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121-670: 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

242-476: 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 870–900 °C (1,600–1,650 °F) have been inferred for dacitic magmas in the Aniakchak II eruption;

363-432: A density current that deposits its sediments as turbidites . When the river water is less dense than the basin water, as is typical of river deltas on an ocean coastline, the delta is characterized by hypopycnal flow in which the river water is slow to mix with the denser basin water and spreads out as a surface fan. This allows fine sediments to be carried a considerable distance before settling out of suspension. Beds in

484-412: A wave-cut terrace is found at the former altitude, but the appearance of the lava domes implies a higher water level. Lake sediments built up in numerous places inside the caldera. Water eventually overflowed a stable sill , thus establishing a constant water level. About 1,860 years before present , it drained catastrophically through a notch in the northeastern rim, forming The Gates gorge, in one of

605-493: A common location for civilizations to flourish due to access to flat land for farming, freshwater for sanitation and irrigation , and sea access for trade. Deltas often host extensive industrial and commercial activities, and agricultural land is frequently in conflict. Some of the world's largest regional economies are located on deltas such as the Pearl River Delta , Yangtze River Delta , European Low Countries and

726-699: A cooling resembling the Little Ice Age , leading to a climate transition around the Mediterranean . Several cultural changes, such as the end of the Arctic Nordic Stone Age and the westward spread of the Seima-Turbino culture in western Eurasia, have been linked to the Aniakchak II eruption, although a direct causal link is unproven. A sulfur yield of 32 ± 11 teragrams has been reconstructed, making Aniakchak II one of

847-418: A decline in human activity 3,500 years ago may have been a consequence of volcanic eruptions like Aniakchak II and the "Jarvis Creek" event of Hayes volcano. The eruption caused a decline in caribou populations of the western Arctic. There is evidence that peat accumulation at Cape Espenberg was interrupted by the eruption, and vegetation growth was slow for up to a century at 1,100 kilometers (680 mi) from

968-545: A delta but enter into the sea in the form of an estuary . Notable examples include the Gulf of Saint Lawrence and the Tagus estuary. In rare cases, the river delta is located inside a large valley and is called an inverted river delta . Sometimes a river divides into multiple branches in an inland area, only to rejoin and continue to the sea. Such an area is called an inland delta , and often occurs on former lake beds. The term

1089-533: A fan. The more often the flow changes course, the shape develops closer to an ideal fan because more rapid changes in channel position result in a more uniform deposition of sediment on the delta front. The Mississippi and Ural River deltas, with their bird's feet, are examples of rivers that do not avulse often enough to form a symmetrical fan shape. Alluvial fan deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape. Most large river deltas discharge to intra-cratonic basins on

1210-420: A few centuries before the eruption new basaltic melts had entered the system. Magma ascended along ring faults on the caldera floor. The eruption was initially magmatic, then became phreatomagmatic when decreasing magma ascent speed allowed water from the lakes in the caldera to flow into the vent. Later, water inflow lessened, and activity returned to magmatic. The magmas became more mafic and less viscous over

1331-399: A few main distributaries. Once a wave-dominated or river-dominated distributary silts up, it is abandoned, and a new channel forms elsewhere. In a tidal delta, new distributaries are formed during times when there is a lot of water around – such as floods or storm surges . These distributaries slowly silt up at a more or less constant rate until they fizzle out. A tidal freshwater delta is

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1452-545: A flat arid area splits into channels that evaporate as it progresses into the desert. The Okavango Delta in Botswana is one example. See endorheic basin . The generic term mega delta can be used to describe very large Asian river deltas, such as the Yangtze , Pearl , Red , Mekong , Irrawaddy , Ganges-Brahmaputra , and Indus . The formation of a delta is complicated, multiple, and cross-cutting over time, but in

1573-573: A hypocynal delta dip at a very shallow angle, around 1 degree. Fluvial-dominated deltas are further distinguished by the relative importance of the inertia of rapidly flowing water, the importance of turbulent bed friction beyond the river mouth, and buoyancy . Outflow dominated by inertia tends to form Gilbert-type deltas. Outflow dominated by turbulent friction is prone to channel bifurcation, while buoyancy-dominated outflow produces long distributaries with narrow subaqueous natural levees and few channel bifurcations. The modern Mississippi River delta

1694-563: A large alluvial fan at the outlet and numerous boulders with sizes of up to 27 meters (89 ft) along the Aniakchak River. The flood destroyed a village on Aniakchak Bay at the Pacific coast, 40 kilometers (25 mi) from the volcano. It appears to have displaced humans from the mouth of the Aniakchak River, where a two-century hiatus in human occupation has been recognized. The lake did not drain entirely during this flood, with

