Mount Okmok is a volcano on eastern Umnak Island, in the central-eastern Aleutian Islands of Alaska. Part of the Aleutian Volcanic Arc, it was formed by the subduction of the oceanic Pacific Plate under the North American Plate. Okmok is a large shield volcano capped by a 10-kilometer (6.2 mi) wide caldera. The caldera contains numerous cinder cones, their lava flows, and a few lakes. Okmok erupts mainly basaltic lava, mostly from the cones within the caldera. Activity began in the Pleistocene. Two large caldera-forming eruptions took place during the Holocene, with a volcanic explosivity index (VEI) of 6; the second of these occurred in 43 BCE and caused a volcanic winter that might have changed the history of Egypt. After this second caldera-forming eruption a crater lake formed in the caldera, and drained in one of the largest known floods of the Holocene. Okmok is one of the most active volcanoes of North America; numerous eruptions have produced lava flows within the caldera, and the 1817 eruption destroyed an Aleut village. The last eruption was in 2008 and produced several new vents in the caldera. This eruption, which occurred with little forewarning, yielded a volcanic cloud that produced volcanic ash fall around Okmok. The volcano is monitored by the Alaska Volcano Observatory (AVO).
Geography and geomorphology Okmok is on northeastern Umnak Island, a remote location in the central Aleutian Islands, 1,400 kilometers (870 mi) from Anchorage. The former Fort Glenn Army Air Base is on the eastern side of the volcano. Some unmaintained trails and dirt roads lead around the mountain. One dirt road leads to the gap in the caldera rim and is the principal way to access the caldera. Umnak Island is largely uninhabited, but fishery vessels sail around it all year round and a major North Pacific aviation route passes in the area. Dutch Harbor on Unalaska Island, the most important production place of seafood in the United States, is 120 kilometers (75 mi) from Okmok. The name "Okmok" was applied to the mountain by Dunn 1908, who had discovered the caldera two years before. The Aleut name of the caldera is Unmagim Anatuu; the second word means "to be thick". An alternative name for the volcano is "Zoomie". Okmok is a 30 kilometers (19 mi) wide shield volcano (sometimes described as "shield-like composite volcano") with gentle (6°) slopes. With a volume of 200–300 cubic kilometers (48–72 cu mi) and an area of about 120 square kilometers (46 sq mi)-870 square kilometers (340 sq mi), it is one of the largest volcanoes in the Aleutians. Its slopes are mostly covered by pyroclastic flow deposits from the second caldera-forming eruption. There are more than twenty Pleistocene-Holocene cones on the outer flank of Okmok, including the 1,268 meters (4,160 ft) high Tulik (highest point of Okmok) on the southern and Jag Peak on the southwestern flank. Glacial valleys, gullies, and amphitheatre-shaped valleys with widths reaching 3.2 kilometers (2 mi) cut into the slopes. Numerous creeks radially drain the edifice to the Bering Sea and Pacific Ocean: From north clockwise they include Crater Creek, Antler Creek, Kansas Creek, Missouri Creek, Ginger Creek, Colorado Creek and Delaware Creek. Crater Creek has formed a fan around Cape Tanak, as mudflows from Okmok are preferentially channeled through this creek.
Caldera The summit is cut by a 10 kilometers (6.2 mi) wide caldera, breached to the northeast by Crater Creek through a notch known as "Gates". Its rim is about 300 meters (980 ft) high above the floor. Arc-shaped structures 1.5 kilometers (0.93 mi) out from the northern and eastern margins indicate the caldera is actually two nested calderas. Lava flows, pyroclastic flows and scoria deposits crop out in the steep cliffs that form the inner wall of the caldera. The highest point on the caldera rim is the 967 meters (3,173 ft) high "Mount Okmok" on the northern rim. The Global Volcanism Program reports a height of 1,073 meters (3,520 ft). Lava flows and several volcanic cones dot the otherwise relatively flat caldera floor. The cones are named A, B, C, D, E, F, G, H and Ahmanilix (formed in 2008, meaning "surprising" in the Atkan language) and form a ring on the caldera floor. Some of the cones are heavily eroded to the point of being unrecognizable. The major cones are the 240 meters (790 ft) high Cone A (two craters) at the southwestern margin, Cone E (a crater with a lake) in the western half of the caldera, Cone D (the largest) in the eastern half of the caldera, Cone F (crater breached to the northwest) at the southern-southeastern margin of the caldera and the 300 meters (980 ft) high, already erosionally modified Ahmanilix just west of Cone D. Meltwater from snowmelt and surface water flow towards the centre of the caldera, generating waterbodies that drain through Crater Creek. As of 2008 there are two major lakes, one north of Cone D and a slightly smaller one at the centr of the caldera; both are partially or entirely within craters excavated by the 2008 eruption. Numerous other lakes are dispersed between the summit craters of cones and plains between the cones. A small debris-covered glacier covers the inward southern/north-facing slopes of the caldera, and rock glaciers have been identified inside the caldera. Past glaciation left small moraines within the caldera.
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