Eruption is uncertain (Smithsonian Institution, 2002-, accessed March 28, 2007).

Reeder (1987) reported that Pilot Everett Skinner observed "an extensive eruption cloud" from Aniakchak on June 25, 1951. This event is generally thought to be a resuspension of ash particles rather than a volcanic eruption.

From McGimsey and others (2007): "Beginning in mid-December 2004 and continuing into January 2005, several series of 10-20 low-frequency seismic events showed up on the Aniakchak seismic network. On January 11, 2005, returning form an observation flight over nearby Veniaminof Volcano AVO staff photographed a partially ice-free Surprise Lake wihtin the Aniakchak caldera (see fig. 29 in original text). A thermal anomaly of unknown source was detected in satellite data on February 1-3, 2005. Subsequent analysis and discussion among AVO scientists regarding these phenomena concluded that nothing unusual was likely occurring, and no further activity was noted in succeeding months."

Seismometers at Aniakchak Volcano appeared to record anomalous seismicty at Aniakchak during the spring of 2008. Further analysis revealed that the signals were a product of icing on the seismometers, and not of volcanic origin.
From Neal and others (2011): "AVO seismic analysts noted a swarm of seismic events at Aniakchak during routine seismic checks of Alaskan volcanoes on March 8. The activity continued intermittently over the next week with events occurring as frequently as 5-10 per hour. Interpreting the significance of the signals was hampered by the fact that only a single station in the Aniakchak network, ANNW, was operating at the time. AVO increased seismic watch frequency for Aniakchak in response and contemplated organizing a winter-time field visit to restore more of the seismic network.
"Subsequently, an evaluation of historical seismicity at Aniakchak during times of more complete network operation suggested that the anomalous signals were likely weather-related (Katrina Jacobs, AVO/UAFGI, written commun., 2008). Similar swarms had occurred on a number of occasions between 2005 and 2008, but none were typical multi-station volcanic events and all showed a strong correlation with time of day. Years of maintenance efforts at the Aniakchak network documented a strong vulnerability of sites to heavy icing that could have been responsible for the intermittent seismic signals. Based on this, an AVO field response was canceled."

From McGimsey and others (2014): "Following several episodes of anomalous seismicity in 2008 (Neal and others, 2011), deep, low-frequency seismic events continued to be recorded at Aniakchak Volcano through the first half of 2009 while the seismic network was still operating. On February 27 and 28, 12 low-frequency events were recorded that had magnitudes of ML 1.0-2.1 and hypocentral depths of about 15-28 km (9.3-17.4 mi), and another sequence of events occurred on March 4 with one locatable low frequency event about 7 km (4.4 mi) depth (Scott Stihler, U.S. Geological Survey, written commun., 2011). Another burst of low-frequency events occurred at Aniakchak on June 3, 2009.
"By September 2009, chronic station outages and data interruption prompted AVO to include Aniakchak in the list of four volcanoes considered for downgrading to UNASSIGNED. On November 17, AVO issued a Volcanic Activity Notice and an Information Statement formally declaring that Aniakchak and three other volcanoes: Veniaminof, Fourpeaked, and Korovin were no longer seismically monitored due to seismic station outages, and thus were changed from volcano alert level NORMAL and Aviation Color Code GREEN to UNASSIGNED. Two seismic stations at Aniakchak became operational by December 8, 209, but would again go out in early 2010. The network would eventually become operational again during the summer of 2010."

From Neal and others (2014): "A low-frequency event followed by a short-lived tremor-like signal was recorded January 15, 2010, on station ANPB. There were no other stations recording data to facilitate further analysis; winters are particularly hard on Aniakchak stations and by January 26, all seismic stations were offline. On July 7, after summer maintenance and repair, two stations (ANPB and ANPK) recorded an approximate 15-minute long swarm. Another brief sequence of low-frequency events was recorded on station ANON on August 7 from about 21:20 to 21:28 UTC. From about 21:28 to 21:35 UTC, a signal resembling tremor occurred, although it also could be interpreted as a series of closely spaced, low-frequency events. This signal was picked up by the other five stations in the Aniakchak network.
"On October 14 from about 22:29 to 22:39 UTC, another sequence of low-frequency events occurred in the Aniakchak area. Only one event could be located, about 6 km (4 mi) east-northeast of Vent Mountain, the most prominent intracaldera cone. Another similar although shallower (9.1 km or 5.6 mi), low-frequency sequence also occurred on October 19. This event was located about 2 km (1.2 mi) south-southeast of Vent Mountain with a local magnitude of 0.5.
"Cluster, low-frequency events occurred on November 5, December 16, and from December 22-24. These small, emergent events may have been related to ice processes in the vicinity of the seismic station (H. Buurman, University of Alaska, written commun., 2010)."