1815-526: A large contiguous magma body under Aniakchak. Specific hazards include: The main danger from future activity at Aniakchak is high ash clouds. Aircraft flying into volcanic ash clouds can suffer engine failures, and Aniakchak is located beneath one of the major air routes of the North Pacific. Precipitating volcanic ash can smother plants and make roads slippery, irritate eyes and lungs, and damage machinery. Ashfall would most likely occur north to east of

1936-411: A larger initial event followed by a lesser one. The column then collapsed, and highly mobile pyroclastic flows consisting of andesite and rhyodacite swept the volcano, filling in valleys, making turns and moving upslope over topography. They had sufficient speed to cross 700 meters (2,300 ft) high topography 20 kilometers (12 mi) away from the vent. During the initial stages of the eruption,

2057-439: A mature delta with a distributary network. Another way these distributary networks form is from the deposition of mouth bars (mid-channel sand and/or gravel bars at the mouth of a river). When this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth bar, which splits the river into two distributary channels. A good example of

2178-423: A sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary". Drowned coastal river valleys that were inundated by rising sea levels during the late Pleistocene and subsequent Holocene tend to have dendritic estuaries with many feeder tributaries. Each tributary mimics this salinity gradient from its brackish junction with the mainstem estuary up to

2299-467: A significant water body remaining inside the caldera that left a terrace 82 meters (269 ft) above the present-day Surprise Lake. The subaqueous eruptions and the abrupt emptying of the lake have drawn scientific interest, and the breakout flood has been compared to similar floods at other volcanoes like Lake Taupō , Lake Tarawera (both New Zealand), Mount Okmok (Alaska) and Ksudach (Russia), and crater breakout floods on Mars . Post-caldera activity

2420-447: A simple delta three main types of bedding may be distinguished: the bottomset beds, foreset/frontset beds, and topset beds. This three-part structure may be seen on small scale by crossbedding . Human activities in both deltas and the river basins upstream of deltas can radically alter delta environments. Upstream land use change such as anti-erosion agricultural practices and hydrological engineering such as dam construction in

2541-483: A topographic barrier may have existed on the southeastern side of Aniakchak. The flows buried a surface of about 2,500 square kilometers (970 sq mi), running over distances exceeding 60 kilometers (37 mi) to Bristol Bay and the Pacific Ocean. When they plunged into the sea, the flows triggered up to 7.8 meters (26 ft) high tsunamis on the northern shore of Bristol Bay. It is possible that there

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2662-511: A western section of the North American Plate ; the northern side is generally flat and marshy, a result of millennia of erosion and general seismic stability. The northern and southern shores are likewise quite different. The northern Bristol Bay coastal side is generally turbid and muddy, experiences tidal extremes, and is relatively shallow; the Pacific side, which is also known as the "ring of fire", has relatively small tidal activity and

2783-551: Is a good example of a fluvial-dominated delta whose outflow is buoyancy-dominated. Channel abandonment has been frequent, with seven distinct channels active over the last 5000 years. Other fluvial-dominated deltas include the Mackenzie delta and the Alta delta. A Gilbert delta (named after Grove Karl Gilbert ) is a type of fluvial-dominated delta formed from coarse sediments, as opposed to gently sloping muddy deltas such as that of

2904-644: Is a misspelling. The name "Aniakchak" is probably Alutiiq and may be related to the Yupik word anyaraq 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

3025-424: Is a triangular landform created by the deposition of the sediments that are carried by the waters of a river , where the river merges with a body of slow-moving water or with a body of stagnant water. The creation of a river delta occurs at the river mouth , where the river merges into an ocean , a sea , or an estuary , into a lake , a reservoir , or (more rarely) into another river that cannot carry away

3146-591: Is called by the Ionians ", including describing the outflow of silt into the sea and the convexly curved seaward side of the triangle. Despite making comparisons to other river systems deltas, Herodotus did not describe them as "deltas". The Greek historian Polybius likened the land between the Rhône and Isère rivers to the Nile Delta, referring to both as islands, but did not apply the word delta. According to

3267-513: Is classified as a "high-threat volcano" by the United States Geological Service . Future eruptions will most likely occur within the caldera, in particular its southwestern sector. Explosive eruptions may occur if the magma is volatile-rich or it interacts with water inside the caldera. Degassed magma would produce lava flows. Large caldera-forming eruptions are improbable in the near future, as there does not seem to be

3388-430: Is mostly by boat plane to Surprise Lake. 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

3509-662: Is one of the most significant tephras of the Northwest Pacific region and has been used as a stratigraphic marker owing to widespread, pristine appearance and characteristic color. Tephra has been found at Chignik Bay , in the Ahklun Mountains , Zagoskin Lake on St. Michael Island, Lake Hill on St. Paul Island , Cape Espenberg and Whitefish Lake on the Seward Peninsula (western Alaska), lakes in

