Misplaced Pages

#704295

123-512: The Cook Inlet and both its arms are bodies of brackish water , containing a turbid mix of ocean salt-water and freshwater runoff from the various rivers and streams. The narrow channel of the inlet funnels the tides creating very fast-moving currents, rip tides , and occasional bore tides . Cook Inlet watershed is the most populated watershed in Alaska. The watershed covers about 100,000 km (39,000 sq mi) of southern Alaska, east of

246-505: A reflexive verb . The lower plate itself is the subject. It subducts, in the sense of retreat, or removes itself, and while doing so, is the "subducting plate". Moreover, the word slab is specifically attached to the "subducting plate", even though in English the upper plate is just as much of a slab. The upper plate is left hanging, so to speak. To express it geology must switch to a different verb, typically to override . The upper plate,

369-406: A consequence of the rigidity of the plate. The point where the slab begins to plunge downwards is marked by an oceanic trench . Oceanic trenches are the deepest parts of the ocean floor. Beyond the trench is the forearc portion of the overriding plate. Depending on sedimentation rates, the forearc may include an accretionary wedge of sediments scraped off the subducting slab and accreted to

492-463: A couple of rivers. Cook sailed up Turnagain Arm in his ship, HMS  Resolution , finding it impossible to navigate against the strong currents and mudflats, and got stuck on a sandbar when he tried to get back out, having to wait for the tide to come in and free his ship. He never actually confirmed it led to a river, which led to a decade of massive speculation until George Vancouver returned to finish

615-460: A larger portion of Earth's crust to deform in a more brittle fashion than it would in a normal geothermal gradient setting. Because earthquakes can occur only when a rock is deforming in a brittle fashion, subduction zones can cause large earthquakes. If such a quake causes rapid deformation of the sea floor, there is potential for tsunamis . The largest tsunami ever recorded happened due to a mega-thrust earthquake on December 26, 2004 . The earthquake

738-416: A mean of 30 feet (9.1 m), and the fourth highest in the world, behind Bay of Fundy (38 ft, 11.7 m), Ungava Bay (32 ft 0 in, 9.75 m), and Bristol Channel (31 ft, 9.6 m). The ocean's natural 12-hour 25-minute tidal cycle is close to Turnagain Arm's natural resonance frequency, which then reinforces the tide similar to water sloshing in a bathtub. Tidal fluctuations in

861-457: A minimum estimate of how far the continent has subducted. The results show at least a minimum of 229 kilometers of subduction of the northern Australian continental plate. Another example may be the continued northward motion of India, which is subducting beneath Asia. The collision between the two continents initiated around 50 my ago, but is still active. Oceanic-Oceanic plate subduction zones comprise roughly 40% of all subduction zone margins on

984-629: A natural defense against hurricane and tsunami damage in particular. The Sundarbans and Bhitarkanika Mangroves are two of the large mangrove forests in the world, both on the coast of the Bay of Bengal . Some seas and lakes are brackish. The Baltic Sea is a brackish sea adjoining the North Sea . Originally the Eridanos river system prior to the Pleistocene , since then it has been flooded by

1107-469: A point of no return. Sections of crustal or intraoceanic arc crust greater than 15 km (9.3 mi) in thickness or oceanic plateau greater than 30 km (19 mi) in thickness can disrupt subduction. However, island arcs subducted end-on may cause only local disruption, while an arc arriving parallel to the zone can shut it down. This has happened with the Ontong Java Plateau and

1230-475: A series of minerals in these slabs such as serpentine can be stable at different pressures within the slab geotherms, and may transport significant amount of water into the Earth's interior. As plates sink and heat up, released fluids can trigger seismicity and induce melting within the subducted plate and in the overlying mantle wedge. This type of melting selectively concentrates volatiles and transports them into

1353-535: A species only found in deep water in the Baltic, while pike are confined to the less saline surface waters. The Caspian Sea is the world's largest lake and contains brackish water with a salinity about one-third that of normal seawater. The Caspian is famous for its peculiar animal fauna, including one of the few non-marine seals (the Caspian seal ) and the great sturgeons , a major source of caviar . Hudson Bay

SECTION 10

#1732787562705

1476-411: A steeper angle is characterized by the formation of back-arc basins . According to the theory of plate tectonics , the Earth's lithosphere , its rigid outer shell, is broken into sixteen larger tectonic plates and several smaller plates. These plates are in slow motion, due mostly to the pull force of subducting lithosphere. Sinking lithosphere at subduction zones are a part of convection cells in

1599-574: A visit with Russian fur trader Gerasim Izmailov in Unalaska , and combined these maps with those of his expedition to create the first Mercator projection of the North Pacific. The inlet was named after Cook in 1794 by George Vancouver , who had served under Cook in 1778. Turnagain Arm was named by Cook, "turnagain" being a moniker he had used before, at his annoyance at having to turn around after exploring another dead end. Upon reaching

1722-510: A zone of shortening and crustal thickening in which there may be extensive folding and thrust faulting . If the angle of subduction steepens or rolls back, the upper plate lithosphere will be put in tension instead, often producing a back-arc basin . The arc-trench complex is the surface expression of a much deeper structure. Though not directly accessible, the deeper portions can be studied using geophysics and geochemistry . Subduction zones are defined by an inclined zone of earthquakes ,

1845-451: Is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second plate, the heavier plate dives beneath the other and sinks into the mantle. A region where this process occurs is known as a subduction zone , and its surface expression is known as an arc-trench complex . The process of subduction has created most of

1968-418: Is "consumed", which happens the geological moment the lower plate slips under, even though it may persist for some time until its remelting and dissipation. In this conceptual model, plate is continually being used up. The identity of the subject, the consumer, or agent of consumption, is left unstated. Some sources accept this subject-object construct. Geology makes to subduct into an intransitive verb and

