Mount Aniakchak () 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 wide caldera with a break to the northeast. The caldera contains Surprise Lake and many volcanic cones, maars and craters, including Vent Mountain. The volcano has erupted mainly calc-alkaline rocks ranging from basalt to rhyolite.
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. Many 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 causing 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.
Geography and geomorphology
Aniakchak is about southwest from Anchorage, Alaska, within the Aniakchak National Monument and Preserve (Bristol Bay Borough) on the Alaska Peninsula between Bristol Bay (Bering Sea) and the Pacific Ocean. Port Heiden is west from the volcano, other towns within from Aniakchak are Chignik Lake, Chignik, Chignik Lagoon, Pilot Point and Ugashik.
The volcano is a wide and deep caldera, formally named Aniakchak Crater. It is surrounded by gently sloping terrain between the Aleutian Range to the southwest and Bristol Bay to the northeast. The Aleutian Range is not high but its mountains rise directly from the sea. Outside of the caldera the volcano is notably asymmetric, with the northwestern side having a less eroded appearance than the southeastern. The highest point of the rim is the high Aniakchak Peak on the southern caldera rim. A deep prominent v-shaped gap in the northeastern caldera rim is known as The Gates. Steep walls cut into fossil-bearing nonvolcanic rocks, with only the top of the cut rock being part of the actual Aniakchak volcano. Outcrops in The Gates bear traces of hydrothermal weathering. There is a single report of volcanic caves at Aniakchak.
A number of secondary cones, lava domes, maars and tuff cones dot the caldera floor, the largest is the wide and - high Vent Mountain just south of the caldera centre. Other craters are the semicircular Half Cone in the northwestern, the 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.
Milky-green Surprise Lake has an area of and abuts the inner northeastern margin of the caldera. Its water is about deep and originates from various hot springs, cold springs and meltwater. Lake waters are continually mixed by strong winds. Hydrothermal inputs give the lake its color. The lake, which formed behind the deltas of several creeks, drains through The Gates valley at elevation above sea level in the eastern caldera rim, the only outlet of the caldera. The outlet forms the Aniakchak River, a National Wild and Scenic River flowing to the Pacific Ocean. In 2010, one of the maars in the caldera broke out, causing a flood in the Aniakchak River. Meshik Lake is south of the caldera. The Meshik and Cinder Rivers drain the rest of the volcanic edifice, to Bristol Bay. A debris-covered glacier is in the southern sector of the caldera and has emplaced moraines. Other small glaciers have developed on Aniakchak Peak and Vent Mountain. Landslides have affected the eastern walls of the caldera.
Geology
Southwest of Aniakchak, the Pacific Plate subducts beneath the North America Plate at a rate of about . This subduction is responsible for the activity of the long Aleutian Volcanic Arc. It extends from Kamchatka 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. The Aleutian Volcanic Arc is part of the wider Pacific Ring of Fire and began erupting during the Tertiary period. Volcanoes close to Aniakchak include Yantarni to the east, Black Peak and Veniaminof to the southwest; Black Peak has emplaced ash layers on Aniakchak. 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. The arc is split by a tectonic discontinuity at the longitude of Aniakchak; east of this discontinuity earthquake activity differs and the arc begins to turn clockwise. Seismic tomography has found evidence that here the boundary between the Pacific Plate and the former Kula Plate was subducted, forming a slab window; the high activity of Aniakchak and neighbouring Veniaminof might be facilitated by magma upwelling in this slab window and its accumulation in the crustal discontinuity.
The volcano grew on a westward-sloping basement formed by Mesozoic-Tertiary sedimentary rocks, which crops out south of the volcano and within the caldera. Chronologically, they are part of the Jurassic Naknek, Cretaceous Staniukovich, Cretaceous Chignik, Paleocene-Eocene Tolstoi, and Eocene-Oligocene Meshik Formations. The crust is mostly andesitic. The Alaska-Aleutian Batholith may extend under the volcano. An aeromagnetic anomaly overlies Aniakchak; similar anomalies are found on neighboring volcanoes but also on much older plutonic complexes in the region.
