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 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 670 kilometers (420 mi) 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 25 kilometers (16 mi) west from the volcano, other towns within 100 kilometers (62 mi) from Aniakchak are Chignik Lake, Chignik, Chignik Lagoon, Pilot Point and Ugashik.
The volcano is a 10 kilometers (6.2 mi) wide and 500–1,000 meters (1,600–3,300 ft) 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 1,341 meters (4,400 ft) high Aniakchak Peak on the southern caldera rim. A 200 meters (660 ft) 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 500 meters (1,600 ft) 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 2.5 kilometers (1.6 mi) wide and 500 meters (1,600 ft)-1 kilometer (0.62 mi) high Vent Mountain just south of the caldera centre. Other craters are the semicircular Half Cone 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. Milky-green Surprise Lake has an area of 2.75 square kilometers (1.06 sq mi) and abuts the inner northeastern margin of the caldera. Its water is about 19.5 meters (64 ft) 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 335 meters (1,099 ft) 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 1 square kilometer (0.39 sq mi) 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
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