From Miller and others (1998): "Veniaminov (1840, p. 18) described an eruption in 1795 on the southwest end of Unimak Island, which most likely occurred at Westdahl. Coats (1950) attributed four eruptions in the late eighteenth century and early nineteenth century to Pogromni volcano. Based on recent observations from aircraft, however, Pogromni does not appear to have been active in historical time. The eruptions should probably be assigned to Westdahl."
From Veniaminov (1840, translated by Lydia T. Black and R.H. Geoghegan, 1984): "In regard to volcanic phenomena, Unimak Island has occupied the first place since antiquity, both for its extraordinary size or the height and position of the mountains located on it. About 1795, with the wind from the SW, the range on the SW end of Unimak blew up with a terrible thunder and an eruption of ash [pepel] or soot [sazha], white in color, in such a great quantity that, for several hours in the middle of the day, not only in the neighboring villages on Aliaksa but even on Unga, there was absolute darkness. The eternal ice, lying on that range, slid down along both sides together with a large quantity of water and burned rocks of different sizes. The last stopped about half-way along and formed a trench or a black belt visible even now. There are still signs in places where the water flows and where the ice, which had slid down the mountain, rested for several years (the vegetation has only just begun to appear there). Nowadays one notices that this very range, which had been at rest after the upheaval, in the last few years, in one place, began to grow or bulge out."
Additionally, Plummer (1898) reports that the southwest crater exploded and fell in.
Sapper (1927) assigns this eruption to his category b1, and estimates more than 1 cubic km (10^9 cubic m) of tephra was erupted.

From Miller and others (1998): "Coats (1950) attributed four eruptions in the late eighteenth century and early nineteenth century to Pogromni volcano. Based on recent observations from aircraft, however, Pogromni does not appear to have been active in historical time. The eruptions should probably be assigned to Westdahl."
Sapper (1917) reports that in 1796 ice and water came pouring down to a town called Nosowskoi. Coats (1950) reports a lava flow or plug dome and some explosive activity.
Simkin and Siebert (1994) assign the eruption a VEI of 0, indicating a non-explosive eruption.