2091-554: Is a specific gravity of between 1.0004 and 1.0226. Thus, brackish covers a range of salinity regimes and is not considered a precisely defined condition. It is characteristic of many brackish surface waters that their salinity can vary considerably over space or time. Water with a salt concentration greater than 30‰ is considered saline . See the salinity table from the Misplaced Pages salinity article . Brackish water condition commonly occurs when fresh water meets seawater. In fact,

2214-555: Is a brackish marginal sea of the Arctic Ocean , it remains brackish due its limited connections to the open ocean, very high levels freshwater surface runoff input from the large Hudson Bay drainage basin , and low rate of evaporation due to being completely covered in ice for over half the year. In the Black Sea the surface water is brackish with an average salinity of about 17–18 parts per thousand compared to 30 to 40 for

2337-499: Is accreted to (scraped off) the continent, resulting in exotic terranes . The collision of this oceanic material causes crustal thickening and mountain-building. The accreted material is often referred to as an accretionary wedge or prism. These accretionary wedges can be associated with ophiolites (uplifted ocean crust consisting of sediments, pillow basalts, sheeted dykes, gabbro, and peridotite). Subduction may also cause orogeny without bringing in oceanic material that accretes to

2460-496: Is characterized by low geothermal gradients and the associated formation of high-pressure low-temperature rocks such as eclogite and blueschist . Likewise, rock assemblages called ophiolites , associated with modern-style subduction, also indicate such conditions. Eclogite xenoliths found in the North China Craton provide evidence that modern-style subduction occurred at least as early as 1.8  Ga ago in

2583-446: Is commonly used as cooling water for power generation and in a variety of ways in the mining, oil, and gas industries. Once desalinated it can also be used for agriculture, livestock, and municipal uses. Brackish water can be treated using reverse osmosis , electrodialysis , and other filtration processes. Subduction zone Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere

SECTION 20

#1732787562705

2706-422: Is exposed, making marine navigation difficult. Historically, ships and boats designed for the area had a relatively flat bottom with a modest centerboard and absence of a large keel because the boats would occasionally become beached at low tide either on purpose or by accident. This design allowed the craft to support its weight on land without sustaining any serious damage to its structure. An example of this design

2829-444: Is fairly well understood, the process by which subduction is initiated remains a matter of discussion and continuing study. Subduction can begin spontaneously if the denser oceanic lithosphere can founder and sink beneath the adjacent oceanic or continental lithosphere through vertical forcing only; alternatively, existing plate motions can induce new subduction zones by horizontally forcing the oceanic lithosphere to rupture and sink into

2952-561: Is found behind the Aleutian Trench subduction zone in Alaska. Volcanoes that occur above subduction zones, such as Mount St. Helens , Mount Etna , and Mount Fuji , lie approximately one hundred kilometers from the trench in arcuate chains called volcanic arcs . Plutons, like Half Dome in Yosemite National Park, generally form 10–50 km below the volcanoes within the volcanic arcs and are only visible on

3075-698: Is frequently prone to earthquake activity. The primary fault, the Aleutian Fault, is found in the nearby Aleutian Trench in the Gulf of Alaska, where a corner of the Pacific Plate is forced underneath Alaska at a 45-degree angle. Thus, most of the uplift force occurs along a line from Kodiak Island and up the Kenai Mountains to the Chugach mountains. The inlet lies in a region where the crust

3198-713: Is listed as critically endangered in the IUCN Red List of Threatened Species. This was perhaps due to local hunt, although an article in the Anchorage Daily News questioned this conclusion. Killer whales commonly feed on belugas in the Cook Inlet, driving them to the upper part of the inlet where they often seek refuge in the shallower waters of the arms. In 2000, the US National Marine Fisheries Service listed

3321-477: Is more buoyant and as a result will rise into the lithosphere, where it forms large magma chambers called diapirs. Some of the magma will make it to the surface of the crust where it will form volcanoes and, if eruptive on earth's surface, will produce andesitic lava. Magma that remains in the lithosphere long enough will cool and form plutonic rocks such as diorite, granodiorite, and sometimes granite. The arc magmatism occurs one hundred to two hundred kilometers from

3444-411: Is old, goes down the subduction zone. As this happens, metamorphic reactions increase the density of the continental crustal rocks, which leads to less buoyancy. One study of the active Banda arc-continent collision claims that by unstacking the layers of rock that once covered the continental basement, but are now thrust over one another in the orogenic wedge, and measuring how long they are, can provide

3567-723: Is ongoing beneath part of the Andes , causing segmentation of the Andean Volcanic Belt into four zones. The flat-slab subduction in northern Peru and the Norte Chico region of Chile is believed to be the result of the subduction of two buoyant aseismic ridges, the Nazca Ridge and the Juan Fernández Ridge , respectively. Around Taitao Peninsula flat-slab subduction is attributed to the subduction of

3690-583: Is primarily used to fuel commercial fertilizer production and a liquified natural gas (LNG) plant and to Anchorage where the gas is consumed largely for domestic uses. Alaska has approximately half the known coal reserves in the U.S. For decades, there has been a proposal to build a large coal mine (the Chuitna Coal Mine ) on the west side of Cook Inlet near the Chuitna River , and the native village of Tyonek, Alaska . American Rivers placed

3813-460: Is the Nomad, a much photographed fishing boat once owned by Joe Reddington which has sat on a mudflat near Knik for several decades. These mudflats can also be dangerous to walk on, exhibiting quicksand -like characteristics, and have claimed the life of at least four people who have wandered out on them, usually tourists, with many more being rescued every year. Cruise ships dock at Seward on

