The Aleutian Arc is a large volcanic arc in the U.S. state of Alaska. It consists of a number of active and dormant volcanoes that have formed as a result of subduction along the Aleutian Trench. Although taking its name from the Aleutian Islands, this term is a geologic grouping rather than a geographic one, and the Aleutian Arc extends through the Alaska Peninsula following the Aleutian Range to the Aleutian Islands.
The Aleutian Arc reflects subduction of the Pacific Plate beneath the North American Plate. It extends 3,000 km (1,900 mi) from the Kamchatka Peninsula in the west to the Gulf of Alaska in the east. Unimak Pass at the southwestern end of the Alaska Peninsula marks the eastward transition from an intra-oceanic in the west to a continental arc in the east. Due to the arcuate geometry of the trench, the relative velocity vector changes from almost trench-normal in the Gulf of Alaska to almost trench-parallel in the west. Along the oceanic part of the subduction zone, convergence varies from 6.3 cm (2.5 in) per year to the north-northwest in the east to 7.4 cm (2.9 in) per year towards the northwest in the west.
Volcanoes within this arc include:
The Alaska Volcano Observatory (AVO) is a joint program of the United States Geological Survey, the Geophysical Institute of the University of Alaska Fairbanks, and the State of Alaska Division of Geological and Geophysical Surveys (ADGGS). AVO was formed in 1988, and uses federal, state, and university resources to monitor and study Alaska's volcanology, hazardous volcanoes, to predict and record eruptive activity, and to mitigate volcanic hazards to life and property. The Observatory website allows users to monitor active volcanoes, with seismographs and webcameras that update regularly. AVO now monitors more than 20 volcanoes in Cook Inlet, which is close to Alaskan population centers, and the Aleutian Arc due to the hazard that plumes of ash pose to aviation.AVO is located in Anchorage on the campus of Alaska Pacific University.Aleutian Basin
The Aleutian Basin is an oceanic basin under the southwestern Bering Sea. While the northeastern half of the Bering Sea overlies the North American Plate in relatively shallow water, the Aleutian Basin consists of oceanic plate—the remnant of the Kula Plate that was mostly subducted under the North American Plate.Subduction of the Kula Plate ceased after the creation of the Aleutian Trench to its south. What remained of the Kula Plate attached to the North American Plate. This former subduction zone is now the Beringian Margin, which now hosts sixteen submarine canyons, including Zhemchug Canyon, the world's largest.The deep-water part of the Bering Sea is separated into the Commander and Bowers basins by the submarine Shirshov Ridge and Bowers Ridge. The Commander Basin occupies the western part of the Bering Sea, with the Shirshov Ridge on its eastern border. The Shirshov Ridge extends 750 km southward from the Russian Olyutorskii Peninsula to connect with Bowers Ridge. The Bowers Ridge extends in the form of an arc over approximately 900 km from the Aleutian Islands Arc to the northwestern termination, where it meets Shirshov Ridge. This former island arc, Bowers Ridge, is a prominent semi-circular-shaped geological that meets the Aleutian arc and, together with the Aleutians, bounds Bowers Basin.
The northern part of the Shirshov Ridge formed 95 My before the present. The ridge grows younger as it goes south, with the southern part of the Shirshov Ridge formed 33 My ago (Early Oligocene). Bowers Ridge was formed 30 My before the present (Late Oligocene).Aleutian Islands
The Aleutian Islands (; Russian: Алеутские острова; Aleut: Tanam Unangaa, literally "Land of the Aleuts", possibly from Chukchi aliat, "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a chain of 14 large volcanic islands and 55 smaller ones belonging to both the U.S. state of Alaska and the Russian federal subject of Kamchatka Krai. They form part of the Aleutian Arc in the Northern Pacific Ocean, occupying an area of 6,821 sq mi (17,666 km2) and extending about 1,200 mi (1,900 km) westward from the Alaska Peninsula toward the Kamchatka Peninsula in Russia, and mark a dividing line between the Bering Sea to the north and the Pacific Ocean to the south. Crossing longitude 180°, at which point east and west longitude end, the archipelago contains both the westernmost part of the United States by longitude (Amatignak Island) and the easternmost by longitude (Semisopochnoi Island). The westernmost U.S. island in real terms, however, is Attu Island, west of which runs the International Date Line. While nearly all the archipelago is part of Alaska and is usually considered as being in the "Alaskan Bush", at the extreme western end, the small, geologically related Commander Islands belong to Russia.
