring capabilities at Shishaldin.
Another episode of eruptive activity occurred over the following weekend, starting late Saturday, July 22. The Aviation Color Code and Volcano Alert Levels were raised to RED/WARNING at 11:43 PM after an ash plume was observed in satellite data. The plume reached altitudes of 37,000 ft asl and drifted to the northeast. Alert levels were returned to ORANGE/WATCH four and a half hours later.
In the early morning hours of Wednesday, July 26, a sustained period of increased eruptive activity began. A more low-level ash plume reached 15,000 ft asl by 3:51 AM and continued for many hours, along with significant explosions. The Aviation Color Code and Volcano Alert Levels remained at ORANGE/WATCH during this event.
On the Thursday of the week following, August 3, volcanic tremor increased into the evening and surface temperatures were strongly elevated. Explosions were detected overnight in infrasound and seismic data. An ash plume reaching up to 31,000 ft erupted starting in the early morning hours of August 4 and continued into the afternoon, declining about 2 PM local time. The Aviation Color Code and Volcano Alert Levels were raised to RED/WARNING at 10:17 AM local time August 4 and were returned to ORANGE/WATCH at 7:55 PM that night.
On Monday, August 15, Shishaldin erupted an ash plume beginning around 2 AM local time that reached about 36,000 ft and generated volcanic lightning. Ash continued to erupt to lower altitudes (~16,000 feet) into August 16. The Aviation Color Code and Volcano Alert Levels remained at ORANGE/WATCH during this event.
On Friday, August 25, seismic unrest ramped up in the early morning hours and the first pilot reports of ash were received at 11:22 AM. The Aviation Color Code and Volcano Alert Levels were raised to RED/WARNING at 12:17 PM, and the ash plume eventually reached up to 32,000 ft and extended up to 280 miles. Ash emissions continued into the night and the Aviation Color Code and Volcano Alert Levels were returned to ORANGE/WATCH at 12:28 AM on August 26.
On Tuesday, September 5, seismicity once again increased in the early morning hours, and an ash plume to 32,000 ft was erupted starting about 8:30 AM. Aviation Color Code and Volcano Alert Levels were raised to RED/WARNING at 8:42 AM local time. The ash altitude and emissions decreased about 11 AM, but still continued at lower altitudes until about 1:30 PM. The Aviation Color Code and Volcano Alert Levels were returned to ORANGE/WATCH at 12:46 PM local time.
On Thursday, September 14, seismicity began ramping up about 6 pm local time, but decreased again without major eruptive activity. However, seismicity increased again on Friday, September 15, and an explosive eruption began about 5:10 PM local time. The Aviation Color Code and Volcano Alert Levels were raised to RED/WARNING at 5:41 PM. The ash plume reached 42,000 ft and abundant volcanic lightning was observed. The ash plume detached from the volcano about 6:30 PM, but more ash erupted starting at 7:30 PM. Trace ash fall was reported in False Pass during this event. The Aviation Color Code and Volcano Alert Levels wee returned to ORANGE/WATCH at 12:44 AM Saturday, September 16, after seismicity returned to pre-event levels.
On Friday, September 22, seismicity began increasing throughout the day without major explosive activity. Lava erupted within the crater starting on September 24, accompanied by low-level ash emissions and flows down the mountain’s flanks. In the early morning hours of Monday, September 25, an ash plume erupted starting at about 5:45 AM local time, reaching 45,000 ft. The Aviation Color Code and Volcano Alert Levels were raised to RED/WARNING at 6:02 AM. The ash plume detached about 7 AM. Trace ash fall was reported in False Pass, King Cove, Cold Bay, and Sand Point. The Aviation Color Code and Volcano Alert Levels were returned to ORANGE/WATCH at 12:34 PM Monday.
After the major event on September 25, seismic activity decreased, but several collapse events occurred in the crater area and steaming pyroclastic flow and lahar deposits were seen on the volcano’s flanks.
On Monday, October 2, seismicity began increasing in the evening starting around 9 PM local time. Lava fountaining and hot avalanches occurred during the night. A significant explosive eruption of Shishaldin started at 5:47 AM AKDT the following morning, October 3. The explosive eruption produced an initial ash cloud as high as 40,000 ft asl, with subsequent ash emissions to between 20,000 and 25,000 ft asl. The eruption was observed in satellite images, webcams, lightning, and seismic and infrasound data. The Aviation Color Code and Volcano Alert Levels were raised to RED/WARNING at 6:12 AM. Ash emissions continued for several hours, dropping down to about 20,000 to 25,000 ft asl. The Aviation Color Code and Volcano Alert Levels were returned to ORANGE/WATCH at 10:36 am October 3 after explosive activity ended, although steaming and small collapse events continued.