Cook Inlet - Misplaced Pages Continue

3936-422: Is very low and the fish fauna consists predominantly of freshwater species such as roach , dace , carp , perch , and pike . The Thames Estuary becomes brackish between Battersea and Gravesend , and the diversity of freshwater fish species present is smaller, primarily roach and dace; euryhaline marine species such as flounder , European seabass , mullet , and smelt become much more common. Further east,

4059-485: Is water occurring in a natural environment that has more salinity than freshwater , but not as much as seawater . It may result from mixing seawater (salt water) and fresh water together, as in estuaries , or it may occur in brackish fossil aquifers . The word comes from the Middle Dutch root brak . Certain human activities can produce brackish water, in particular civil engineering projects such as dikes and

4182-480: Is wrinkled under this force and the tectonic forces push the ground downward. In 1964, a megathrust earthquake occurred when over 600 miles of the Aleutian fault ruptured, uplifting the Kenai Mountains 60 feet (20 meters) in under 5 minutes. With a magnitude of 9.2, the earthquake was the fourth largest ever recorded. It devastated Anchorage where much of downtown dropped several stories, and the mountains surrounding

4305-571: The Alaska Railroad along the eastern shores of Turnagain Arm and Knik Arm of Cook Inlet around 1915. The natives of the Eklutna village are the descendants of the residents of eight native villages around upper Cook Inlet. During the 1964 Alaska earthquake , areas around the head of Turnagain Arm near Girdwood and Portage dropped as much as 8 feet (2.4 m) by subsidence and subsequent tidal action. Both hamlets were destroyed. Girdwood

4428-616: The Aleutian Islands and Koniag natives from Kodiak , to hunt for sea otters and other marine mammal species for trade with China via Russia's then-exclusive inland port of trade at Kiakhta . Other Europeans to visit Cook Inlet include the 1778 expedition of James Cook , its namesake, who sailed into it while searching for the Northwest Passage . Cook received maps of Alaska, the Aleutians, and Kamchatka during

4551-721: The Aleutian Range , south and east of the Alaska Range , receiving water from its tributaries , which include the Knik River , the Little Susitna River , the Susitna and Matanuska rivers , Eagle River , Ship Creek , Resurrection Creek , Portage Creek, Kenai River , and many others. The watershed includes the drainage areas of Denali (formerly named Mount McKinley) and is the collection point for

4674-528: The Cascade Volcanic Arc , that form along the coast of continents. Island arcs (intraoceanic or primitive arcs) are produced by the subduction of oceanic lithosphere beneath another oceanic lithosphere (ocean-ocean subduction) while continental arcs (Andean arcs) form during the subduction of oceanic lithosphere beneath a continental lithosphere (ocean-continent subduction). An example of a volcanic arc having both island and continental arc sections

4797-774: The Chile Rise , a spreading ridge . The Laramide Orogeny in the Rocky Mountains of the United States is attributed to flat-slab subduction. During this orogeny, a broad volcanic gap appeared at the southwestern margin of North America, and deformation occurred much farther inland; it was during this time that the basement -cored mountain ranges of Colorado, Utah, Wyoming, South Dakota, and New Mexico came into being. The most massive subduction zone earthquakes, so-called "megaquakes", have been found to occur in flat-slab subduction zones. Although stable subduction

4920-420: The Chuitna River on its list of America's Ten Most Endangered Rivers of 2007, based on the threat of this mine. Turnagain Arm is one of only about 60 bodies of water worldwide to exhibit a tidal bore . The bore may be more than six feet (1.8 m) high and travel at 15 miles per hour (24 km/h) on high spring tides and opposing winds. Turnagain Arm sees the largest tidal range in United States, with

5043-578: The Gulf of Alaska or Whittier in Prince William Sound and transport passengers via bus or train to Anchorage. However, over 95% of freight entering Alaska comes through the Port of Anchorage , which is served by major container ship companies and other carriers. The Cook Inlet beluga whale is a genetically distinct and geographically isolated stock. The population fell to 278 in 2005 and it

Cook Inlet - Misplaced Pages Continue

5166-630: The Paleoproterozoic Era . The eclogite itself was produced by oceanic subduction during the assembly of supercontinents at about 1.9–2.0 Ga. Blueschist is a rock typical for present-day subduction settings. The absence of blueschist older than Neoproterozoic reflects more magnesium-rich compositions of Earth's oceanic crust during that period. These more magnesium-rich rocks metamorphose into greenschist at conditions when modern oceanic crust rocks metamorphose into blueschist. The ancient magnesium-rich rocks mean that Earth's mantle

5289-491: The Vitiaz Trench . Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process create and destroy water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding

5412-533: The Wadati–Benioff zone , that dips away from the trench and extends down below the volcanic arc to the 660-kilometer discontinuity . Subduction zone earthquakes occur at greater depths (up to 600 km (370 mi)) than elsewhere on Earth (typically less than 20 km (12 mi) depth); such deep earthquakes may be driven by deep phase transformations , thermal runaway , or dehydration embrittlement . Seismic tomography shows that some slabs can penetrate

5535-406: The core–mantle boundary . Here the slabs are heated up by the ambient heat and are not detected anymore ~300 Myr after subduction. Orogeny is the process of mountain building. Subducting plates can lead to orogeny by bringing oceanic islands, oceanic plateaus, sediments and passive continental margins to convergent margins. The material often does not subduct with the rest of the plate but instead