The islands, with their 57 volcanoes, form the northernmost part of the Pacific Ring of Fire. Physiographically, they are a distinct section of the larger Pacific Border province, which in turn is part of the larger Pacific Mountain System physiographic division.
These Islands are most known for the battles and skirmishes that occurred there during the Aleutian Islands Campaign of World War II. It was one of only two attacks on the United States during that war.Aleutian Range
The Aleutian Range is a major mountain range located in southwest Alaska. It extends from Chakachamna Lake (80 miles/130 km southwest of Anchorage) to Unimak Island, which is at the tip of the Alaska Peninsula. It includes all of the mountains of the Peninsula. The Aleutian Range is special because of its large number of active volcanoes, which are also part of the larger Aleutian Arc. The mainland part of the range is about 600 miles (1000 km) long. The Aleutian Islands are (geologically) a partially submerged western extension of the range that stretches for another 1,600 km (1000 mi). However the official designation "Aleutian Range" includes only the mainland peaks and the peaks on Unimak Island. The range is almost entirely roadless wilderness. Katmai National Park and Preserve, a large national park within the range, must be reached by boat or plane.
The core Aleutian Range can be divided into three mountain groups. Listed from southwest to northeast, they are:
Mountains of the Alaska Peninsula and Unimak Island
Neacola MountainsSee Aleutian Islands for the continuation of the range to the west of Unimak Island. Just to the north of the Aleutian Range are the Tordrillo Mountains, the southeasternmost extent of the Alaska Range.Selected mountains:
Mount Redoubt (3,108 m), Chigmit Mountains
Iliamna Volcano (3,054 m), Chigmit Mountains
Mount Neacola, (2,873 m), Neacola Mountains
Mount Shishaldin (2,857 m), Unimak Island
Mount Pavlof (2,715 m), Alaska Peninsula
Mount Veniaminof (2,508 m), Alaska Peninsula
Isanotski Peaks (2,446 m), Unimak Island
Mount Denison (2,318 m), Alaska Peninsula
Mount Griggs, (2,317+ m), Alaska Peninsula
Mount Douglas (2,153 m), Alaska Peninsula
Mount Chiginagak (2,134 m), Alaska Peninsula
Double Peak (2,078 m), Chigmit Mountains
Mount Katmai (2,047 m), Alaska Peninsula
Pogromni Volcano (2,002 m), Unimak Island
Mount Okmok (1,073 m), Fox IslandsAleutian Ridge
The Aleutian Ridge is located in the western part of the Aleutian Arc in the Bering Sea.Aleutian Trench
The Aleutian Trench (or Aleutian Trough) is an oceanic trench along a convergent plate boundary which runs along the southern coastline of Alaska and the Aleutian islands. The trench extends for 3,400 km from a triple junction in the west with the Ulakhan Fault and the northern end of the Kuril–Kamchatka Trench, to a junction with the northern end of the Queen Charlotte Fault system in the east. It is classified as a "marginal trench" in the east as it runs along the margin of the continent. The subduction along the trench gives rise to the Aleutian arc, a volcanic island arc, where it runs through the open sea west of the Alaska Peninsula. As a convergent plate boundary, the trench forms part of the boundary between two tectonic plates. Here, the Pacific Plate is being subducted under the North American Plate at a dip angle of nearly 45°. The rate of closure is 3 inches (76 mm) per year.Aleutian subduction zone
The Aleutian subduction zone is a ~2500 mile-long convergence boundary between the North American Plate and the Pacific Plate, that extends from the Alaska Range to the Kamchatka Peninsula. Here, the Pacific Plate is being subducted underneath the North American plate and the rate of subduction changes from west to east from 7.5 cm/yr to 5.1 cm/yr. The Aleutian subduction zone includes two prominent features, the Aleutian arc and the Aleutian trench. The island arc was created via volcanic eruptions from dehydration of the subducting slab at ~100 km depth. The trench is a narrow and deep morphology that occurs between the two converging plates as the subucting slab dives beneath the overriding plate.Andesite line
The andesite line is the most significant regional geologic distinction in the Pacific Ocean basin. It separates the mafic basaltic volcanic rocks of the Central Pacific Basin from the partially submerged continental areas of more felsic andesitic volcanic rock on its margins. The andesite line parallels the subduction zones and deep oceanic trenches around the Pacific basin. It is the surface expression of melting within and above the plunging subducting slab. It follows the western edge of the islands off California and passes south of the Aleutian Arc, along the eastern edge of the Kamchatka Peninsula, the Kuril Islands, Japan, the Mariana Islands, Yap, Palau, the Solomon Islands, Fiji, Tonga, and New Zealand's North Island. The dissimilarity continues northeastward along the western edge of the Andes mountains of South America to Mexico, returning then to the islands off California. Indonesia, the Philippines, Japan, New Guinea, and New Zealand lie outside the andesite line.