After the event on October 3, seismic activity remained elevated for two weeks. Pyroclastic flow and lahar deposits were observed, along with explosion craters at their toe, as was evidence for more collapses of the crater rim. Shishaldin emitted increased levels of sulfur dioxide (SO2) during the weeks of October 9 and October 16. Seismicity and sulfur dioxide emissions began to decline the week of October 23.
On Tuesday, October 31, AVO changed the Aviation Color Code and Volcano Alert Levels to YELLOW/ADVISORY, citing the decrease in seismicity and sulfur dioxide emissions over the past week.
Two days later, on Thursday, November 2, volcanic tremor increased again. The Aviation Color Code and Volcano Alert Levels were raised back to ORANGE/WATCH at 8:31 PM AKDT that night, with AVO stating that an eruption was likely underway. Satellite images showed ash emissions started at about 8:00 PM local time. The emissions lasted until Friday morning, but as the ash plume didn’t exceed 20,000 ft above sea level, the Aviation Color Code and Volcano Alert Levels remained at ORANGE/WATCH during this event.
After the explosive event on November 3, much smaller explosions and collapse events around the summit crater rim occurred frequently for about a week, along with above-background levels of seismicity, elevated surface temperatures, and steam and gas emissions. During the weeks following, the frequency of small explosions detected in infrasound data dropped off, and gas emissions and elevated surface temperatures were less frequently detected. Seismicity remained somewhat elevated, and steam plumes and collapse events were sometimes observed. There was a small cluster of earthquakes starting on December 25 about 4-6 miles southwest of Shishaldin's summit.
On January 2, AVO lowered the alert levels to YELLOW/ADVISORY, stating that activity had declined over the two months prior, and that there had not been any evidence of eruption-driven changes to the volcano outside of the crater or evidence of lava within the crater. However, unrest continued with the small events taking place deep in the vent.
Minor steam emissions and seismicity (including occasional tremor) continued in January and February, but explosion signals dropped off. Steam emissions were observed on and off in webcam views.
On February 11, a minor ash plume was observed in a webcam view, prompting AVO to raise the alert levels to ORANGE/WATCH at 12:46 PM AKST. Analysis of seismic, infrasound, and thermal data indicated no evidence of explosive activity, which led AVO to conclude that this event was most likely caused by a small rockfall or debris flow. Therefore, the alert levels were returned to YELLOW/ADVISORY on February 17. Satellite data on February 17 and 19 showed that additional collapse events had occurred on the upper southeast and northeast flanks, but otherwise activity remained low.
In the latter part of February, Shishaldin was fairly quiet, with barely elevated seismicity and intermittent steaming.
On March 7, an SO2 plume was noted in satellite data. Through the end of April, roughly 1-2 SO2 observations per week were accompanied by some periods of seismic tremor, occasional gas-bubble burst signals noted in infrasound, occasional weakly elevated surface temperatures, and a robust steam/gas plume.
On May 10 through May 16, a few new areas of ash were spotted in webcam images of the snow-covered summit. These were likely caused by minor collapses of the inner crater wall that were observed in radar satellite data. Weak infrasound, occasional tremor, and steam plumes were also observed.
Between May 16 and June 7, volcanic earthquakes and occasional tremor were observed, along with steaming when the weather was clear. SO2 gas was observed on six days during this period.
Between June 7 and July 12, sulfur dioxide gas emissions were observed in satellite data several times each week, along with a fairly persistent steam plume, earthquakes, occasional tremor, and weak infrasound explosion signals when observation conditions were good.
Between July 12 and July 26, sulfur dioxide gas observations were less frequent, though other activity remained approximately the same.
Over the week of August 2, an AVO field crew conducted fieldwork at Shishaldin, including helicopter overflights on July 28 and 30 and August 1. Thermal imaging revealed a warm central vent deep within the summit crater, and gas instruments detected sulfur dioxide within the persistent steam and gas plume.
Over the month of August, the typical steam plume was observed during clear weather, along with small volcanic earthquakes and weak seismic tremor and infrasound signals.
On August 30, 2024, AVO returned the Aviation Color Code and Volcano Alert Level to GREEN and NORMAL. The accompanying statement specified that while seismicity was still slightly elevated and gas emissions and small infrasound signals continued at Shishaldin, these signals had declined over time since the last explosive event in November and were expected to persist for a good while longer in the post-eruptive period. In AVO’s opinion, the volcano no longer posed any immediate threat of eruptive activity, and any resumption of activity would be preceded by detectable signals.