5658-406: The crab-eating frog , Fejervarya cancrivora (formerly Rana cancrivora ). Mangroves represent important nesting sites for numerous birds groups such as herons, storks, spoonbills, ibises, kingfishers, shorebirds and seabirds. Although often plagued with mosquitoes and other insects that make them unpleasant for humans, mangrove swamps are very important buffer zones between land and sea, and are

5781-411: The lower mantle and sink clear to the core–mantle boundary . Here the residue of the slabs may eventually heat enough to rise back to the surface as mantle plumes . Subduction typically occurs at a moderately steep angle by the time it is beneath the volcanic arc. However, anomalous shallower angles of subduction are known to exist as well as some that are extremely steep. Flat-slab subduction

5904-416: The zeolite , prehnite-pumpellyite, blueschist , and eclogite facies stability zones of subducted oceanic crust. Zeolite and prehnite-pumpellyite facies assemblages may or may not be present, thus the onset of metamorphism may only be marked by blueschist facies conditions. Subducting slabs are composed of basaltic crust topped with pelagic sediments ; however, the pelagic sediments may be accreted onto

6027-506: The Alaskan crust. The concept of subduction would play a role in the development of the plate tectonics theory. First geologic attestations of the "subduct" words date to 1970, In ordinary English to subduct , or to subduce (from Latin subducere , "to lead away") are transitive verbs requiring a subject to perform an action on an object not itself, here the lower plate, which has then been subducted ("removed"). The geological term

6150-588: The Alps. The chemistry of the inclusions supports the existence of a carbon-rich fluid in that environment, and additional chemical measurements of lower pressure and temperature facies in the same tectonic complex support a model for carbon dissolution (rather than decarbonation) as a means of carbon transport. Elastic strain caused by plate convergence in subduction zones produces at least three types of earthquakes. These are deep earthquakes, megathrust earthquakes, and outer rise earthquakes. Deep earthquakes happen within

6273-413: The Cook Inlet beluga whale population as depleted and began development of a conservation plan. On October 22, 2008 the Cook Inlet beluga whale was put onto the endangered species list. Cook Inlet activities include commercial fishing, oil and gas development, release of treated sewage , noise from aircraft and ships, shipping traffic, and tourism. However, it is not known what impact these activities had on

SECTION 50

#1732787562705

6396-411: The Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year. Subduction is possible because the cold and rigid oceanic lithosphere is slightly denser than the underlying asthenosphere , the hot, ductile layer in the upper mantle . Once initiated, stable subduction is driven mostly by the negative buoyancy of

6519-527: The Kenai rely on income generated from fisheries in the Cook Inlet. The west side of the Inlet is not connected to any major road systems and is typically accessed by boat or plane, including the village of Tyonek, Alaska , some oil camps, and many seasonal fishing camps. The Cook Inlet Basin contains large oil and gas deposits including several offshore fields. As of 2005 there were 16 platforms in Cook Inlet,

6642-484: The North Sea but still receives so much freshwater from the adjacent lands that the water is brackish. As seawater is denser, the water in the Baltic is stratified, with seawater at the bottom and freshwater at the top. Limited mixing occurs because of the lack of tides and storms, with the result that the fish fauna at the surface is freshwater in composition while that lower down is more marine. Cod are an example of

6765-779: The Turnagain Arm subsided 8 feet (2.4 meters), submerging the towns of Portage and Girdwood, as well as long stretches of the Seward Highway. Because it lies along a subduction zone , the Cook Inlet region contains active volcanoes, including Augustine Volcano and Mount Redoubt , and is part of the Pacific Ring of Fire . Hot magma and steam from the subducting ocean floor build up within these volcanoes, which tend to erupt in fairly regular cycles with very explosive force, often spewing volcanic ash tens of thousands of feet (several kilometers) high. Volcanic eruptions in

6888-472: The asthenosphere. Both models can eventually yield self-sustaining subduction zones, as the oceanic crust is metamorphosed at great depth and becomes denser than the surrounding mantle rocks. The compilation of subduction zone initiation events back to 100 Ma suggests horizontally-forced subduction zone initiation for most modern subduction zones, which is supported by results from numerical models and geologic studies. Some analogue modeling shows, however,

7011-619: The beluga whale population. The Coastal Marine Institute at the University of Alaska Fairbanks 's School of Fisheries and Ocean Sciences working with the U.S. Department of the Interior's Minerals Management Service (MMS), began a three-year project in 2003 focusing on the water circulation in Cook Inlet. Cook Inletkeeper , a member of the Waterkeeper Alliance , began in 1994 when a group of Alaskans became concerned about

7134-523: The colder oceanic lithosphere is, on average, more dense. Sediments and some trapped water are carried downwards by the slab and recycled into the deep mantle. Earth is so far the only planet where subduction is known to occur, and subduction zones are its most important tectonic feature. Subduction is the driving force behind plate tectonics , and without it, plate tectonics could not occur. Oceanic subduction zones are located along 55,000 km (34,000 mi) convergent plate margins, almost equal to

7257-435: The continent, away from the trench, and has been described in western North America (i.e. Laramide orogeny, and currently in Alaska, South America, and East Asia. The processes described above allow subduction to continue while mountain building happens concurrently, which is in contrast to continent-continent collision orogeny, which often leads to the termination of subduction. Continents are pulled into subduction zones by

7380-579: The crust would be melted and recycled into the Earth's mantle . In 1964, George Plafker researched the Good Friday earthquake in Alaska . He concluded that the cause of the earthquake was a megathrust reaction in the Aleutian Trench , a result of the Alaskan continental crust overlapping the Pacific oceanic crust. This meant that the Pacific crust was being forced downward, or subducted , beneath

