Coombs and others (2008) provide information about the 1929 eruption as follows: "The 1929 eruption is the largest historical eruption of Gareloi and also unique in its eruptive style and composition. The most detailed description of the eruption is given by oats (1959) who summarized the first-hand observations of William H. Dirks, Sr. (Coats, 1959; p. 2): 'In April of 1929, there was an eruption. A terrible quake * * * split the mountain from its crater right to the beach. Ashes fell, completely covering their cabins and boats* * * '
"Coats further elaborated on the series of events from observations of the deposits, which were 17 years old when he visited the island and still unmodified either by vegetation or further eruptive activity. According to Coats, the eruption excavated 13 explosion craters, ranging in diameter from 80 to 1,600 m, along a 4.3-km-long, south-southeast trending fissure [see figs. 5 and 7 in original text]. On the basis of work by Coats (1959) and our fieldwork, the eruption began with formation of the current South Peak crater and a possible directed blast to the south. Following this, an explosive eruption covered the southern half of the island in several meters of tephra fall [see fig. 8 in original text]. At this time, the upper craters were excavated. Small pyroclastic flow deposits are interbedded with tephra-fall deposits, indicating that pyroclastic flows accompanied tephra fall.
"During the second half of the eruption, the composition of the juvenile clasts in the fall deposits became more mafic; craters formed lower on the flank, and small lava flows effused from them. Sometime after explosive activity ceased, debris flows from the summit carried hydrothermally altered blocks down the volcano's southern slope.
"The eruption's total volume is unknown, as much of the ejecta were deposited offshore. Onland pyroclastic deposits represent about 16,000 cubic m, and that of lava is about 2,500 cubic m."

Miller and others (1998), also summarize Coats (1956): "In April of 1929, a phreatic eruption opened an elongate crater 1600 m in maximum diameter just below the southern summit; further explosions produced 12 smaller craters aligned along a south- to southeast-trending fissure. Glassy pumice, lapilli, scoria, and accidental rocks were then ejected from the lower craters, blanketing an area roughly 2.5 by 5 km on the southeast slope. Ash layers up to 2 m thick on Ogliuga Island, located about 16 km southeast, may be attributable at least in part to this eruption (Coats, 1956, p. 92) and several centimeters of pyroclastic debris are known to have fallen on Atka Island (about 300 km eastward) during the event. Extrusion of lava, which formed short steep flows, occurred at four sites along the fissure after the tephra eruptions. Various metallic oxides and halides (including atacamite, paratacamite and hematite) were deposited in several of the lower craters. Activity may have continued into 1930.
"Juvenile material erupted in 1929 may be significantly more siliceous than the older basaltic or basaltic andesite rocks. The lava was apparently highly viscous and generally formed blocky, vesicular, or scoriaceous flows, one of which is 75 m wide and 15 m thick. Orthopyroxene, brown hornblende, plagioclase (average composition about An48), and clinopyroxene were observed in lava flows and pyroclastics of the 1929 eruption (Coats, 1959, p. 254-255)."

Finch (1931): "Gareloi volcano, a peak 5,334 feet high far to the west in the Aleutian Islands, was very active during the spring and summer of 1930. The appearance of half of the island was said to be changed by lava flows from fissures, and a hut was destroyed belonging to fox farmers."

Jaggar (1932): "Gareloi in the western Aleutians was said to erupt so violently in 1930 as to change its appearance by lava flows from fissures."

Coombs and others (2008) summarize the 1980 eruption: "On August 8, the crew of a Northwest Orient Airlines plane spotted a plume of ash that rose 35,000-40,000 ft above sea level (Anchorage Times, August 16, 1980, p. A3). David Evans of the USGS reported earthquakes whose epicenters were west of the Adak seismic network on August 8 and 9. Ash fall was not reported at Adak. Cloud cover mostly obscured further observations after August 10, although a photograph of the summit from August 13 shows a light-gray plume rising from the North Peak crater [see fig. 9 in original text]. Evans said, 'As far as we know, there was no lava eruption' (Anchorage Times, August 16, 1980, p. A3). On August 10, airborne stratospheric sampling detected ash and elevated sulfate concentrations between 54 degrees N and 60 degrees N and 150 degrees W (about 2,000 km away) at an altitude of 63,000 ft asl (Sedlacek and others, 1981). On the basis of wind direction and NOAA-6 satellite imagery, the plume was a plausible product of the August 8 eruption (Sedlacek and others, 1981). The Smithsonian reported renewed ash emissions to 6 km asl in September 1980."

From Cameron and others, 2023: "The typical seismicity of Mount Gareloi is characterized by many low-frequency earthquakes that are commonly too small to be located by its seismic network, which comprises six stations on Gareloi Island and nearby Kavalga Island. Although the frequency of located seismic events varied in the past, seismicity at Mount Gareloi was fairly constant in recent years: 265 earthquakes were located in 2016 and 269 were located in 2017. In the summer of 2018, however, AVO located an anomalously high number of earthquakes, with 190 located in June and 178 in July. AVO then located 80 earthquakes in August and 77 in September before the volcano returned to typical activity levels in the fall. This period of increased activity was dominated by low-frequency earthquakes at depths of 5–15 km, which is typical for seismicity at Mount Gareloi. AVO paid close attention to the elevated seismicity rates but kept the Aviation Color Code and Volcano Alert Level at GREEN and NORMAL.
"In 2022, while retrospectively reviewing high-resolution satellite imagery of Mount Gareloi, AVO found a 2018 image showing an approximately 5-kilometer-long ash deposit extending north-northwest from the south fumarole field of the volcano. This image, taken by the WorldView-2 satellite on June 12, appears to record a previously unrecognized small ash emission (eruption). Satellite imagery acquired by Planet Labs PBC first shows the ash streak on June 6; the actual emission may have taken place on June 5."

From Orr and others, 2024: "Beginning on May 18, 2021, anomalous seismicity was recorded at the Mount Gareloi. In particular, the low-energy LP events commonly observed at the volcano overlapped with a harmonic tremor recorded on every station on the island. The dominant tremor frequencies peaked at ~3.4 hertz (Hz), ~6.4 Hz, ~8.3 Hz, and ~10.7 Hz. These harmonic signals, which were intermittent during at least the previous day, became conspicuous after a regional ML 5.0 earthquake at 20:42 HADT on May 17 (05:42 UTC on May 18), though this change might be coincidental. Harmonic tremor took place intermittently until May 27, when highly frequent and highly energetic LP events emerged, sometimes merging into continuous broadband tremor. An ~2-Hz infrasound signal was also recorded locally on June 3, although it was unclear if the signal was of volcanic origin. A monochromatic, ~7 Hz seismic signal lasting 15-20 min was recorded on June 6, while elevated-amplitude seismicity, ongoing since about May 27, continued. Hence, on June 8, AVO raised the Aviation Color Code and Volcano Alert Level to YELLOW and ADVISORY. The increase in seismic activity likely reflected a change to the magmatic-hydrothermal system interaction. No indications of unrest, however, were observed by an AVO field crew that flew over the summit on May 23.
"Over the following two months, the elevated and anomalous seismicity continued intermittently; SO2 was detected in satellite imagery but was consistent with measurements from the last several years; light steaming was observed in partly cloudy satellite and webcam views of the volcano; and weakly elevated surface temperatures were sometimes observed in satellite data. No eruptive activity was observed in satellite views of the volcano and no activity was detected by local or regional infrasound sensors. Starting in the second week of July, seismic activity gradually diminished and returned to background levels. AVO lowered the Aviation Color Code and Volcano Alert Level to GREEN and NORMAL on July 28, 2021."