Mount Aniakchak (Russian: Аниакчак) is a volcano on the western Alaska Peninsula. Part of the Aleutian Volcanic Arc, it was formed by the subduction of the oceanic Pacific Plate under the North American Plate. Aniakchak is a 10 kilometers (6.2 mi) wide caldera with a break to the northeast. The caldera contains Surprise Lake and numerous volcanic cones, maars and craters, including Vent Mountain. The volcano has erupted mainly calc-alkaline rocks ranging from basalt to rhyolite.

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Aniakchak Caldera
Mount Aniakchak caldera
Highest point
Elevation	1,341 m (4,400 ft)
Coordinates	56.88°N 158.15°W / 56.88; -158.15[1]
Geography
Aniakchak Caldera Location in Alaska
Location	Aniakchak National Monument and Preserve, Alaska, US
Parent range	Aleutian Range
Topo map	USGS Chignik D-1
Geology
Mountain type	Caldera (Stratovolcano)
Volcanic arc/belt	Aleutian Arc
Last eruption	May to June 1931
U.S. National Natural Landmark
Designated	November 1967

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Activity began in the Pleistocene. Aniakchak is one of the most active volcanoes in Alaska and underwent several significant caldera-forming eruptions. The largest eruption is known as Aniakchak II and took place in 1628/1627 BCE. During this eruption, pyroclastic flows swept all the flanks of the volcano and caused a tsunami in Bristol Bay. Tephra from the eruption rained down over Alaska, with noticeable deposits being left as far as northern Europe. The eruption depopulated the central Alaska Peninsula and caused cultural changes in Alaska. Together with other volcanic eruptions at that time, Aniakchak II may have caused climatic anomalies. The present-day caldera formed during this eruption. A lake formed in the caldera, which drained in one of the largest known floods of the Holocene. Numerous lava domes and cones were emplaced within the caldera after the Aniakchak II eruption, with some events depositing ash over Alaska.

The last eruption took place in 1931. It was intense, forming a new crater in the caldera and ash fallout over numerous towns in Alaska. The volcano is monitored by the Alaska Volcano Observatory (AVO). The area around the volcano is the Aniakchak National Monument and Preserve, maintained by the National Park Service.

Aniakchak is about 670 kilometers (420 mi) southwest from Anchorage, Alaska, within the Aniakchak National Monument and Preserve^[2] (Bristol Bay Borough^[3]) on the Alaska Peninsula between Bristol Bay (Bering Sea) and the Pacific Ocean.^[4] Port Heiden is 25 kilometers (16 mi) west from the volcano,^[5] other towns within 100 kilometers (62 mi) from Aniakchak are Chignik Lake, Chignik, Chignik Lagoon, Pilot Point and Ugashik.^[6]

The volcano is a 10 kilometers (6.2 mi) wide and 500–1,000 meters (1,600–3,300 ft) deep caldera,^[7] formally named Aniakchak Crater.^[8] It is surrounded by gently sloping terrain^{[lower-alpha 1]} between the Aleutian Range to the southwest and Bristol Bay to the northeast.^[10] The Aleutian Range is not high but its mountains rise directly from the sea.^[11] Outside of the caldera the volcano is notably asymmetric, with the northwestern side having a less eroded appearance than the southeastern.^[12] The highest point of the rim is the 1,341 meters (4,400 ft) high Aniakchak Peak on the southern caldera rim.^[13][14][15] A 200 meters (660 ft) deep^[16] prominent v-shaped gap in the northeastern caldera rim is known as The Gates.^[17][18] Steep walls^[19] cut into fossil-bearing nonvolcanic rocks,^[15][20] with only the top 500 meters (1,600 ft) of the cut rock being part of the actual Aniakchak volcano.^[21] Outcrops in The Gates bear traces of hydrothermal weathering.^[22] There is a single report of volcanic caves at Aniakchak.^[23]