7503-597: The crust, megathrust earthquakes on the subduction interface near the trench, and outer rise earthquakes on the subducting lower plate as it bends near the trench. Anomalously deep events are a characteristic of subduction zones, which produce the deepest quakes on the planet. Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than twenty kilometers. However, in subduction zones quakes occur at depths as great as 700 km (430 mi). These quakes define inclined zones of seismicity known as Wadati–Benioff zones which trace

SECTION 60

#1732787562705

7626-609: The crust, through hotspot magmatism or extensional rifting, would the crust be able to break from its continent and begin subduction. Subduction can continue as long as the oceanic lithosphere moves into the subduction zone. However, the arrival of buoyant continental lithosphere at a subduction zone can result in increased coupling at the trench and cause plate boundary reorganization. The arrival of continental crust results in continental collision or terrane accretion that may disrupt subduction. Continental crust can subduct to depths of 250 km (160 mi) where it can reach

7749-408: The cumulative plate formation rate 60,000 km (37,000 mi) of mid-ocean ridges. Sea water seeps into oceanic lithosphere through fractures and pores, and reacts with minerals in the crust and mantle to form hydrous minerals (such as serpentine) that store water in their crystal structures. Water is transported into the deep mantle via hydrous minerals in subducting slabs. During subduction,

7872-448: The degree of lower plate curvature of the subducting plate in great historical earthquakes such as the 2004 Sumatra-Andaman and the 2011 Tōhoku earthquake, it was determined that the magnitude of earthquakes in subduction zones is inversely proportional to the angle of subduction near the trench, meaning that "the flatter the contact between the two plates, the more likely it is that mega-earthquakes will occur". Outer rise earthquakes on

7995-446: The dense subducting lithosphere. The down-going slab sinks into the mantle largely under its own weight. Earthquakes are common along subduction zones, and fluids released by the subducting plate trigger volcanism in the overriding plate. If the subducting plate sinks at a shallow angle, the overriding plate develops a belt of deformation characterized by crustal thickening, mountain building , and metamorphism . Subduction at

8118-440: The descending slab. Nine of the ten largest earthquakes of the last 100 years were subduction zone megathrust earthquakes. These included the 1960 Great Chilean earthquake which at M 9.5 was the largest earthquake ever recorded, the 2004 Indian Ocean earthquake and tsunami , and the 2011 Tōhoku earthquake and tsunami . The subduction of cold oceanic lithosphere into the mantle depresses the local geothermal gradient and causes

8241-455: The different regimes present in this setting. The models are as follows: In their 2019 study, Macdonald et al. proposed that arc-continent collision zones and the subsequent obduction of oceanic lithosphere was at least partially responsible for controlling global climate. Their model relies on arc-continent collision in tropical zones, where exposed ophiolites composed mainly of mafic material increase "global weatherability" and result in

8364-530: The flooding of coastal marshland to produce brackish water pools for freshwater prawn farming . Brackish water is also the primary waste product of the salinity gradient power process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it can be damaging to the environment (see article on shrimp farms ). Technically, brackish water contains between 0.5 and 30 grams of salt per litre—more often expressed as 0.5 to 30 parts per thousand (‰), which

8487-420: The forearc-hanging wall and not subducted. Most metamorphic phase transitions that occur within the subducting slab are prompted by the dehydration of hydrous mineral phases. The breakdown of hydrous mineral phases typically occurs at depths greater than 10 km. Each of these metamorphic facies is marked by the presence of a specific stable mineral assemblage, recording the metamorphic conditions undergone but

8610-555: The head of Cook Inlet, Cook was of the opinion that both Knik Arm and Turnagain Arm were the mouths of rivers and not the opening to the Northwest Passage. Under orders by the British Crown to ignore any such rivers and inlets, he had initially planned to pass it by, but at the insistence of John Gore and many others of his crew, he reluctantly agreed to explore the area. Cook anchored at Ship Creek and encountered

8733-430: The idea of subduction initiation at passive margins is popular, there is no modern day example for this type of subduction nucleation. This is likely due to the strength of the oceanic or transitional crust at the continental passive margins, suggesting that if the crust did not break in its first 20 million years of life, it is unlikely to break in the future under normal sedimentation loads. Only with additional weaking of

8856-432: The inlet and its orientation with respect to the lunar orbit causes the tide to come in and go out very rapidly. As the inlet narrows, the speed of the water increases, creating very powerful currents with speeds of up to 6 knots (7 miles per hour). While lined with large areas of silt and mudflats , the central and upper inlet is filled with narrow troughs that may be 150 to 300 feet (50 to 100 meters) deep. These, along with

8979-514: The interior of the state, arrived sometime between 500 and 1600 AD. In the 18th century, Russian fur hunters ( promyshlenniki ) were among the first European visitors. The Lebedev Lastochkin Company leader Stepan Zaikov established a post at the mouth of the Kenai River, Fort Nikolaevskaia , in 1786. These fur trappers used Siberian Native and Alaska Native people, particularly Aleuts from

9102-433: The local Natives for the first time when two men approached in kayaks, invited them to come ashore. Under Cook's orders, William Bligh , of HMS  Bounty fame, organized a party to travel up Knik Arm. Bligh served as Cook's Sailing Master on this, his 3rd and final voyage, the aim of which was discovery of the Northwest Passage . After meeting with some local Dena'ina, Bligh returned to report Knik Arm indeed led only to

9225-574: The lower plate occur when normal faults oceanward of the subduction zone are activated by flexure of the plate as it bends into the subduction zone. The 2009 Samoa earthquake is an example of this type of event. Displacement of the sea floor caused by this event generated a six-meter tsunami in nearby Samoa. Seismic tomography has helped detect subducted lithospheric slabs deep in the mantle where no earthquakes occur. About one hundred slabs have been described in terms of depth and their timing and location of subduction. The great seismic discontinuities in