3630-604: Is possible that the Brooks River Archeological District on the northern Alaska Peninsula became the destination of survivors. Over Alaska and Beringia , it is conceivable that the eruption forced humans to rely more on marine resources and thus prompted the archaeological transition from the Arctic small tool tradition to the Norton tradition and human migration to coastal sites. In Central Alaska,

3751-474: Is rarely visited; on average there are fewer than 300 visitors every year. 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. Access

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3872-548: Is so named because the shape of the Nile Delta approximates the triangular uppercase Greek letter delta . The triangular shape of the Nile Delta was known to audiences of classical Athenian drama ; the tragedy Prometheus Bound by Aeschylus refers to it as the "triangular Nilotic land", though not as a "delta". Herodotus 's description of Egypt in his Histories mentions the Delta fourteen times, as "the Delta, as it

3993-481: Is still active. Sometimes volcanic ash is blown away by wind. There are active fumaroles and hot springs in the caldera, mostly around the 1931 vents and along Surprise Lake respectively. Water temperatures in the hot springs reach 21–25 °C (70–77 °F). Helium and carbon dioxide emissions have been noted from a spring next to Surprise Lake. The magma chamber of Aniakchak is estimated to hold about 129 × 10 calories (5.4 × 10 J) of heat. The volcano

4114-487: 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

4235-525: Is the largest known eruption at Aniakchak, and one of the largest Holocene eruptions in North America, comparable with the 1912 Katmai and early Holocene Mount Mazama events. A volcanic explosivity index of 6 or 7 has been assigned to the eruption. It yielded more than 50 cubic kilometers (12 cu mi) in rock (pyroclastic flows and tephra), and total tephra volume may have reached 114 cubic kilometers (27 cu mi). The initial stage of

4356-502: 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

4477-725: The Alaska Peninsula National Wildlife Refuge , and the Izembek National Wildlife Refuge . The most active volcano along the volcanic mountain range is Pavlof Volcano which is more than 8,251 feet (2,515 meters)(see also: Aleutian Arc ). The southern side of the Alaska Peninsula is rugged and mountainous, created by the uplifting tectonic activity of the North Pacific Plate subsiding under

4598-648: The British Isles , and in Finland . The Aniakchak tephra has been used to date sediments and scientific findings between Greenland, Canada and the Chukchi Sea. Vegetation and human populations on the Alaska Peninsula were devastated by the eruption. The pyroclastic flows would have killed everything in their path and buried the remains. The landscape remained unvegetated for more than a millennium. Together with eruptions of neighboring Black Peak and Veniaminof,

4719-662: The Greater Tokyo Area . The Ganges–Brahmaputra Delta , which spans most of Bangladesh and West Bengal and empties into the Bay of Bengal , is the world's largest delta. The Selenga River delta in the Russian republic of Buryatia is the largest delta emptying into a body of fresh water, in its case Lake Baikal . Researchers have found a number of examples of deltas that formed in Martian lakes . Finding deltas

4840-849: The Kenai Peninsula and Seward Peninsula . The base of the Alaska Peninsula extends outward from the end of the Alaska Range . The Aleutian Range is a very active volcanic mountain range which runs along the entire length of the Peninsula. Within it lie Wildlife Refuges, including the Katmai National Park and Preserve , the Aniakchak National Monument and Preserve and the Becharof National Wildlife Refuge ,

4961-719: The Moon . The eruption sterilized much of the caldera and killed numerous animals, with Hubbard noting dead birds in the caldera, and ash ingestion resulted in numerous casualties among caribou and reindeer. The total volume of rock reached 0.9 cubic kilometers (0.22 cu mi), making this eruption one of the largest eruptions in Alaska during the 20th century. It consists of three separate tephra units formed by various ash-to-lapilli sized rocks and three lava flows consisting of trachydacite , basaltic andesite and andesite. Several different sources of magma contributed to this eruption, and

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5082-532: The Nile Delta and Colorado River Delta are some of the most extreme examples of the devastation caused to deltas by damming and diversion of water. Historical data documents show that during the Roman Empire and Little Ice Age (times when there was considerable anthropogenic pressure), there was significant sediment accumulation in deltas. The industrial revolution has only amplified the impact of humans on delta growth and retreat. Ancient deltas benefit

5203-593: 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

5324-529: The Saint Elias Mountains of Yukon , Canada. In June, the new vents were still emitting volcanic gases with a smell of sulfur, and Surprise Lake and Aniakchak River were discolored. Lava stopped flowing in July. According to Hubbard, the pre-eruption caldera was a "wonderland" with plants and springs, while describing the post-eruption caldera as "an abomination of desolation" and comparing it to

5445-516: The 11 and 20 May, accompanied by sounds of explosions. The eruption was observed by Jesuit priest and geologist Bernard R. Hubbard , who visited the caldera after the eruption, making this eruption well-documented. It was both explosive and effusive: Explosions at the 1931 Main Crater produced tephra fallout, reaching thicknesses of 40 meters (130 ft) mostly to the northwest. Lava flows issued from Doublet Crater, Main Crater and Slag Heap and filled

