Mount Okmok
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 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 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.
Geography and geomorphology
Okmok is on northeastern Umnak Island, a remote location in the central Aleutian Islands, 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 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 wide shield volcano with gentle slopes. With a volume of and an area of about -, 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 high Tulik on the southern and Jag Peak on the southwestern flank.
Glacial valleys, gullies, and amphitheatre-shaped valleys with widths reaching 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 wide caldera, breached to the northeast by Crater Creek through a notch known as "Gates". Its rim is about high above the floor. Arc-shaped structures 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 high "Mount Okmok" on the northern rim. The Global Volcanism Program reports a height of.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 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 high Cone A at the southwestern margin, Cone E in the western half of the caldera, Cone D in the eastern half of the caldera, Cone F at the southern-southeastern margin of the caldera and the 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. 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.
Geology
The Pacific Plate subducts beneath the North American Plate at a rate of, giving rise to the long Aleutian Volcanic Arc. The arc has about forty volcanoes between Alaska and Kamchatka. In the central Aleutians these include from west to east Seguam, Amukta, Chagulak, Yunaska, Herbert, Carlisle, Cleveland, Uliaga, Kagamil, Vsevidof, Recheshnoi, Okmok, Bogoslof, Makushin, Mount Gilbert, Westdahl, Fisher, Shishaldin, Isanotski and Roundtop. Apart from volcanoes, subduction in the Aleutians produces frequent earthquakes.Umnak island is on Cretaceous seafloor; the submerged Umnak oceanic plateau might extend under the island. Southwest of Okmok, a ridge formed by Tertiary volcanic rocks joins the volcano to the rest of Umnak island and the volcanoes Recheshnoi and Vsevidof. Northeast of Okmok is the Idak plateau, an uplifted older volcano. There is no evidence of faulting at Okmok. There are two tectonic stress regimes at Okmok, a northwest-southeast regional regime and a radial local one. Coastal uplift has occurred during historical times.
Surface deformation has been recorded before and after eruptions, often continuing for years, implying underground magma movements centered at depth. The deformation source constitutes the magma chamber. Episodically recharged from deeper reservoirs, the magma chamber has a temperature exceeding and is thought to be insulated by accumulated crystals. Most eruptions arise from the magma chamber, sometimes through sills or shallower chambers that feed the intracaldera cones like Cone A. With exceptions, magma is only briefly stored underground before erupting to the surface. Shallower pools of basaltic andesite magma are rarely involved, but played a role during the 2008 eruption. With rare exceptions, the magma is deflected sideward before reaching the surface via ring faults, thus erupting at the margin of the caldera instead of at its center.
Composition
Okmok has erupted basalt and basaltic andesite, which define a tholeiitic rock suite with systematic variations of silicon dioxide and trace element contents over time. The caldera-forming eruptions initially produced some rhyodacite and rhyolite, but most of their products are basaltic andesite. 20th century activity produced mostly basaltic rocks until 2008, when basaltic andesite reappeared. Vents in the northwestern sector of the caldera produce more mafic magmas than those in the southeastern. Okmok is the principal source of prehistoric obsidians in the Aleutians, as far as the Alaska Peninsula more than from the volcano; the so-called "Group I" obsidian in Alaskan archaeological sites may come from there.The Okmok rocks contain only small quantities of phenocrysts, which include clinopyroxene, olivine and plagioclase; rhyolites also contain augite pyroxene, hypersthene and titanomagnetite. Water and ice interaction has converted some of the basalts to palagonite. The rocks have a typical composition for volcanic arc magmas, with enrichment of elements presumably derived from subducted sediments and sediment-derived fluids. The subducting plate releases fluids into the overlying mantle, which ascends under Okmok and melts at temperatures of to form basaltic magmas. The melts are relatively water-poor and reduced, which along with other factors results in tholeiitic magmas. Older models have a tectonic boundary close to Okmok facilitate the ascent of tholeiitic melts. The andesite and rhyolite probably form through fractional crystallization of the basalts.
Climate, vegetation and fauna
The Aleutian Islands have cloudy and rainy weather, with frequent storms in winter and fog in summer. Mean annual temperatures are. There is frequent snow cover, except on recent lava flows. The mountain obstructs the airflow, thus on the eastern side cloud cover is less. The closest weather station is at Dutch Harbor, and may not reflect the climate at Okmok. During the ice ages >55,000 and 24,000–12,000 years ago, the mountain was covered by glaciers. Minor glacier advances may have taken place between 7,500–5,500 and 3,500–2,000 years ago.Tussock grass and tundra covers the lower parts of the volcano, with numerous flowers budding during late summer. The upper parts of the edifice above are bare. Animals include red foxes, reindeer and numerous birds, and marine life occurs in the seas surrounding Umnak.
Eruption history
Volcanic activity on northeastern Umnak Island commenced about 2.1–1.7 million years ago, eventually giving rise to Okmok volcano. A single rhyolitic flow was emitted on the northern flank during the Pleistocene. Older volcanic features, such as the Pleistocene Tulik, either formed in ice or show traces of glacial erosion. Effusive eruptions characterize the activity of the volcano, except for the two large caldera-forming eruptions during the Holocene: "Okmok I" about 12,000 years ago and "Okmok II" in 43 BCE. The occurrence of these explosive eruptions may be due to the accumulation of volatile-rich basaltic andesite magmas under the volcano.The pre-Holocene volcanic history of Okmok is poorly known. Tephra from Okmok has been recovered in marine sediment cores from the Bering Sea. A large eruption 64,500 years ago produced about of dense rock equivalent, which forms the "SR4" tephra in the Bering Sea. The volcano was probably glaciated during that time.
Twelve separate vents erupted inside the caldera since the last caldera-forming eruption, forming tuff cones, maars and cinder cones. Some eruptions began underwater and produced hyaloclastite and pillow lavas. The intracaldera cones are not precisely dated but Cone D is the oldest vent, at 2,000–1,000 years. Subsequent activity formed tuff cones until about 1,000 years before present, Cone F probably between 400 and 1,000 years before present and Cone E 400 years ago. Outside of the caldera, a thick base surge deposit was emplaced on the western side of Okmok 1,500 years ago and mudflows 400–300 years ago. Deposits close to Kettle Cape imply that eruptions capable of depositing ash there took place on average every 150 years during the Holocene. After the 43 BCE eruption, the rate was about one eruption every 75 years. The magma supply rate since that eruption amounts to. More than 60 tephra layers were emplaced after the Okmok I eruption. Seismic swarms and increased seismicity occurred in 2001 and 2009.