9348-434: The main body of Cook Inlet, while not as extreme as the shallow and narrow Turnagain Arm, regularly reach 25 feet (7.6 m) or more and exhibit currents in excess of 5 knots (9.3 km/h; 5.8 mph) at full tidal flow. The inlet and its arms have been proposed as a potentially attractive site for the generation of tidal power . Turnagain Arm and Knik Arm are known for large areas of silt . At low tide, much of this

9471-415: The mantle and is recycled. They are found at convergent plate boundaries, where the heavier oceanic lithosphere of one plate is overridden by the leading edge of another, less-dense plate. The overridden plate (the slab ) sinks at an angle most commonly between 25 and 75 degrees to Earth's surface. This sinking is driven by the temperature difference between the slab and the surrounding asthenosphere, as

9594-484: The mantle, at 410 km (250 mi) depth and 670 km (420 mi), are disrupted by the descent of cold slabs in deep subduction zones. Some subducted slabs seem to have difficulty penetrating the major discontinuity that marks the boundary between the upper mantle and lower mantle at a depth of about 670 kilometers. Other subducted oceanic plates have sunk to the core–mantle boundary at 2890 km depth. Generally, slabs decelerate during their descent into

9717-463: The mantle, from typically several cm/yr (up to ~10 cm/yr in some cases) at the subduction zone and in the uppermost mantle, to ~1 cm/yr in the lower mantle. This leads to either folding or stacking of slabs at those depths, visible as thickened slabs in seismic tomography. Below ~1700 km, there might be a limited acceleration of slabs due to lower viscosity as a result of inferred mineral phase changes until they approach and finally stall at

9840-566: The map. Having been in a bad mood since first agreeing to explore the area, and as a result of this frustration, the second body of water was given the disingenuous name "Turn Again". Early maps label Turnagain Arm as the "Turnagain River". The SS Farallon was a wooden Alaskan Steamship Company liner that struck Black Reef in the Cook Inlet on January 5, 1910. All thirty-eight men on board survived, and were rescued twenty-nine days later. Few white people visited upper Cook Inlet until construction of

9963-467: The most extensive brackish water habitats worldwide are estuaries , where a river meets the sea. The River Thames flowing through London is a classic river estuary. The town of Teddington a few miles west of London marks the boundary between the tidal and non-tidal parts of the Thames, although it is still considered a freshwater river about as far east as Battersea insofar as the average salinity

10086-609: The ocean floor during the Cretaceous period . This area, where a corner of the Pacific Plate subducts under the North American Plate , uplifted the ocean floor and compressed it, causing the crust to thicken and scrunch . Tectonic forces lifted the mountains and peninsula out of the water, forming a valley that was cut off from the ocean to the north and the west. The rest of Alaska's mountains were formed in

10209-492: The ocean floor, studied the Mid-Atlantic Ridge and proposed that hot molten rock was added to the crust at the ridge and expanded the seafloor outward. This theory was to become known as seafloor spreading . Since the Earth's circumference has not changed over geologic time, Hess concluded that older seafloor has to be consumed somewhere else, and suggested that this process takes place at oceanic trenches , where

10332-549: The oceans. The deep, anoxic water of the Black Sea originates from warm, salty water of the Mediterranean . Lake Texoma , a reservoir on the border between the U.S. states of Texas and Oklahoma , is a rare example of a brackish lake that is neither part of an endorheic basin nor a direct arm of the ocean, though its salinity is considerably lower than that of the other bodies of water mentioned here. The reservoir

10455-429: The oldest oceanic lithosphere. Continental lithosphere is up to 200 km (120 mi) thick. The lithosphere is relatively cold and rigid compared with the underlying asthenosphere , and so tectonic plates move as solid bodies atop the asthenosphere. Individual plates often include both regions of the oceanic lithosphere and continental lithosphere. Subduction zones are where cold oceanic lithosphere sinks back into

10578-504: The oldest of which is the XTO A platform first installed by Shell in 1964, and newest of which is the Osprey platform installed by Forest Oil in 2000. Most of the platforms are operated by Union Oil, which was acquired by Chevron in 2005. There are also numerous oil and gas pipelines running around and under the Cook Inlet. The main destinations of the gas pipelines are to Kenai where the gas

10701-412: The overlying plate. If an eruption occurs, the cycle then returns the volatiles into the oceans and atmosphere. The surface expressions of subduction zones are arc-trench complexes. On the ocean side of the complex, where the subducting plate first approaches the subduction zone, there is often an outer trench high or outer trench swell . Here the plate shallows slightly before plunging downwards, as

10824-399: The overriding continent. When the lower plate subducts at a shallow angle underneath a continent (something called "flat-slab subduction"), the subducting plate may have enough traction on the bottom of the continental plate to cause the upper plate to contract by folding, faulting, crustal thickening, and mountain building. Flat-slab subduction causes mountain building and volcanism moving into

10947-510: The overriding plate. However, not all arc-trench complexes have an accretionary wedge. Accretionary arcs have a well-developed forearc basin behind the accretionary wedge, while the forearc basin is poorly developed in non-accretionary arcs. Beyond the forearc basin, volcanoes are found in long chains called volcanic arcs . The subducting basalt and sediment are normally rich in hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as

11070-431: The pens the farmed fish are kept in. Another important brackish water habitat is the mangrove swamp or mangal . Many, though not all, mangrove swamps fringe estuaries and lagoons where the salinity changes with each tide. Among the most specialised residents of mangrove forests are mudskippers , fish that forage for food on land, and archer fish , perch-like fish that "spit" at insects and other small animals living in