5566-431: The Alaska Peninsula also is home to several well-known villages: Cold Bay , King Cove , Perryville , Chignik , Chignik Lake , Chignik Lagoon , and Port Moller . Each is primarily inhabited by Alaska Natives and each, likewise, is mostly dependent on the fishing industry for sustenance. 56°30′N 158°45′W  /  56.5°N 158.75°W  / 56.5; -158.75 River delta A river delta

5687-705: The Alaskan Brooks Range, the Mount Logan icefield at the Alaska-Canada border, and the Bering and Chukchi Seas northwest of Alaska. Thinner tephra has been recovered more than 4,500 kilometers (2,800 mi) from the volcano, in numerous ice cores of Greenland , in Nordan's Pond on Newfoundland , in marine sediment cores east of Greenland, in sediments from Northern Ireland and Wales in

5808-700: The Alaskan Peninsula. The largest post-caldera eruption at Aniakchak took place 400 years ago. It had a volcanic explosivity index of 3–4, destroying Half Cone in a series of Plinian eruptions and emplacing the Cobweb lava flow. Pyroclastic flows and ash fallout reached thicknesses of 40 meters (130 ft), with ash falling 330 kilometers (210 mi) away in north-northeastern direction. The layered eruption deposits crop out in Half Cone. Inflow of new magma and crystallization of old magma probably triggered

5929-479: 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"

6050-515: The Aniakchak II eruption might have depopulated part of the area around Aniakchak. Earthquakes might have alerted inhabitants of the impending catastrophe but they may not have had time to escape to safe distance. The resulting gap in settlement between the eastern and western parts of the Alaska Peninsula may explain why the Alutiiq people and Aleut people are separate. Areas close to the volcano remained uninhabited for more than two millennia, and it

6171-411: The Aniakchak II eruption, and the caldera has been compared to Ceraunius Tholus on Mars . Before the eruption, Aniakchak was a 2,300 meters (7,500 ft) high deeply eroded stratovolcano with two separate magma bodies, one andesitic and the other rhyodacitic , under Aniakchak at least 4.1–5.5 kilometers (2.5–3.4 mi) in depth. These two magma bodies had evolved independently in the time before

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6292-537: The Aniakchak II event, a smaller eruption may have emplaced a tephra layer in the Brooks Range of northern Alaska. Other large caldera-forming eruptions in Alaska took place at Mount Okmok , Fisher Caldera , and Veniaminof, with lesser events at Kaguyak and Black Peak. Unlike them, before the caldera-forming eruption, Aniakchak was a small volcanic edifice. Various dating methods, mostly relying on radiocarbon , have yielded ages of around 3,000–4,000 years for

6413-404: 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

6534-700: The Greek geographer Strabo , the Cynic philosopher Onesicritus of Astypalaea , who accompanied Alexander the Great 's conquests in India , reported that Patalene (the delta of the Indus River ) was "a delta" ( Koinē Greek : καλεῖ δὲ τὴν νῆσον δέλτα , romanized:  kalei de tēn nēson délta , lit.   'he calls the island a delta'). The Roman author Arrian 's Indica states that "the delta of

6655-503: The Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta. It is commonly a result of homopycnal flow. Such deltas are characterized by a tripartite structure of topset, foreset, and bottomset beds. River water entering the lake rapidly deposits its coarser sediments on the submerged face of the delta, forming steeping dipping foreset beds. The finer sediments are deposited on

6776-472: The ability to pile up and accumulate due to the sediment traveling into a steep subduction trench rather than a shallow continental shelf . There are many other lesser factors that could explain why the majority of river deltas form along passive margins rather than active margins. Along active margins, orogenic sequences cause tectonic activity to form over-steepened slopes, brecciated rocks, and volcanic activity resulting in delta formation to exist closer to

6897-441: The area. The bears of the peninsula and Bristol Bay are so numerous because they feed on the world's largest sockeye salmon ( Oncorhynchus nerka ) runs, which occur here in large part because the many large lakes of the peninsula are an important element in their lifecycle. These salmon, after returning from their brief time at sea, swim into the lakes and their contributing streams to spawn. Their offspring, or fry , overwinter in

7018-1256: The basins feeding deltas have reduced river sediment delivery to many deltas in recent decades. This change means that there is less sediment available to maintain delta landforms, and compensate for erosion and sea level rise , causing some deltas to start losing land. Declines in river sediment delivery are projected to continue in the coming decades. The extensive anthropogenic activities in deltas also interfere with geomorphological and ecological delta processes. People living on deltas often construct flood defences which prevent sedimentation from floods on deltas, and therefore means that sediment deposition can not compensate for subsidence and erosion . In addition to interference with delta aggradation , pumping of groundwater , oil , and gas , and constructing infrastructure all accelerate subsidence , increasing relative sea level rise. Anthropogenic activities can also destabilise river channels through sand mining , and cause saltwater intrusion . There are small-scale efforts to correct these issues, improve delta environments and increase environmental sustainability through sedimentation enhancing strategies . While nearly all deltas have been impacted to some degree by humans,