From Miller and others (1998): "Coats (1950) attributed four eruptions in the late eighteenth century and early nineteenth century to Pogromni volcano. Based on recent observations from aircraft, however, Pogromni does not appear to have been active in historical time. The eruptions should probably be assigned to Westdahl."
The eruption of 1820 is a little more confusing, as reports are unclear as to whether the event occurred on Unimak Island (containing the volcanoes of Westdahl, Pogromni, Fisher Caldera, Shishaldin, Isanotski, and Roundtop) or Umnak Island (containing the volcanoes Okmok, Recheshnoi, and Vsevidof). Modern compilers have attributed this event to Westdahl.
Soloviev and Go (1972) report: "1820, March 1 or the night of 2-3. A powerful volcanic eruption occurred on the northern tip of Umnak Island. The ashes spread as far as Unalaska and Unimak Islands. The eruption was accompanied by a strong earthquake. 'At dawn, it was observed that the sea had become more agitated' (Perrey, 1865 [in French])." Lander (1996) attributes this eruption to Pogromni, which is on southwest Unimak Island. Okmok Volcano is on the northeast of Umnak Island.
Lander (1996) writes "1820, March 1-2. Due to a strong eruption of Pogromni Volcano on the northern tip of Umnak Island [typographical error? Pogromni and Westdahl are on the southwestern side of Unimak Island], ashes injected into the air were observed as far away as Unalaska and Unimak [Another error? Westdahl and Pogromni are on Unimak] Islands. There was a strong earthquake on the night of March 1. The sea was observed to have became highly disturbed by dawn (Soloviev and Go, 1975: in Russian). Mushketov and Orlov (1893, p. 207-208: in Russian) describe the eruption has having taken place on February 19-20 (March 1-2 in Gregorian date) and another account dated March 1 referring to ash fall clouding the sea. Although the highly disturbed sea is suggestive of a tsunami, there is no specific mention of waves."
Mushketov and Orlov (1893, translated in 1994 by Katherine Arndt) wrote: "In 1820, on 19 February (Old Style) (in the night on the 20th), in a severe SE wind, on certain islands of the Aleutian Archipelago there were felt strong tremblings of the earth with a subterranean rumble. Immediately after, the air, it seemed, caught fire and clouds of ash and sand began to fall during the whole night. With the approach of day the wind changed, the volcanic matter stopped falling, but the sea was highly agitated. At the same time as these phenomena occurred on Unalaska, the volcano of Urimak [sic] Island, located 197 kilometers from Unalaska, began to produce eruptions which continued until August. (According to Postels, the active volcano was located on the north end of Umnak Island.) People who set out to look for the crater, due to the stinking steams tha thad spread for a verst, could not approach it and were convinced only that the island had increased in size and that the sea had rushed back for a considerable distance. Volcanic products erupted in such quantity that Urimak Island was covered with them for a radius of three miles around the crater. Postels describes these phenomena in the following manner: on 1 March on the north end of Umnak Island there occurred an eruption in which the ash reached Unalaska and Unimak. A strong earthquake, accompanied by a severe storm from the SE, plunged the inhabitants of Unalaska into terror. With the rising of the sun they saw that in certain places the earth was covered with ash more than a foot deep; the springs were choked with it, [and] the sea became cloudy, so that for a whole year fish were not seen in it and even whales appeared more rarely than usual. Not far from the site of eruption the Aleuts found amber in the loosened earth which covered the cliff that is washed by the lake." Mushketov and Orlov cite Malte Brun, Nouv. annales des voyages, XV, p. 131; Arago, Ann. de chimie et de phys., XXI, p. 396; and Postels, Voyage autour du monde, III, p. 24."
Coats (1950) reports a minor explosive eruption and Powers (1958) reports an ash eruption.

From Miller and others (1998): "Coats (1950) attributed four eruptions in the late eighteenth century and early nineteenth century to Pogromni volcano. Based on recent observations from aircraft, however, Pogromni does not appear to have been active in historical time. The eruptions should probably be assigned to Westdahl."
Grewingk (1850, translated 2003 by Fritz Jaensch) reports that Isenbeck said that Pogromnaia was belching fire in 1827, and "Lastly, on the southern side of Unimak, on the west side of Tugamak Gulf [Unimak Bight], the location of which is not accurately fixed, but probably not far from Pogromnoi Volcano, there, in September of 1827, according to the log of the corvette Moller, a 'lava brulante' was observed, meaning most likely a not-yet-cooled lava stream. Dr. Isenbeck reports (oral report) that during the passage through the Strait of Unimak, a fiery column constantly ascended from the volcano closest to the coast. This was probably Pogromnoi Volcano, although Veniaminov's indications, according to which the Shishaldin erupted at the same time, speak against it."
Powers (1958) reports an ash eruption.

From Miller and others (1998): "Coats (1950) attributed four eruptions in the late eighteenth century and early nineteenth century to Pogromni volcano. Based on recent observations from aircraft, however, Pogromni does not appear to have been active in historical time. The eruptions should probably be assigned to Westdahl."
Petroff (1884) reports an eruption in autumn of 1830, but Grewingk (1850, translated 2003 by Fritz Jaensch) suggests the eruption is from Shishaldin. Becker (1898) lists an 1830 eruption with ashes.

From Miller and others (1998): "Based on recent observations from aircraft, however, Pogromni does not appear to have been active in historical time. The eruptions should probably be assigned to Westdahl."
Jones (1952) reports that an officer of one of the TAKL ships reported that Pogromni volcano was not active on July 2, but it was smoking on July 5 to 7.