A number of secondary cones, lava domes, maars and tuff cones dot the caldera floor,^[21] the largest is the 2.5 kilometers (1.6 mi) wide^[24] and 500 meters (1,600 ft)^[25]-1 kilometer (0.62 mi) high Vent Mountain^{[lower-alpha 2]} just south of the caldera centre.^[26] Other craters are the semicircular^[27] Half Cone^{[lower-alpha 3]} in the northwestern, the 1 kilometer (0.62 mi) wide 1931 Main Crater and West Dome in the western, Slag Heap and Doublet Crater in the western-southwestern, New Cone, Breezy Cone, Windy Cone and two water-filled maars in the southeastern, and Surprise Cone, Bolshoi Dome, Vulcan Dome and Pumice Dome in the eastern sectors of the caldera.^[26][28]

Milky-green^[29] Surprise Lake^{[lower-alpha 4]} has an area of 2.75 square kilometers (1.06 sq mi)^[15] and abuts the inner northeastern margin of the caldera.^[26][28] Its water is about 19.5 meters (64 ft) deep^[15] and originates from various hot springs, cold springs and meltwater.^[30] Lake waters are continually mixed by strong winds.^[31] Hydrothermal inputs give the lake its color.^[32] The lake, which formed behind the deltas of several creeks,^[29] drains through^[15] The Gates valley at 335 meters (1,099 ft) elevation above sea level in the eastern caldera rim,^[13][15] the only outlet of the caldera.^[22] The outlet forms the Aniakchak River,^[15] a National Wild and Scenic River^[33] flowing to the Pacific Ocean.^[34] In 2010, one of the maars in the caldera broke out, causing a flood in the Aniakchak River.^[35] Meshik Lake is south of the caldera.^[36] The Meshik and Cinder Rivers drain the rest of the volcanic edifice, to Bristol Bay.^[37] A 1 square kilometer (0.39 sq mi) debris-covered glacier^[15] is in the southern sector of the caldera and has emplaced moraines.^[28] Other small glaciers have developed on Aniakchak Peak and Vent Mountain.^[35] Landslides have affected the eastern walls of the caldera.^[28]

Southwest of Aniakchak, the Pacific Plate subducts beneath the North America Plate^{[lower-alpha 5]} at a rate of about 65 millimeters per year (2.6 in/year). This subduction is responsible for the activity^[39] of the 4,000 kilometers (2,500 mi) long Aleutian Volcanic Arc. It extends from Kamchatka^[40] across the Aleutian Islands to Alaska and features more than forty active volcanoes. It is one of the most active volcanic arcs in the world, with multiple eruptions each year.^[15] The Aleutian Volcanic Arc is part of the wider Pacific Ring of Fire^[41] and began erupting during the Tertiary period.^[42] Volcanoes close to Aniakchak include Yantarni to the east, Black Peak and Veniaminof to the southwest;^[35] Black Peak has emplaced ash layers on Aniakchak.^[42] The segment of the Aleutian Volcanic Arc from the central Aleutian Islands to the western Alaska Peninsula, which includes Aniakchak, features some of the largest volcanoes of the arc;^[43] the formation of the Aniakchak caldera may be facilitated by a tectonic discontinuity that allows magma to accumulate in the crust.^[44]

The volcano grew on a westward-sloping^[19] basement formed by Mesozoic-Tertiary sedimentary rocks,^[21] which crops out south of the volcano and within the caldera.^[45] Chronologically, they are part of the Jurassic Naknek, Cretaceous Staniukovich, Cretaceous Chignik, Paleocene-Eocene Tolstoi, and Eocene-Oligocene Meshik Formations.^[46] The crust is mostly andesitic.^[47] The Alaska-Aleutian Batholith may extend under the volcano.^[10] An aeromagnetic anomaly overlies Aniakchak; similar anomalies are found on neighboring volcanoes but also on much older plutonic complexes in the region.^[48]

During the last glacial maximum more than 11,700 years ago, the region was covered by ice. When the glaciers retreated at the end of the ice age, they left numerous elongated moraines, U-shaped valleys, and various kinds of lakes (including kettle lakes and proglacial lakes). Two separate glaciations have been defined at Aniakchak.^[49][50]