7139-422: The bird's-foot of the Mississippi or Ural river deltas), pushing its mouth into the standing water. As the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope ). As the gradient of the river channel decreases, the amount of shear stress on the bed decreases, which results in the deposition of sediment within

7260-522: The bottom of the Main Crater. Volcanic lightning was reported during the eruption. Ash fell in various communities, including Chignik, Kanakanak , Katmai National Park , Kodiak Island , Nushagak Peninsula , Port Heiden, and Holy Cross 600 kilometers (370 mi) north of the volcano. The ash clouds were thick enough to plunge the land into darkness, and there were widespread problems with radio communications. Ash has been recovered from ice cores in

7381-453: 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

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7502-491: The caldera lake. The Pumice Dome eruption crossed the caldera rim to produce a 3 kilometers (1.9 mi) long lava flow outside the caldera. Distal andesitic tephras found in Greenland ice strata provide possible evidence of two unidentified explosive Aniakchak eruptions, one in 88 CE and another in 536 CE. About 900 ± 80 years ago, Surprise Cone was emplaced inside the remnant caldera lake. Conceivably, its eruption and that of

7623-443: 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. The Alaskan Peninsula was settled about 7,000 years ago by people who practiced hunter-gatherer lifestyles. After being driven away by

7744-601: The caldera. Aniakchak is monitored by the Alaska Volcano Observatory since 1997 through seismometers and satellite images , and collects reports from visitors to the caldera and aircraft to detect renewed activity. The observatory publishes a volcano hazard level ; in case of an eruption, it would coordinate with government agencies and publish updates through the Internet and other means. Alaska Peninsula In literature (especially Russian),

7865-607: The caldera. Southwest of Aniakchak, the Pacific Plate subducts beneath the North America Plate at a rate of about 65 millimeters per year (2.6 in/year). This subduction is responsible for the activity of the 4,000 kilometers (2,500 mi) 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

7986-505: 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

8107-401: The caldera. They are slow, but steam explosions or pyroclastic flows caused by collapses of lava domes can amplify their threat. Snow and ice within the caldera – and during larger eruptions, outside – can melt when impacted by the fallout of hot rocks. The loose volcanic ash on the slopes of Aniakchak can be liquefied by rainfall. Either can produce mudflows , which threaten valleys running from

8228-406: The caldera. While the hot springs and fumaroles are not a threat by themselves, in case of the ascent of new magma, temperatures and carbon dioxide concentrations may rise to dangerous levels. The vents can eject volcanic bombs, large blocks that fall down close to the vent. Landslides or subaqueous explosions can cause floods or local tsunamis from the lakes in the caldera. Hazards exist mainly within

8349-558: 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

8470-476: The central Aleutian Islands to the western Alaska Peninsula, which includes Aniakchak, features some of the largest volcanoes of the arc; the formation of the Aniakchak caldera may be facilitated by a tectonic discontinuity that allows magma to accumulate in the crust. The volcano grew on a westward-sloping basement formed by Mesozoic -Tertiary sedimentary rocks, which crops out south of the volcano and within

8591-450: The channel and a rise in the channel bed relative to the floodplain . This destabilizes the river channel. If the river breaches its natural levees (such as during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a steeper, more stable gradient. Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. Repeated channel-switching events build up

8712-436: The course of the eruption, causing a transition to Strombolian eruptions . Aniakchak is dormant , with occasional seismic activity clustered at shallow depths under the caldera. Satellite imagery has noted ongoing sinking of the caldera floor, with the rate (a few millimeters per year) decreasing over time. The subsidence may be due to the degassing and cooling of magma under the volcano. The magmatic system under Aniakchak

8833-519: The deep and food-abundant depths of these lakes until their migration to the sea in one or two years. Exceptionally large seabird colonies exist along the coast. Additionally, there are large populations of sea mammals in the North Pacific Ocean between the Alaska Peninsula and Kamchatka . This includes harbor seals , ringed seals , northern fur seals , whales , porpoises , sea otters and sea lions . The rugged southern half of

8954-619: The economy due to their well-sorted sand and gravel . Sand and gravel are often quarried from these old deltas and used in concrete for highways , buildings, sidewalks, and landscaping. More than 1 billion tons of sand and gravel are produced in the United States alone. Not all sand and gravel quarries are former deltas, but for ones that are, much of the sorting is already done by the power of water. Urban areas and human habitation tend to be located in lowlands near water access for transportation and sanitation . This makes deltas