Juergen Kienle's compiled notes on file at the Geophysical Institute, University of Alaska Fairbanks, state that Skinner reported in 1979 that during late 1964 to early 1965, 7 separate "blow holes" of ash on E-W line were active at Westdahl. He also states that across a plateau south of Pogromni, just before it slopes off to Bering Sea, there was activity for 1 week, there was a deep gully connecting all blow holes in snow field, and at the end of the week, the main fissure had completely stopped, but a new cone had formed 3 miles from the Bering Sea beach (with lava flow).
Reeder and Doukas (1994) report that Westdahl erupted in 1964-65, producing a fissure-fed lava flow that eventually covered 35 square kilometers.
Coats (1964) writes that tremors were reported at Scotch Cap, and that although there was lava, the composition is unknown. Coarse tephra was also produced. On April 16, the vent was only steaming slightly. Secondary fumaroles were seen on the lava flow.
The Associated Press produced an article on March 14, 1964: "A major new volcanic eruption was reported late Friday on Unimak Island in the Aleutian chain, with a two-mile river of lava flowing from the mountain's crater and debris hurled 2,000 feet into the air.
"The lava flow and spewing debris was reported by a Coast Guard plane flying a low-level survey about five miles east of Westdahl peak, on Pogromni volcano at the west end of Unimak."

Around 1:15 pm on February 4, 1978, Clark (1978) observed "great billowing clouds of steam with a drifting black backdrop that suggested falling ash. * * * This was accompanied by lightning, thunder and the smell of sulphur. * * * By 4:00 pm the cloud had become much broader and dark * * * and Clark observed 'swaths of melting snow coming down the hillside.' By 4:30 pm the ash was thick at Scotch Cap. At 6:45 pm Clark 'experienced hail; small stones. The stones are dark. The ash has actually been seeding the clouds! We have noticed small bits of cinder from the cores of the hailstones.' At 7:35 pm a foot of ash had built up. At 11:35 pm lightning continued, and appeared red. Hail and cinder storms continued on and off at Scotch Cap, along with lightning and thunder, until the afternoon of February 5, when the cloud traveled to the southwest. Clark also reports, on February 6, that the road from Scotch Cap to Cape Serichef was washed out, leaving a 30 foot drop off, at least 100 yards across.
From Krafft and others (1980): The U.S. Coast Guard reported on 6 February that ash, accompanied by a sulfur odor, was falling on a station located at the foot of Westdahl. Lightning was observed above the summit, accompanied by thunder and rumbling. Reeve Aleutian Airways personnel report an ash cloud rising to 8,000 -10,000 m altitude, including some large blocks visible above the 3000 m cloud layer. Snow contaminated by dark ash fell on the freight vessel UNITED SPIRT between 12:00 and about midnight on 7 February, as it steamed from 48.8 degrees N, 152.5 degrees W to 49.2 degrees N, 156.3 degrees W, about 1000 km SE of Westdahl. A plume was visible in a satellite image taken at 1129 on 9 February. After the 9th, activity declined to steaming. The new crater formed by the February eruption is about 1.5 km in diameter and 0.5 km deep, located at about 1450 m elevation. Its upper portion cuts through glacial ice, which reaches a thickness of 200 m on the N rim. The bottom of the vertical-walled crater is filled with blocks, ash, ice, and talus. A lahar deposit, originating on the WSW flank of the new crater, extends down the glacier on Westdahl's flank to the sea, cutting the road from Cape Sarichef to Scotch Cap. The thickness of the upper portion of the deposit averages about 50 cm, increasing to 1-3 m near the lower end (Data from: SEAN Bulletin, vol. 3, n. 1, p. 7, n.2, p. 3-6, n. 9, p. 9-11)."