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.
Composition
Aniakchak has erupted rocks ranging from basalt to rhyolite, which define a calc-alkaline rock suite typical for volcanic arc rocks. Phenocrysts are rare, they include amphibole, augite, clinopyroxene, hornblende, hypersthene, ilmenite, iron sulfide, magnetite, olivine, orthopyroxene, plagioclase and quartz, depending on the rock unit. Temperatures of have been inferred for dacitic magmas in the Aniakchak II eruption; the temperature of the andesite is unknown.
None of the Aniakchak volcanic rocks are derived directly from the mantle. Rather, mantle-derived basaltic melts, enriched by fluids produced during subduction, ascend into the crust into a "mush"-like region above depth at Aniakchak. They receive a contribution from subducted sediments. Magmas differentiate within this mush region at low pressures and high temperatures, where fractional crystallization and melting of crustal rocks modify their chemistry. The rhyodacitic and rhyolitic rocks form in such mush regions. Separate magma bodies can form and absorb melts from surrounding rock. Part of the mush region was emptied during the Aniakchak II eruption. After the caldera-forming eruptions, fractional crystallization of newly arrived andesitic magmas yielded the silicic magmas erupted later.
Climate, fauna and vegetation
The climate east of the Aleutian Range is wet and mild, while west of the mountains, there is less precipitation and higher temperature variation. The closest weather stations to Aniakchak are at Kodiak and Cold Bay, close to sea level. They show mean annual temperatures of and mean annual precipitation reaching .
The main vegetation in the region is tundra. It consists of ericaceous heath, forbs, lichens, mosses and shrubs. Meadows grow on mountain ridges and in moist valleys; the latter include wetlands formed by forbs, grasses and sedges The cinder and ash cones are sparsely covered with grasses, forbs and lichens, while meadows and herbs cover the caldera floor. Some ash-covered terrain is barren of vegetation, but features wind-blown dunes.
Kodiak bears, foxes and caribou populate the region, 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. Sockeye salmons and Dolly Varden trouts occur in Surprise Lake, and a population of less than 20,000 salmon spawns there. The salmon arrived in Surprise Lake after it overflowed the caldera rim and connected with the ocean, and since then evolved into two distinct populations that reproduce in different parts of Surprise Lake. Some of these fish species migrate between the sea and the rivers; they provide nutrients to waterbodies they ascend into and are economically important.
Human history, name and use
The Alaskan Peninsula was settled about 7,000 years ago by people who practiced hunter-gatherer lifestyles. 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. The central Alaskan Peninsula is inhabited by the Alutiiq people. Beginning in 1741, Russians and later Americans visited the region and left their cultural imprint on the native population.
The volcano was discovered in 1922 and originally named "Old Crater"; "Aniakshak" is a misspelling. The name "Aniakchak" is probably Alutiiq and may be related to the Yupik word which means "the way to go out". It was deemed a National Natural Landmark in 1967 and became part of the Aniakchak National Monument and Preserve in 1980 after the passage of the Alaska National Interest Lands Conservation Act. Owing to its remote location and hostile climate, Aniakchak is rarely visited; on average there are fewer than 300 visitors every year, making it the perhaps least visited national park in the NPS system. The area is noted for dinosaur fossils, including fossilized footprints, recovered from the Chignik Formation. They have provided insights into the ancient environment. Recreational activities include backpacking, camping, fishing, hunting, and rafting. There are seasonal hunting and fishing lodges around Aniakchak, but otherwise there is little infrastructure. Access is mostly by boat plane to Surprise Lake.
Eruption history
Aniakchak began erupting at least 850,000 years ago. Two stages of early activity (850,000–550,000 and 440,000–10,000 years ago) built a composite volcano formed by lava flows and rock fragments produced by a central vent. 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. 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, but if such a caldera formed, its explosive activity left no traces.