9075-586: 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. At least forty eruptions took place during the Holocene , half before and half after the second caldera-forming eruption, equivalent to one eruption every 340 years after

9196-418: The eruption produced rhyodacitic rocks, then both andesite and rhyodacite erupted, and at the end it was andesitic. The pyroclastic flow deposits are rich in pumice and scoria and mostly unwelded. They reach thicknesses exceeding 100 meters (330 ft) where they ponded against pre-existing topography. The eruption produced more than 100 cubic kilometers (24 cu mi) tephra , which fell out north of

9317-418: The eruption, increasing the pressure in the magmatic system until magma began to propagate to the surface. During the course of the eruption, magma composition changed from dacite to basaltic andesite, a typical phenomenon at Alaskan volcanoes and other eruptions of Aniakchak; however, the distinction between the "pink" and "brown" pumices is not due to this compositional gap. Another eruption may have occurred at

9438-479: The eruption. Owing to the multitude of methods, the dates span a wide range, but consensus has developed around a 1628/1627 BCE date derived from ice cores. Other Alaskan volcanoes erupting around that time are Veniaminof and Hayes . Numerous scientific efforts, investigating caldera formation in the Aleutian Volcanic Arc and geology, geophysics, petrology and volcanology, have been carried out on

9559-478: The eruption. Triggered either by the failure of the magma chamber roof or by an earthquake, one of the two magma bodies leaked into the other. At least ten smaller explosive eruptions occurred before the climactic event, which probably occurred during summer. A more than 25 kilometers (16 mi) high eruption column rose over the volcano and produced a lapilli and volcanic ash fallout. Data from ice cores imply that there may have been more than one explosion, with

9680-414: The flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity , which diminishes the ability of the flow to transport sediment . As a result, sediment drops out of the flow and is deposited as alluvium , which builds up to form the river delta. Over time, this single channel builds a deltaic lobe (such as

9801-506: The fresh stream feeding the head of tidal propagation. As a result, the tributaries are considered to be "subestuaries". The origin and evolution of a tidal freshwater delta involves processes that are typical of all deltas as well as processes that are unique to the tidal freshwater setting. The combination of processes that create a tidal freshwater delta result in a distinct morphology and unique environmental characteristics. Many tidal freshwater deltas that exist today are directly caused by

9922-466: The geomorphology and ecosystem. Deltas are typically classified according to the main control on deposition, which is a combination of river, wave , and tidal processes, depending on the strength of each. The other two factors that play a major role are landscape position and the grain size distribution of the source sediment entering the delta from the river. Fluvial-dominated deltas are found in areas of low tidal range and low wave energy. Where

10043-406: The lake bottom beyond this steep slope as more gently dipping bottomset beds. Behind the delta front, braided channels deposit the gently dipping beds of the topset on the delta plain. While some authors describe both lacustrine and marine locations of Gilbert deltas, others note that their formation is more characteristic of the freshwater lakes, where it is easier for the river water to mix with

10164-533: 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

10285-594: The lakewater faster (as opposed to the case of a river falling into the sea or a salt lake, where less dense fresh water brought by the river stays on top longer). Gilbert himself first described this type of delta on Lake Bonneville in 1885. Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into Okanagan Lake in British Columbia and form prominent peninsulas at Naramata , Summerland , and Peachland . In wave-dominated deltas, wave-driven sediment transport controls

10406-583: The land of the Indians is made by the Indus river no less than is the case with that of Egypt". As a generic term for the landform at the mouth of the river, the word delta is first attested in the English-speaking world in the late 18th century, in the work of Edward Gibbon . River deltas form when a river carrying sediment reaches a body of water, such as a lake, ocean, or a reservoir . When

10527-417: The largest known floods (peak discharge of 1,100,000 cubic meters per second (39,000,000 cu ft/s)) of the Holocene. The overflow was caused either by headward erosion of a river outside the caldera, capturing it, or a consequence of eruptions that stirred the lake and formed waves which overtopped its rim. The resulting flood scoured the river valley, forming a scabland , and deposited gravel bars,

10648-414: The largest sulfur-producing volcanic eruptions of the late Holocene. Babylonians observing Venus during the reign of King Ammi-Saduqa reported a haze which may have been from the Aniakchak eruption. Within a few decades, the caldera filled with water until more than half of the caldera floor was submerged. Water levels may have reached 490 meters (1,610 ft) or 610 meters (2,000 ft) elevation;

10769-456: 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

10890-679: 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. 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 3–5 °C (37–41 °F) and mean annual precipitation reaching 870–1,380 millimeters (34–54 in). The main vegetation in

11011-524: The onset of or changes in historical land use, especially deforestation , intensive agriculture , and urbanization . These ideas are well illustrated by the many tidal freshwater deltas prograding into Chesapeake Bay along the east coastline of the United States. Research has demonstrated that the accumulating sediments in this estuary derive from post-European settlement deforestation, agriculture, and urban development. Other rivers, particularly those on coasts with significant tidal range , do not form

