Alaska Volcano Observatory

From Orr and others, 2024: "Mount Veniaminof showed no indication of activity at the start of 2021. In response to a prolonged local seismic network outage that started in December 2020, AVO changed the volcano’s Aviation Color Code and Volcano Alert Level from GREEN and NORMAL to UNASSIGNED on January 15, 2021, reflecting the observatory’s limited ability to detect volcanic activity there.
"On March 1, the TROPOspheric Monitoring Instrument (TROPOMI) onboard the Copernicus Sentinel-5 Precursor satellite identified unrest at Mount Veniaminof in the form of elevated sulfur dioxide (SO2) emissions. The TROPOMI sensor measures variations in the wavelength-dependent absorption of ultraviolet energy due to the presence of gases (such as ozone and SO2) and retrieves the amount of gas present in the total atmosphere to account for the observed absorption. On March 2, satellite imagery again showed SO2. Elevated surface temperatures began to appear in satellite imagery early on March 4, and then a few hours later, at 05:13 AKST (14:13 UTC), regional infrasound instruments detected an explosion. Satellite imagery recorded a corresponding ash plume shortly thereafter. An ashy plume from cone A was seen in webcam imagery after sunrise, confirming that an eruption was underway. AVO responded later that morning by increasing the Aviation Color Code and Volcano Alert Level to ORANGE and WATCH. A retrospective analysis of high-resolution satellite imagery identified ash deposition near cone A and melting of glacial ice ~1 km [0.6 mi] to its east on February 28, but no signs of activity on February 25, suggesting that the eruption began between those two dates. Figure 18 [in reference] shows a timeline of the activity observed at Mount Veniaminof in 2021.
"The eruption reached its climax on March 4-10, producing moderately to strongly elevated surface temperatures, ash emissions, and frequent explosions detected by infrasound. Several explosions on March 6 were heard and even felt in Perryville, ~35 km [22 mi] away. Webcam and satellite data from this period showed ash emissions from cone A and a steam plume from the growing melt pit east of the cone. Between March 6 and March 10, the ash plumes reached as high as ~15,000 ft (~4,600 m) ASL and traveled as far as 220 km [140 mi] from the volcano. Owing to the height and extent of these ash plumes, local flight restrictions were set in place on March 9. Tephra deposits from the ash plumes were mostly confined to the caldera, but some ash fell outside the caldera on March 8, reaching as far as 22 km [14 mi] from the vent. The ash during this interval was blown predominantly southeastward.
"Lava effusion from vents within the glacial melt pit on the east flank of cone A first appeared in satellite imagery on March 7. The sub-circular melt pit continued to widen as the lava spread away from the flank vents and encompassed an area of nearly 700,000 square meters (m^2) [0.2 square miles] by late March.
Less intense activity characterized the second half of March at Mount Veniaminof. Two seismic stations at the volcano came back online on March 12, allowing AVO geophysicists to detect tremor and LP earthquakes once again. Tremor was detected daily, whereas explosions and ash plumes were detected less frequently. Thermal anomalies from lava effusion were weaker in the second half of the month than in the first half. Lava effusion and minor ash emissions likely continued within the caldera throughout much of this time, although observations were limited to those on clear weather days. Ash plume altitudes remained lower than ~10,000 ft (~3,000 m) ASL. Although not reported, webcam imagery suggested that trace amounts of ash fell in Perryville on March 27, making it the only day during the eruption when a community may have been impacted by ash.
"On April 1, the eruption paused and the volcano showed only slightly elevated surface temperatures, which were probably related to cooling lava flows. The Aviation Color Code and Volcano Alert Level were lowered to YELLOW and ADVISORY on April 2 in response to this decreased activity. However, an ash plume on April 5 prompted AVO to raise the Aviation Color Code and Volcano Alert Level back to ORANGE and WATCH. A second ash plume was observed on April 6. No eruptive activity was observed thereafter, so the Aviation Color Code and Volcano Alert Level were lowered to YELLOW and ADVISORY on April 21, then back to UNASSIGNED on May 12. Elevated surface temperatures detected in the weeks after the pause were most likely related to a warm summit cone and cooling lava flows. Altogether, the three lava flows that erupted within the glacial melt pit covered a combined area of ~2.7×10^4 m^2 [2.9X10^5 sq ft] (Waythomas, 2021).
"AVO workers fully restored the Mount Veniaminof local seismic network during a field campaign in late June and early July. The Aviation Color Code and Volcano Alert Level were consequently changed to GREEN and NORMAL on July 8. Tremor, steam plumes, SO2 emissions, and thermal anomalies were detected in geophysical and remote sensing data shortly before and during the field campaign, which ran from June 28 to July 5. However, the field crew itself observed no noteworthy activity. No further unrest took place at Mount Veniaminof for the remainder of 2021.
"Tephra samples were collected from a snow pit ~2 km [~1.2 mi] east of cone A, adjacent to the melt pit, during the field campaign. Four distinct tephra layers were sampled; each consisted of black and red-oxidized grains intermixed with loose plagioclase crystals. Each layer had a modal grain size of 0.25–0.5 millimeter (mm) [0.01-0.02 in] and a maximum grain size of 2 mm [0.08 in]. One layer also had apparent accretionary lapilli. Petrographic analysis of the samples showed that the phenocrysts, like those sampled from the 2018 eruption (Loewen and others 2021), comprised normally zoned plagioclase, olivine, and rare clinopyroxene. The groundmass was a mixture of microlitic sideromelane, tachylyte, and lithic components. The glass composition was similar to that erupted in 2018, although with slightly lower silica concentrations (like the 2013 eruption glass)."