Within the closed loop of the andesite line are most of the deep troughs, submerged volcanic mountains, and oceanic volcanic islands that characterize the Pacific basin. It is here that basaltic lavas gently flow out of rifts to build huge dome-shaped volcanic mountains whose eroded summits form island arcs, chains, and clusters. Outside the andesite line, volcanism is of the explosive type. The Pacific Ring of Fire runs parallel to the line and is the world's foremost belt of explosive volcanism.
The term andesite line predates the geologic understanding of plate tectonics. The term was first used in 1912 by New Zealand geologist Patrick Marshall to describe the distinct structural and volcanologic boundary extending from east of New Zealand to Fiji and north of the New Hebrides and the Solomon Islands.Bowers Ridge
The Bowers Ridge is located in the southern part of the Aleutian Basin. It extends over 900 km in an arc, starting in the southeast at the Aleutian Arc and terminating to the northwest at the Shirshov Ridge. The Bowers Ridge arc separates the Aleutian Basin from the Bowers Basin, which it encloses. The ridge is not currently seismically active. The northern slope of the ridge is steeper than the southern slope. On the Aleutian side, the ridge is rimmed by a trough filled with a sedimentary sequence 9–10 km thick.The average age of the Bowers Ridge is about 30 My (Late Oligocene). Rock from the upper part of the Bowers Ridge was determined to be of Miocene age. Hence the Bowers Ridge and the southern part of the Shirshov Ridge are of roughly the same age since the adjacent region of the Shirshov Ridge is ddated at 33 My B.P. (Early Oligocene).Commander Basin
The Commander Basin (alternately Komandorsky Basin) is located between the Shirshov Ridge and the Kamchatka Peninsula. Its southern boundary is the Aleutian arc (see figure) and occupies the western part of the Bering Sea. The Kamchatka Strait provides a deep water access to the basin from the southwest.
The basins sedimentary cover is less than 2 km thick. In the southwestern portion of the basin near the Ulakahn Fault, linear magnetic anomalies associated with the Early Miocene have been identified. The magnetic anomalies support a sequential opening of the Commander Basin resulting from stresses on the interface between the Eurasian and Pacific plates. The structures of the Bering Sea floor at the Commander Basin were created 17 to 21 Million years before the present.The Commander Basin floor is a horizontal plain 3800–3900 m deep. It is covered with 2000–6000 m of sediment overlying an oceanic crust which is 12–14 km thick. Active spreading in the Commander Basin occurred between 40 and 10 Myr ago, with subduction of the basin floor along the Ulakhan fault underneath the Kamchatka peninsula. There are four major fracture zones in the basin and magnetic lineations have been detected in the basin. Except in the southern end of the basin, the spreading center has been subducted.The water in the basin circulates in a cyclonic gyre, with the western Kamchatka Current flowing southward along the Kamchatka Peninsula. The northward leg of the gyre is outside of the Commander Basin, and is carried by the Bering Slope Current, which flows along the edge of the continental shelf at the eastern edge of the Aleutian Basin. To the south, the Commander Basin connects to the North Pacific through the 4.4 km wide, 4,420 meter deep Kamchatka Strait and the 2.0 km wide, 2,000 meter deep Near Strait.Flinn–Engdahl regions
The Flinn–Engdahl regions (or F–E regions) are a division of the Earth into seismic zones. In seismology, it is the standard of localizing earthquakes. The scheme was proposed in 1965 by Edward A. Flinn and E. R. Engdahl. The first official definition was published in 1974 and a revision in 1995. Because each F-E region is composed of 1x1 degree blocks with integer latitudes and longitudes, the borders of the F–E regions may differ from political boundaries. For instance, the F–E region 545 ("Northern Italy") also includes parts of France, Switzerland, Austria and Slovenia. After the 1995 revision there are 754 F–E regions, subsequently numbered from 1 to 757 with three gaps (172, 299 and 550) at dissolved regions. The regions are grouped into 50 larger seismic regions.Korovin Volcano
Korovin Volcano is the highest point on Atka Island in the Aleutian Islands chain Alaska, United States. Korovin is a side vent to the main Atka shield volcano. However, Korovin is the highest point on the island.Mount Cleveland (Alaska)
Mount Cleveland (also known as Cleveland Volcano) is a nearly symmetrical stratovolcano on the western end of Chuginadak Island, which is part of the Islands of Four Mountains just west of Umnak Island in the Fox Islands of the Aleutian Islands of Alaska. Mt. Cleveland is 5,675 ft (1,730 m) high, and one of the most active of the 75 or more volcanoes in the larger Aleutian Arc. Aleutian natives named the island after their fire goddess, Chuginadak, who they believed inhabited the volcano. In 1894 a team from the U.S. Coast and Geodetic Survey visited the island and gave Mount Cleveland its current name, after then-president Grover Cleveland.