11132-434: The other tuff cones was triggered by the drainage of the caldera lake, which depressurized the magma system. Half Cone erupted 840 ± 30 and 570 ± 40 years ago and activity alternated between Vent Mountain and Half Cone. Vent Mountain emplaced lava flows and tephra on the caldera floor. One pyroclastic flow crossed the northern caldera rim. The post-caldera activity has resulted in widespread ashfall over southwestern Alaska and

11253-482: The peninsula, and also the Kodiak Archipelago which lie off the south coast of the peninsula and are home to even more bears, constitute the Alaska Peninsula montane taiga ecoregion and contain a number of protected areas such as Katmai National Park . Vegetation on the Peninsula consists mostly of shrub-lands, grassy meadows, or wet tundra. Besides the communities on the coast ( see: Bristol Bay ),

11374-487: 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

11495-568: 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

11616-435: 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. Aniakchak has erupted rocks ranging from basalt to rhyolite , which define

11737-487: The result of this process is the Wax Lake Delta . In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition. This results in the smoothing of the planform (or map-view) shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates

11858-579: The river delta, causing the delta to retreat. For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels. Erosion is also an important control in tide-dominated deltas, such as the Ganges Delta , which may be mainly submarine, with prominent sandbars and ridges. This tends to produce a "dendritic" structure. Tidal deltas behave differently from river-dominated and wave-dominated deltas, which tend to have

11979-414: The river water is nearly equal in density to the basin water, the delta is characterized by homopycnal flow , in which the river water rapidly mixes with basin water and abruptly dumps most of its sediment load. Where the river water has a higher density than basin water, typically from a heavy load of sediment, the delta is characterized by hyperpycnal flow in which the river water hugs the basin bottom as

12100-492: The same time on Vent Mountain, and a tephra in Skilak Lake may also come from this eruption. Plummer et al. 2012 suggested this eruption as the 1453 mystery eruption . There may or may not have been another eruption before the 1931 event. The last eruption began on 1 May 1931. Initially, white clouds rose over the volcano at 10:00 am 1 May, followed by ash at 12:00 pm. Intense explosions occurred that day and on

12221-640: The second caldera-forming event. This rate is the highest of all volcanoes in the eastern Aleutian volcanic arc. Most Holocene eruptions have not produced known tephra deposits. There is evidence that after several eruptions, humans abandoned sites close to the volcano. Lava flows were emplaced on the northern flank of the volcano. Three major eruptions took place during the Holocene: The Aniakchak I, Black Nose Pumice, and Aniakchak II eruptions. The Aniakchak I eruption took place 9,500–7,500 years ago, and emplaced volcanic bombs and ignimbrites on

12342-399: The sediment is never piled up in thick sequences due to the sediment traveling and depositing in deep subduction trenches. At the mouth of a river, the change in flow conditions can cause the river to drop any sediment it is carrying. This sediment deposition can generate a variety of landforms, such as deltas, sand bars, spits, and tie channels. Landforms at the river mouth drastically alter

12463-514: The sediment source. When sediment does not travel far from the source, sediments that build up are coarser grained and more loosely consolidated, therefore making delta formation more difficult. Tectonic activity on active margins causes the formation of river deltas to form closer to the sediment source which may affect channel avulsion , delta lobe switching, and auto cyclicity. Active margin river deltas tend to be much smaller and less abundant but may transport similar amounts of sediment. However,

12584-814: The sediment supplied by the feeding river. Etymologically, the term river delta derives from the triangular shape (Δ) of the uppercase Greek letter delta . In hydrology , the dimensions of a river delta are determined by the balance between the watershed processes that supply sediment and the watershed processes that redistribute, sequester, and export the supplied sediment into the receiving basin. River deltas are important in human civilization , as they are major agricultural production centers and population centers. They can provide coastline defence and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position. On geologic timescales , they are also important carbon sinks . A river delta

12705-424: The shape of the delta, and much of the sediment emanating from the river mouth is deflected along the coastline. The relationship between waves and river deltas is quite variable and largely influenced by the deepwater wave regimes of the receiving basin. With a high wave energy near shore and a steeper slope offshore, waves will make river deltas smoother. Waves can also be responsible for carrying sediments away from

12826-456: 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

12947-551: 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

13068-679: 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 15 kilometers (9.3 mi) 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

13189-549: The term "Alaska Peninsula" was used to denote the entire northwestern protrusion of the North American continent , or all of what is now the state of Alaska, exclusive of its panhandle and islands . The Lake and Peninsula borough , the Alaskan equivalent of a county , is named after the peninsula. The Alaska/Aleutian Peninsula is also grouped into Southwest Alaska . The other largest peninsulas in Alaska include

