Alaska Volcano Observatory

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)."