11193-404: The planet. The ocean-ocean plate relationship can lead to subduction zones between oceanic and continental plates, therefore highlighting how important it is to understand this subduction setting. Although it is not fully understood what causes the initiation of subduction of an oceanic plate under another oceanic plate, there are three main models put forth by Baitsch-Ghirardello et al. that explain

11316-590: The possibility of spontaneous subduction from inherent density differences between two plates at specific locations like passive margins and along transform faults . There is evidence this has taken place in the Izu-Bonin-Mariana subduction system. Earlier in Earth's history, subduction is likely to have initiated without horizontal forcing due to the lack of relative plate motion, though a proposal by A. Yin suggests that meteorite impacts may have contributed to subduction initiation on early Earth. Though

11439-520: The pressures and temperatures necessary for this type of metamorphism are much higher than what is observed in most subduction zones. Frezzoti et al. (2011) propose a different mechanism for carbon transport into the overriding plate via dissolution (release of carbon from carbon-bearing minerals into an aqueous solution) instead of decarbonation. Their evidence comes from the close examination of mineral and fluid inclusions in low-temperature (<600 °C) diamonds and garnets found in an eclogite facies in

11562-515: The rapid ecological changes in the inlet. The group formally incorporated the following year using settlement proceeds from a Clean Water Act lawsuit against Cook Inlet oil and gas producers. Their stated mission is to "protect Alaska's Cook Inlet watershed and the life it sustains," with a focus on clean water, healthy habitat, local economies, and energy. They are based in Homer . Brackish water Brackish water , sometimes termed brack water ,

11685-458: The region have been associated with earthquakes and tsunamis , and debris avalanches have resulted in tsunamis also. There was an earthquake of the magnitude of 7.1 on December 31, 1901 generated by an eruption that caused several tsunamis. In 2009 a lahar from Mt. Redoubt threatened the Drift River oil terminal . Cook Inlet has the fourth largest tidal range in the world. The shape of

11808-500: The river. This type of ecological succession from freshwater to marine ecosystem is typical of river estuaries. River estuaries form important staging points during the migration of anadromous and catadromous fish species, such as salmon , shad and eels , giving them time to form social groups and to adjust to the changes in salinity. Salmon are anadromous, meaning they live in the sea but ascend rivers to spawn; eels are catadromous, living in rivers and streams, but returning to

11931-444: The rocks of the mantle. The mantle-derived magmas (which are initially basaltic in composition) can ultimately reach the Earth's surface, resulting in volcanic eruptions. The chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with (melts) Earth's crust or undergoes fractional crystallization . Arc volcanoes tend to produce dangerous eruptions because they are rich in water (from

12054-421: The runoff from many surrounding glaciers, leading to the high turbidity and large silt deposits. Within the watershed there are several national parks and the active volcano Mount Redoubt , along with three other historically active volcanoes . Cook Inlet provides navigable access to the port of Anchorage at the northern end, and to the smaller Homer port further south. Before the growth of Anchorage, Knik

12177-412: The salinity increases and the freshwater fish species are completely replaced by euryhaline marine ones, until the river reaches Gravesend, at which point conditions become fully marine and the fish fauna resembles that of the adjacent North Sea and includes both euryhaline and stenohaline marine species. A similar pattern of replacement can be observed with the aquatic plants and invertebrates living in

12300-479: The same manner, in cyclic events, and the mountain ranges become progressively older the further north they lie. Thus, the Cook Inlet and its surrounding land masses are rather young compared to those of the Alaska Range , which formed around 126 million years ago, or the Brooks Range which formed around 256 million years ago. The valley left by this continental compaction remains open to the Gulf of Alaska to

12423-727: The sea to breed. Besides the species that migrate through estuaries, there are many other fish that use them as "nursery grounds" for spawning or as places young fish can feed and grow before moving elsewhere. Herring and plaice are two commercially important species that use the Thames Estuary for this purpose. Estuaries are also commonly used as fishing grounds and as places for fish farming or ranching. For example, Atlantic salmon farms are often located in estuaries, although this has caused controversy, because in doing so, fish farmers expose migrating wild fish to large numbers of external parasites such as sea lice that escape from

12546-436: The sedimentary and volcanic cover is mostly scraped off to form an orogenic wedge. An orogenic wedge is larger than most accretionary wedges due to the volume of material there is to accrete. The continental basement rocks beneath the weak cover suites are strong and mostly cold, and can be underlain by a >200 km thick layer of dense mantle. After shedding the low density cover units, the continental plate, especially if it

12669-450: The sinking oceanic plate they are attached to. Where continents are attached to oceanic plates with no subduction, there is a deep basin that accumulates thick suites of sedimentary and volcanic rocks known as a passive margin. Some passive margins have up to 10 km of sedimentary and volcanic rocks covering the continental crust. As a passive margin is pulled into a subduction zone by the attached and negatively buoyant oceanic lithosphere,

12792-453: The slab and sediments) and tend to be extremely explosive. Krakatoa , Nevado del Ruiz , and Mount Vesuvius are all examples of arc volcanoes. Arcs are also associated with most ore deposits. Beyond the volcanic arc is a back-arc region whose character depends strongly on the angle of subduction of the subducting slab. Where this angle is shallow, the subducting slab drags the overlying continental crust partially with it, which produces

12915-472: The south, forming the inlet as it exists today. The surrounding mountains were host the large ice sheets and glaciers during the Pleistocene epoch (the Ice Ages, around 2 million to 11,000 years ago) which scoured the land and formed vast plains and moraines surrounding much of the upper inlet from glacial till that was deposited. Cook Inlet exists in a subsidence zone which contains many faults , and