From McGimsey and others (2014): "In late August 2011, AVO was alerted by Troy Hamon, Chief of Resource Management, Katmai National Park, that the larger of two maar crater lakes on the caldera floor of Aniakchak Volcano had recently drained, leaving a gaping notch in the crater rim. The flood had ravaged the downstream drainage and alluvial fan as well as the upper reaches of the Aniakchak River, which drains Surprise Lake and the eastern caldera floor through The Gates. During September 8-9, 2011, authors Tina Neal and Game McGimsey, accompanied by Troy Hamon (on September 8), visited the site to document and study the activity.
"Aniakchak caldera formed about 3,600 years ago in a colossal eruption (Neal and others, 2001). Subsequent eruptive activity has occurred from many intracaldera vents, including eruptions that formed two maar craters, subsequently filling with water, that are situated on the southeastern caldera floor between the central Vent Mountain cone and the sheer inner east caldera wall. The larger maar is about 450 m (1,476 ft) by 515 m (1,690 ft), as measured rim to rim in Google EarthTM imagery (fig. 3); the smaller maar is 170 m (558 ft) in diameter at the rim. The maars formed subsequent to the draining of a caldera-wide lake, at least 1,000 years before present (BP), but they may be much younger, perhaps less than 400 years old.
"Geological investigations in 1992 documented that a low point on the maar rim was about 10-15 m (33-49 ft) above the lake surface, although higher strand lines were visible in the deposits surrounding the maar, one of which was located nearly at the low point (Neal and others, 2001). On the outboard flank of the maar-rim low-point, a seep - piping through the maar crater wall deposits to the lake - occurred near the base of the flank and formed the headwater for the stream that courses between a prominent lava flow from Vent Mountain and the colluvial slopes extending out from the eastern caldera wall. This stream flows about 1.7 km (1 mi) northward from its source at the maar to the Aniakchak River immediately inboard of The Gates, a prominent notch in the caldera wall through which the river drains the entire caldera.
"The maar breakout flood scoured and excavated the channel and banks of the stream north of the maar, cutting deeply into the colluvial slopes on the eastern side of the drainage. The previously well-vegetated alluvial fan near The Gates was entirely covered by flood deposits up to 1.5 m (4.9 ft) thick. Clumps of vegetation, up to about 1 m (3.3 ft) in length, were strewn on and within the deposit. The Aniakchak River was pressed even more tightly against the northern wall opposite the fan, causing some bank erosion, and the downstream river channel, sand bars, and banks were noticeably disrupted for more than 20 km (12.4 mi) beyond The Gates. The area of the breach on the maar rim was dramatically altered as downcutting progressed until a new base level was established. Discharge through the outflow at the northern end of the maar now approximates inflow at the southern end (primarily at the waterfall.
"Field measurements and observations indicate that the maar lake level declined 4.65 m (15.3 ft) from its recent high stand (determined from photographs taken July 18, 2010), as well as measurements made in the field in September 2011); this high stand was below the low-point on the northern rim where the breach occurred. The volume of water lost was at least 645,000 m3 (844,000 cubic yards), but this represents a minimum volume because there is no way of knowing how much additional water flooded into the maar immediately prior to initiation of the breakout flooding.
"Based on analysis of photographs, weather data, and observations from a local guide, the maar flood likely occurred between late July and late September 2010. Evidence of recent flooding on the southern floor and wall of the caldera, which drain into the maar, suggests that the maar flood was likely caused by the rapid and voluminous influx of water into the maar lake during a period of unusually heavy rainfall. The lake level rose rapidly and the low point on the maar rim may have been reached and spilled over. The increased hydraulic head may have simultaneously initiated failure of the crater wall at that location where groundwater seepage and possibly piping within the mantling deposit was occurring. The heavy precipitation may have further increased saturation of the deposit. Once the breach began, by either rapid downcutting, or structural failure of the wall, or both, rapid release of water occurred, resulting in a short-lived but massive flood down the drainage to - and then down - the Aniakchak River."

From Cameron and others, 2020: "Volcanic activity did not occur at Aniakchak volcano (an informal designation used by AVO to refer to the volcano that comprises Aniakchak Crater and the volcanic features in its vicinity) in 2016, but a magnitude 6.2 earthquake occurred 20 km east-southeast of Aniakchak caldera on April 2. This earthquake was followed by an energetic aftershock sequence, and most earthquakes were located in the upper 20 km of the crust. The earthquake and associated aftershock sequence were thrust events and nonvolcanic, although AVO seismologists closely monitored the activity in case a volcanic event was triggered by the regional earthquake sequence. The Aviation Color Code and Volcano Alert Level remained at GREEN and NORMAL, respectively."