From Pamenter and Kienle (1981): "Ash cloud about 8 km high. "A cloud apparently erupted from Westdahl was present on NOAA weather satellite imagery for more than 30 hours on 8 and 9 February. The cloud was first observed on an infrared image taken at 03:52 on 8 February, about 17 hours after the previous image, on which no eruption cloud could be seen. It occurred on 08:42 and 10:37 (infrared) but was not present at 19:58 on the 9th of 09:52 on the 10th. The cloud was no more than 50 km in longest dimension on any of the images, nor was it elongated into a typical volcanic plume. The height of the cloud was calculated separately from infrared and visual images taken at 09:26 on 8 February. Analysis of the infrared image gives a temperature of -53 degrees C at the top of the cloud, corresponding to an altitude of slightly more than 8 km. On the visual image, measurements of the shadow cast on the weather cloud deck by the volcanic cloud results in an estimated altitude of 7.7 +/- 1 km (Data from SEAN Bulletin, vol. 4, p. 2-3, 1979)."

From Neal and McGimsey (1997): "AVO detected a plume-like cloud with a negative Band 4-5 signature on an AVHRR image of Unimak Island on the morning of March 1, 1996. Subsequent analysis with NWS colleagues and lack of any confirmation of an eruption by pilots let to the conclusion that the cloud was meterologic in nature. The cloud suggested possible activity at Westdahl, which makes up much of the southwest portion of Unimak Island, 50 miles west of False Pass and 125 miles northeast of Akutan."

From Neal and others (2005): "On January 7, 2004, 90 earthquakes occurred over a period of 12 hours beneath Westdahl Volcano on Unimak Island in the eastern Aleutians. Since the short-period seismic network was installed on this volcano in 1998, the majority of background seismicity has occurred in the vicinity of Faris Peak, a young, intracaldera cone 4 km (2.5 mi) east-northeast of Westdahl Peak (fig. 21B.) The 2004 swarm consisted of earthquakes ranging in size from ML (local or Richter magnitude) = 0.2 to ML = 1.6. The largest earthquakes (ML = 1.6) occurred during the second half of the swarm. Depths ranged from sea level to 8 km (5 mi) below sea level. Over the next 10 months, AVO detected several deep, long-period events below the volcano. Given the location of this swarm beneath the general area of the most recent historical eruptions and the subsequent swarm of deep, long-period earthquakes, this seismicity most likely represents a magmatic intrusion. Due to the short duration and abrupt termination of this swarm, AVO did not raise the level of concern color code for Westdahl, nor did AVO mention the activity in its weekly updates."

From Cameron and others, 2023: "Throughout 2018, Westdahl volcano (an informal name that herein includes Westdahl Peak, Westdahl caldera, and associated intra- and extra-caldera features) continued its long-term steady inflation, which has persisted since the deformation was first observed after the installation of a GPS network on Unimak Island in 2008. An analysis of Westdahl volcano InSAR data by Lu and others (2003) indicated a shallow magma reservoir exists beneath the volcano; the continued inflation is consistent with an ongoing accumulation of magma at shallow depths. Westdahl volcano remained at GREEN and NORMAL throughout 2018."
From Orr and others, 2023: "In 2019, Westdahl volcano continued its long-term trend of steady inflation, which has persisted since the trend was first observed after the installation of a GNSS network on Unimak Island in 2008. An analysis of Westdahl volcano InSAR data by Lu and others (2003) indicated a magma reservoir exists beneath the volcano at a depth of about 6 km [3.7 mi] below sea level. Continued inflation of Westdahl volcano is consistent with an ongoing accumulation of melt at relatively shallow depths. The volcano remained at GREEN and NORMAL throughout 2019."

From Neal and others (2014): "In late July, AVO seismologists noted a marked increase in lower crustal seismicity at Westdahl. Most of the seismicity was long period in character however some volcano-tectonic events also were recorded.
Deep (>10 km or >6.2 mi), low frequency events located with the Westdahl seismograph network tend to cluster in an area northwest of the volcano's summit. Past analysis of InSAR results for Westdahl by Lu and others (2003) suggests a shallow magma reservoir beneath the volcano. Continued inflation of the volcano is consistent with ongoing accumulation of melt at shallow levels. Such ascent of magma from depth may explain the 2010 seismicity at Westdahl."