The climate east of the Aleutian Range is wet and mild, while west of the mountains, there is less precipitation and higher temperature variation.^[67] The closest weather stations to Aniakchak are at Kodiak and Cold Bay, close to sea level. They show mean annual temperatures of 3–5 °C (37–41 °F) and mean annual precipitation reaching 870–1,380 millimeters (34–54 in).^[68]

The main vegetation in the region is tundra. It consists of ericaceous heath, forbs, lichens, mosses and shrubs.^[69] Meadows grow on mountain ridges^[70] and in moist valleys;^[71] the latter include wetlands formed by forbs, grasses and sedges^[72] The cinder and ash cones are sparsely covered with grasses, forbs^[69] and lichens,^[73] while meadows and herbs cover the caldera floor.^[74] Some ash-covered terrain is barren of vegetation,^[37] but features wind-blown dunes.^[75]

Kodiak bears, foxes and caribou populate the region,^[76] while Alaska blackfish, Chinook salmon, chum salmon, coastrange sculpin, Coho salmon, Dolly Varden trout, ninespine stickleback, Pacific staghorn sculpin, pink salmon, rainbow trout, sockeye salmon, starry flounder and threespine stickleback occur in the rivers, including Aniakchak River.^[77] Sockeye salmons and Dolly Varden trouts occur in Surprise Lake,^[78] and a population of less than 20,000 salmon^[79] spawns there.^[3] The salmon arrived in Surprise Lake after it overflowed the caldera rim and connected with the ocean,^[80] and since then evolved into two distinct populations that reproduce in different parts of Surprise Lake.^[81] Some of these fish species migrate between the sea and the rivers;^[82] they provide nutrients to waterbodies they ascend into^[83] and are economically important.^[82]

The Alaskan Peninsula was settled about 7,000 years ago^[84] by people who practiced hunter-gatherer lifestyles.^[85] After being driven away by the Aniakchak II eruption and eruptions of neighboring volcanoes, humans resettled the region beginning 1,600 years ago, building numerous villages.^[86] The central Alaskan Peninsula is inhabited by the Alutiiq people. Beginning in 1741, Russians and later Americans visited the region^[87] and left their cultural imprint on the native population.^[88]

The volcano was discovered in 1922^[25] and originally named "Old Crater";^[89] "Aniakshak" is a misspelling.^[90] The name "Aniakchak" is probably Alutiiq and may be related to the Yupik word anyaraq which means "the way to go out".^[91] It was deemed a National Natural Landmark in 1967^[3] and became part of the Aniakchak National Monument and Preserve in 1980^[92] after the passage of the Alaska National Interest Lands Conservation Act.^[93] Owing to its remote location and hostile climate, Aniakchak is rarely visited;^[26] on average there are fewer than 300 visitors every year.^[94] Recreational activities include backpacking, camping, fishing, hunting, and rafting.^[5] There are seasonal hunting and fishing lodges around Aniakchak.^[5] Access is mostly by boat plane to Surprise Lake.^[94]

Aniakchak began erupting at least 850,000 years ago.^[21] Two stages of early activity (850,000–550,000 and 440,000–10,000 years ago)^[5] built a composite volcano formed by lava flows and rock fragments^[2] produced by a central vent.^[95] Tephra layers on St. Michael Island imply that Aniakchak erupted 15,505 ± 312 years ago, but any evidence close to the volcano has been erased by erosion.^[96] At the end of the Pleistocene, Aniakchak was a glacially eroded mountain with its summit south of the present-day caldera. An ancestral caldera may have been the source of a significant glacier in the Birthday Creek drainage,^[97] but if such a caldera formed, its explosive activity left no traces.^[95]

At least forty eruptions took place during the Holocene,^[1] half before^[98] and half after the second caldera-forming eruption,^[21] equivalent to one eruption every 340 years after the second caldera-forming event.^[99] This rate is the highest of all volcanoes in the eastern Aleutian volcanic arc.^[5] Most Holocene eruptions have not produced known tephra deposits.^[100] There is evidence that after several eruptions, humans abandoned sites close to the volcano.^[101] Lava flows were emplaced on the northern flank of the volcano.^[102]