13310-656: The trailing edges of passive margins due to the majority of large rivers such as the Mississippi , Nile , Amazon , Ganges , Indus , Yangtze , and Yellow River discharging along passive continental margins. This phenomenon is due mainly to three factors: topography , basin area, and basin elevation. Topography along passive margins tend to be more gradual and widespread over a greater area enabling sediment to pile up and accumulate over time to form large river deltas. Topography along active margins tends to be steeper and less widespread, which results in sediments not having

13431-443: The volcano and in surrounding valleys. They are similar in appearance and chemistry to the Aniakchak II deposits, but can be distinguished with the help of trace element data. A tephra layer in central Alaska has been attributed to the Aniakchak I eruption. How the volcano appeared after the Aniakchak I eruption is unclear; conceivably, either a small caldera formed or the caldera rapidly filled with ice. The so-called Black Nose Pumice

13552-410: The volcano but can occur in any direction Pyroclastic flows and pyroclastic surges are fast-flowing avalanches/clouds of hot rock. Owing to their enormous speed and high temperature, they tend to kill everything in their path. Future eruptions would most likely create such flows within the caldera, but only larger events would pose a threat outside of it. Lava domes and lava flows can be extruded within

13673-523: The volcano in an elongated area extending across western Alaska, including the Alaska Peninsula, Bristol Bay, the Kuskokwim and Yukon River Deltas , Norton Sound and the Seward Peninsula . Tephra thickness decreases from 1 meter (3 ft 3 in) 300 kilometers (190 mi) from the vent to 1 centimeter (0.39 in) 1,500 kilometers (930 mi) from the vent. The Aniakchak II tephra

13794-458: The volcano. The Aniakchak II eruption took place during the 17th century BCE, an era with numerous volcanic eruptions; other volcanoes that erupted at that time are Mount St. Helens , Vesuvius and in particular the Minoan eruption of Santorini and separating their dates and respective influences is difficult. The eruptions caused a volcanic winter at a time when global climate was undergoing

13915-668: The water is deep and clear. All of the Peninsula is organized as a part of four adjacent boroughs ; the Aleutians East Borough , Bristol Bay Borough , Kodiak Island Borough , and Lake and Peninsula Borough . The Lake and Peninsula Borough includes most of the peninsula's territory. Average annual precipitation ranges from 24–65 in (610–1,650 mm). Coastal areas are subject to intense storms, wind, and rain. Winter temperatures average between 12 and 34 °F (−11 and 1 °C), and in summer between 43 and 59 °F (6 and 15 °C). Frosts can occur any day of

14036-510: The year at higher elevations. The climate can be compared to that of parts of Scotland , the Aleutian Islands , Iceland , and Tierra del Fuego . The Alaska Peninsula is home to some of the largest populations of native and undisturbed wildlife in the United States. Besides the famous McNeil River and Katmai Alaskan brown bear populations, large herds of caribou , moose , wolves , waterfowl , and willow ptarmigan inhabit

14157-502: Was a strait connecting the Pacific Ocean and Bristol Bay before the eruption, where the Meshik River exists today, and this was filled in by rocks during the Aniakchak II eruption. The volcano collapsed like a piston, forming the caldera. Landslides on its inner walls enlarged the caldera depression. The eruption evacuated the magmatic system of Aniakchak, and subsequent eruptions had a different chemistry. The Aniakchak II eruption

14278-494: Was emplaced 7,000 years ago during several closely spaced Plinian eruptions and consists of two pumice fallout layers, separated by an ignimbrite. It is partly eroded or buried by products of the Aniakchak II eruption. A tephra layer in Southeastern Alaska was attributed to an unidentified eruption of Aniakchak 5,300–5,030 years before present , but may have originated at Mount Edgecumbe instead. Shortly before

14399-732: Was first coined by Alexander von Humboldt for the middle reaches of the Orinoco River , which he visited in 1800. Other prominent examples include the Inner Niger Delta , Peace–Athabasca Delta , the Sacramento–San Joaquin River Delta , and the Sistan delta of Iran. The Danube has one in the valley on the Slovak–Hungarian border between Bratislava and Iža . In some cases, a river flowing into

14520-515: Was in roughly equal degrees explosive and effusive ; many eruptions were both. Nine separate structures were emplaced in the caldera, partly in or under the lake. Half Cone and Vent Mountain were the site of multiple eruptions. Most of the vents are located at the caldera margin, probably along a ring fracture on the caldera floor. A first post-caldera explosive eruption occurred 2,300 years ago. Afterwards, several undated lava domes (Bolshoi Dome, Pumice Dome, West Dome, and Vulcan Dome) were emplaced in

14641-532: 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 . Aniakchak is about 670 kilometers (420 mi) southwest from Anchorage, Alaska , within the Aniakchak National Monument and Preserve ( Bristol Bay Borough ) on

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