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."
On January 3, 2020, beginning at about 9:30 am AKST, seismicity at the volcano began increasing over a period of several hours and eventually led to a brief period of sustained ash emission resulting in an ash cloud that reached as high as 27,000 feet above sea level according to reports from passing pilots. The ash cloud consisted of a linear, directed ash plume that extended from the volcano to the southeast at least 75 miles. The ash cloud also produced minor amounts of volcanic lightning. Seismicity then abruptly decreased.