Three major eruptions took place during the Holocene: The Aniakchak I, Black Nose Pumice, and Aniakchak II eruptions.^[35] The Aniakchak I eruption took place 9,500–7,500 years ago,^[103] and emplaced volcanic bombs^[95] and ignimbrites^[2] on the volcano and in surrounding valleys.^[104] They are similar in appearance and chemistry to the Aniakchak II deposits, but can be distinguished with the help of trace element data.^[97] A tephra layer in central Alaska has been attributed to the Aniakchak I eruption.^[96] How the volcano appeared after the Aniakchak I eruption is unclear; conceivably, either a small caldera formed or the caldera rapidly filled with ice.^[105] The so-called Black Nose Pumice was emplaced 7,000 years ago during several closely spaced Plinian eruptions^[106] and consists of two pumice fallout layers, separated by an ignimbrite. It is partly eroded or buried by-products of the Aniakchak II eruption.^[107][25] A tephra layer in Southeastern Alaska was attributed to an unidentified eruption of Aniakchak 5,300–5,030 years before present,^[108] but may have originated at Mount Edgecumbe instead.^[109][110] Shortly before the Aniakchak II event, a smaller eruption may have emplaced a tephra layer in the Brooks Range of northern Alaska.^[111]

Other large caldera-forming eruptions in Alaska took place at Mount Okmok, Fisher Caldera, and Veniaminof, with lesser events at Kaguyak and Black Peak.^[112] Unlike them, before the caldera-forming eruption, Aniakchak was a small volcanic edifice.^[45]

The volcano is classified as a "high-threat volcano"^{[lower-alpha 15]} by the United States Geological Service.^[235] Future eruptions will most likely occur within the caldera, in particular its southwestern sector. Explosive eruptions may occur if the magma is volatile-rich or it interacts with water inside the caldera. Degassed magma would produce lava flows. Large caldera-forming eruptions are improbable in the near future, as there does not seem to be a large contiguous magma body under Aniakchak.^[228]

Specific hazards include:

• The main danger from future activity at Aniakchak is high ash clouds.^[236] Aircraft flying into volcanic ash clouds can suffer engine failures,^[237] and Aniakchak is located beneath one of the major air routes of the North Pacific. Precipitating volcanic ash can smother plants and make roads slippery, irritate eyes and lungs, and damage machinery. Ashfall would most likely occur north to east of the volcano but can occur in any direction.^[238]
• Pyroclastic flows and pyroclastic surges are fast-flowing avalanches/clouds of hot rock. Owing to their enormous speed and high temperature, they tend to kill everything in their path. Future eruptions would most likely create such flows within the caldera, but only larger events would pose a threat outside of it.^[239]
• Lava domes and lava flows can be extruded within the caldera. They are slow, but steam explosions or pyroclastic flows caused by collapses of lava domes can amplify their threat.^[240]
• Snow and ice within the caldera – and during larger eruptions, outside – can melt when impacted by the fallout of hot rocks. The loose volcanic ash on the slopes of Aniakchak can be liquefied by rainfall. Either can produce mudflows, which threaten valleys running from the caldera.^[241]
• While the hot springs and fumaroles are not a threat by themselves, in case of the ascent of new magma, temperatures and carbon dioxide concentrations may rise to dangerous levels.^[242]
• The vents can eject volcanic bombs, large blocks that fall down close to the vent.^[238] Landslides or subaqueous explosions can cause floods^[243] or local tsunamis from the lakes in the caldera.^[244] Hazards exist mainly within the caldera.^[239]

Aniakchak is monitored by the Alaska Volcano Observatory^[45] since 1997^[245] through seismometers and satellite images, and collects reports from visitors to the caldera and aircraft to detect renewed activity. The observatory publishes a volcano hazard level; in case of an eruption, it would coordinate with government agencies and publish updates through the Internet and other means.^[246]

• List of National Natural Landmarks
• List of volcanoes in the United States