One of the most active volcanoes in the Aleutian Arc, Cleveland has erupted at least 22 times in the last 230 years. A VEI 3 eruption in 1944 produced the arc's only known volcanic fatality. Most recently Mount Cleveland has erupted three times in 2009, twice in 2010, once in 2011 and in 2016 and 2017. The volcano's remoteness limits opportunities for its study, and the Alaska Volcano Observatory relies heavily on satellites for monitoring. The volcano is primarily hazardous to aircraft; many of the flights over the north Pacific approach the vicinity of the volcano, and volcanic ash released from eruptions can damage sensitive electronic equipment and sensors.Mount Spurr
Mount Spurr is a stratovolcano in the Aleutian Arc of Alaska, named after United States Geological Survey geologist and explorer Josiah Edward Spurr, who led an expedition to the area in 1898. The Alaska Volcano Observatory (AVO) currently rates Mount Spurr as Level of Concern Color Code Green. The mountain is known aboriginally by the Dena'ina Athabascan name K'idazq'eni, literally 'that which is burning inside'.Mount Spurr, the highest volcano of the Aleutian arc, is a large lava dome constructed at the center of a roughly 5 km-wide horseshoe-shaped caldera that is open to the south. The volcano lies 80.87 miles west of Anchorage and NE of Chakachamna Lake. The caldera was formed by a late-Pleistocene or early Holocene debris avalanche and associated pyroclastic flows that destroyed an ancestral Spurr volcano. The debris avalanche traveled more than 15.5 miles to the SE, and the resulting deposit contains blocks as large as 100m in diameter. Several ice-carved post-caldera domes lie in the caldera. Present Mt. Spurr is the highest of the post-caldera. This regrown summit peak of Spurr experienced a heating event in 2004 which created a small crater lake. By 2008, the summit crater had cooled enough to have begun to have accumulated significant amounts of snow again. The youngest post-caldera dome, Crater Peak (2309m, 7575 ft), formed at the breached southern end of the caldera about 3.2 km south of Spurr, has been the source of about 40 identified Holocene tephra layers. Spurr's two historical eruptions, from Crater Peak in 1953 and 1992, deposited ash on the city of Anchorage. Crater Peak has a summit crater that is itself slightly breached along the south rim; the north wall of the crater exposes the truncated remains of an older dome or lava lake. Before the 1992 eruption, a small crater lake occupied the bottom of Crater Peak's crater.
As with other Alaskan volcanoes, the proximity of Spurr to major trans-Pacific aviation routes means that an eruption of this volcano can significantly disrupt air travel. Volcanic ash can cause jet engines to fail.Pogromni Volcano
Pogromni Volcano is a stratovolcano on Unimak Island in the Aleutian Islands. Near it are 5 cinder cones, and a mountain called Pogromni's Sister.
Pogromni is old and eroded with a single glacier on its flank and base. Eruptions have been attributed to it in 1795, 1796, 1826, 1827, and 1830, but historic eruptions attributed to it may have come from nearby Westdahl volcano.The volcano's name comes from the Russian Погромный, meaning "characterised by violent outbreak." This naming refers to a volcanic eruption, but shares the same etymology as pogrom, a Russian word referring to violent riots against an ethnic group.Robert R. Coats
Robert Roy Coats (1910–1995) was an American geologist known for his studies of the Aleutian Islands and his exhaustive report of Elko County, Nevada. He was born in Toronto, Canada, and grew up in Marshalltown, Iowa and Seattle, Washington. He graduated valedictorian of his high school class in Seattle at the age of 16, and attended the University of Washington, where he received both a B.S. and M.S. degree in Geology and Mining (1931 and 1932). He continued graduate work at the University of California, Berkeley, receiving his doctorate in 1938, with a thesis on the ore bodies of the Virginia City mining district in Nevada. He was known as an eccentric and brilliant student.
