Description: From Neal and others (2001): "In the last 200 years, Aniakchak volcano is known to have erupted once - during about six weeks in May - June 1931. Documentation of the event is limited; the following summary is derived principally from the writings of University of Santa Clara missionary and explorer Hubbard (1931; Jan 2 and 16, 1932), who visited Aniakchak in 1930 and 1931.
The 1931 eruption was violent, included both explosive and effusive phases, and sent ash at least 600 km north of the volcano. The first sign of activity was noted about 10 a.m. on May 1, 1931, when residents of the former Meshik (now part of Port Heiden) saw a vigorous, white column of steam ascending above the crater. By noon, residents reported ground shaking, rumbling noises, and the beginning of tephra fall from a large, black mushroom cloud, intermittently illuminated by lightning, over the caldera. Cloud height was estimated to be more than 6 km above sea level. Fallout in Meshik in the early stages of eruption included ash and pea- to egg-sized, frothy, black pumice that pelted homes. Radio communications with Chignik and other communities in southwestern Alaska were hampered repeatedly by static caused by ash in the atmosphere (Anchorage Daily Times, 1931; Fairbanks Daily News-Miner, May 4, 1931). Ash fall was noted at Kanakanak (near Dillingham), 225 km north of the volcano [see fig. 2 in original text] (Fairbanks Daily News-Miner, May 2, 1931). Observers describe a constant level of eruption until May 11, when an extremely violent explosion rocked the volcano. Heavy ash fall produced total darkness for several hours near the volcano. As much as several millimeters of black ash accumulated at Chignik, and greater amounts were recorded at Ugashik [see fig. 2 in original text]. Rafts of pumice containing individual fragments as much as 20 cm across reportedly were floating in Bristol Bay west of Meshik. A 10-km-wide swath of black ash and 'almost complete darkness' observed from a boat in Bristol Bay attest to the severity of the fallout (Seward Daily Gateway, May 28, 1931).
After May 11, the eruption apparently diminished in intensity until May 20, when explosions were heard at Ugashik (75 km northeast) and at an unspecified location more than 300 km away. Beginning on May 26, intermittent small ash plumes were reported over the caldera and Chignik residents reported 'rumbling' like distant surf in the direction of Aniakchak. Several earthquakes, some described as 'severe,' were felt in Chignik and Hook Bay in late May.
Father Hubbard flew over the volcano on June 10, while the eruption was still in progress. His party first hiked to the caldera on June 13 and discovered moving, blocky lava flows at the bottom of two new explosion pits [see figs. 6A, B in original text]. A third small lava flow issued from a knob slightly above the base of the west caldera wall [see fig. 6C in original text]. Steam explosions had reamed a shallow pit in coarse ash and lapilli that blanketed a lava-flow field inside Half Cone [see fig. 6D in original text]. Although accumulation of fallout was heaviest in the western and northwestern parts of the caldera, nearly all vegetation inside the caldera was destroyed or buried. Three small lakes in the western part of the caldera (Knappen, 1929) were filled completely with ash and lapilli, and Surprise Lake was cloudy with suspended ash. Hubbard also reported dead birds, presumably killed by carbon dioxide that had accumulated in low areas near the vent. The north rim of Vent Mountain's summit crater reportedly was steaming (Regan, 1987)."
"* * * Earthquakes during the 1931 eruption were strong enough to be felt in Chignik, 65 km away, and to destabilize the precipitous caldera walls. Hubbard (Jan. 2. 1932) reported avalanches in progress inside Aniakchak in mid-June. The rock-avalanche lobes that extend from the south wall inside the caldera [see fig. 5 in original text] may have formed in 1931.
"Beyond the caldera rim, fallout from the 1931 eruption affected several hundred thousand square kilometers of southwestern Alaska. As much as 1 to 2 cm of ash may have fallen in Chignik (Fairbanks Daily News-Miner, May 14, 1931). Ash reportedly was about 6 mm thick on Kodiak Island (presumably in the village of Kodiak), in Katmai National Monument2, and on the Nushagak Peninsula, and a fine dusting was reported at Holy Cross, 600 km north of the volcano. Light ash fall was reported also at Squaw Harbor on Unga Island, 140 km southwest of Aniakchak. Reindeer and caribou losses from fallout were reported to be 'heavy' at Nushagak, and dead swans and geese, believed to have died from ash ingestion, were noted at Ugashik (Hubbard, Jan. 2, 1932). From these scattered reports, we constructed a map showing the area most likely to have received noticeable amounts of ash fall [see fig. 7 in original text].
"The volume of material erupted in 1931 is difficult to determine by traditional field methods because of the widespread dispersal of fine ash, much of it over water. In many places, because strong winds and rain have stripped the 1931 deposit completely, original thickness on land is difficult to measure accurately. Furthermore, eyewitness accounts are few and, from our experience, prone to exaggeration. Using limited field measurements and interpretation of written accounts of ash fall during and after the eruption, we estimate the total bulk volume of the 1931 deposits to be about 0.3 to 0.5 cubic km.
"The interaction of erupting magma and abundant water in part explains why the 1931 eruption was explosive. The conclusion that the eruption was hydrovolcanic was based on the presence of accretionary lapilli, as much as several centimeters in diameter (Hubbard, Jan. 2, 1932); rhythmic surge-and-fall deposits, exposed in the walls of the main vent; blocky lithic ejecta; and widely dispersed fine-grained ash. Also, Knappen (1929) noted standing water in the western part of the caldera prior to the eruption. That this shallow ground-water system persists is indicated by the presence of Surprise Lake and by the abundance of springs on the floor in the eastern part of the caldera. Future eruptions may include a similar strong hydrovolcanic component as hot rising magma mixes explosively with water."
Bacon and others (2014) estimate the total volume of erupted products at about 0.9 cubic km, less than 1 percent of which was lava.