In 1937, Coats took a teaching job at the University of Alaska Fairbanks. He left that job for a post with the U.S. Geological Survey in 1939, in Washington, D.C.. As part of the Alaska Branch of the USGS, he continued working in Alaska, mapping in the Chichagof, Anikovik, Nome, Solomon, Kigluaik and Kobuk River areas, among others. During World War II, he spent time in the Aleutian Islands, returning in 1946 as part of the Survey’s Volcano Project. His field work in the Aleutians led to his 1962 paper (see references) on the origin of the Aleutian island arc. That prescient synthesis of tectonics and magmatism of the Aleutian arc contained several of the essential ideas of the subsequent paradigm of plate tectonics. He correctly interpreted:
that the origin of the dipping zone of seismicity beneath the Aleutian arc occurs along a megathrust above underthrusting oceanic crust and its sedimentary cover;
the relation between the position of the active volcanoes and depth to the underthrust oceanic crust;
the role of fluids derived from the down-going slab and magmatic differentiation in determining compositions of erupted volcanic rocks (See Figure 1).Coats’s colleagues from the Alaska Branch of the USGS have commented on his extraordinary ability to comprehend the detailed geologic history of an area with only the most cursory examination. Visiting the Aleutian islands of Buldir, Adak, Kanaga, Semisophochnoi, Gareloi, Kiska and Amchitka (among others), typically for 1–2 days in notoriously bad weather, he produced geologic reports of such detail, insight and quality that they remain today, even in the face of much additional work, the prime source for understanding Aleutian geology.
From 1951 to 1954, Coats was engaged in the search for radioactive granitic rocks, and in field studies of rhyolitic extrusive rocks. His field work took him to New England, and to all of the western states. In 1954, he began a long and productive period of work in Elko County, Nevada, where he studied rhyolites, stratigraphy and structure, and mapped the geology of the Jarbidge, Owyhee, Mountain City and Tuscarora 15’ quadrangles. He relocated to Menlo Park, California in 1956, but spent most summers with his family in a field camp in Nevada. Many of his 70 publications were based on his work in this period, which culminated in his 1987 synthesis report on the Geology of Elko County, a complex area the size of Connecticut and Massachusetts together. The 1:250,000 scale map for the report has a total of 109 mapping units, with areas mapped as small as 10 ha.
Although most of his professional career was spent outside of academia, Coats enjoyed mentoring and teaching one-on-one. Many of his former field assistants and protégés have gone on to illustrious careers in universities and government service.
Coats was married for 58 years to Elizabeth Robinson Coats. He died peacefully at his home in Aptos, California, on January 12, 1995. He was survived by two sons (David of Minneapolis and Robert of Berkeley) and a daughter (Katherine Mrache, of Aptos), and seven grandchildren.Segula Island
Segula Island (Aleut: Chiĝulax̂) is an island in the Rat Islands archipelago of the western Aleutian Islands, Alaska. It consists of a Holocene stratovolcano, called Segula Volcano.
Segula Island is three to four miles in diameter, and is located about 15 miles (24 km) east of Kiska Island. The island supports a large auklet colony; one of only nine in the Aleutian Island chain.Shirshov Ridge
The Shirshov Ridge is located on the eastern border of the Commander Basin below the Kamchatka Peninsula. It extends directly southward for a distance of 750 km toward the Aleutian arc in the eastern part of the Bering Sea (see figure).The Shirshov Ridge rises 2–2.5 km above the surrounding basins. Although not immediately evident from the morphology, the Shirshov Ridge southern end extends to meet the northwestern-most portion of Bowers Ridge. The continuity is evident in structural maps of basement rocks, in magnetic surveys, and in free air gravity field surveys. The connection has been distorted by a sinistral shear fault, which has displaced the point of connection by ~350 km.Tanaga (volcano)
Tanaga (Aleut: Kusuuginax̂) is a 5,924-foot (1,806 m) stratovolcano in the Aleutian Range of the U.S. state of Alaska. There have been three known eruptions since 1763. The most recent was in 1914 and produced lava flows.