13038-447: The storage of carbon through silicate weathering processes. This storage represents a carbon sink , removing carbon from the atmosphere and resulting in global cooling. Their study correlates several Phanerozoic ophiolite complexes, including active arc-continent subduction, with known global cooling and glaciation periods. This study does not discuss Milankovitch cycles as a driver of global climate cyclicity. Modern-style subduction

13161-481: The stratosphere during violent eruptions can cause rapid cooling of Earth's climate and affect air travel. Arc-magmatism plays a role in Earth's Carbon cycle by releasing subducted carbon through volcanic processes. Older theory states that the carbon from the subducting plate is made available in overlying magmatic systems via decarbonation, where CO 2 is released through silicate-carbonate metamorphism. However, evidence from thermodynamic modeling has shown that

13284-526: The subducting slab bends downward. During the transition from basalt to eclogite, these hydrous materials break down, producing copious quantities of water, which at such great pressure and temperature exists as a supercritical fluid . The supercritical water, which is hot and more buoyant than the surrounding rock, rises into the overlying mantle, where it lowers the melting temperature of the mantle rock, generating magma via flux melting . The magmas, in turn, rise as diapirs because they are less dense than

13407-509: The subducting slab. Transitions between facies cause hydrous minerals to dehydrate at certain pressure-temperature conditions and can therefore be tracked to melting events in the mantle beneath a volcanic arc. Two kinds of arcs are generally observed on Earth: island arcs that form on the oceanic lithosphere (for example, the Mariana and the Tonga island arcs), and continental arcs such as

13530-451: The subject, performs the action of overriding the object, the lower plate, which is overridden. Subduction zones are important for several reasons: Subduction zones have also been considered as possible disposal sites for nuclear waste in which the action of subduction itself would carry the material into the planetary mantle , safely away from any possible influence on humanity or the surface environment. However, that method of disposal

13653-454: The surface once the volcanoes have weathered away. The volcanism and plutonism occur as a consequence of the subducting oceanic slab dehydrating as it reaches higher pressures and temperatures. Once the oceanic slab reaches about 100 km in depth, hydrous minerals become unstable and release fluids into the asthenosphere. The fluids act as a flux for the rock within the asthenosphere and cause it to partially melt. The partially melted material

13776-505: The temperatures in the Cook Inlet region fairly moderate compared to the extremes found in other parts of the state. The inlet was first explored and settled by Alutiiq people, tribes of coastal-dwelling Pacific Eskimos , beginning around 6000 years ago. The Chugach arrived around the first century and were the last of the Alutiiq people to settle in the area, but abandoned it after tribes of Dena'ina people , an Athabaskan people from

13899-601: The tide comes in all at once. Large bores are less frequent, and are typically formed during a new or full moon, especially when the moon is at perigee . Unlike areas around the Bering Sea where weather is largely affected by sea ice, the waters of the inlet are warmed by the Alaska Current in the Gulf of Alaska, part of the North-Pacific Subpolar Gyre , which affects the climate and keeps

14022-411: The tides, provide a challenge to ships navigating through the waters. The strong tides create powerful rip tides and bore tides which are sometimes among the largest in the world. Tidal bores occur within the inlet, and especially Turnagain Arm, almost daily, but are usually too small to notice. Large bores tend to occur after extreme tidal lows, appearing as a wall of water sometimes over 10 feet high as

14145-439: The timing and conditions in which these dehydration reactions occur is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust. A metamorphic facies is characterized by a stable mineral assemblage specific to a pressure-temperature range and specific starting material. Subduction zone metamorphism is characterized by a low temperature, high-ultrahigh pressure metamorphic path through

14268-420: The trees, knocking them into the water where they can be eaten. Like estuaries, mangrove swamps are extremely important breeding grounds for many fish, with species such as snappers , halfbeaks , and tarpon spawning or maturing among them. Besides fish, numerous other animals use mangroves, including such species as the saltwater crocodile , American crocodile , proboscis monkey , diamondback terrapin , and

14391-444: The trench and approximately one hundred kilometers above the subducting slab. Arcs produce about 10% of the total volume of magma produced each year on Earth (approximately 0.75 cubic kilometers), much less than the volume produced at mid-ocean ridges, but they have formed most continental crust . Arc volcanism has the greatest impact on humans because many arc volcanoes lie above sea level and erupt violently. Aerosols injected into

14514-400: The underlying ductile mantle . This process of convection allows heat generated by radioactive decay to escape from the Earth's interior. The lithosphere consists of the outermost light crust plus the uppermost rigid portion of the mantle . Oceanic lithosphere ranges in thickness from just a few km for young lithosphere created at mid-ocean ridges to around 100 km (62 mi) for

14637-628: Was caused by subduction of the Indo-Australian plate under the Euro-Asian Plate, but the tsunami spread over most of the planet and devastated the areas around the Indian Ocean. Small tremors which cause small, nondamaging tsunamis, also occur frequently. A study published in 2016 suggested a new parameter to determine a subduction zone's ability to generate mega-earthquakes. By examining subduction zone geometry and comparing

14760-519: Was created by the damming of the Red River of the South , which (along with several of its tributaries) receives large amounts of salt from natural seepage from buried deposits in the upstream region. The salinity is high enough that striped bass , a fish normally found only in salt water, has self-sustaining populations in the lake. Brackish water is being used by humans in many different sectors. It

14883-744: Was later relocated inland and Portage was abandoned. About 20 miles (32 km) of the Seward Highway sank below the high-water mark of Turnagain Arm; the highway and its bridges were raised and rebuilt in 1964–66. Most of Alaska's population is in the Cook Inlet area, with highest concentration in Anchorage . Along the East side of the Cook Inlet, the Kenai Peninsula is host to many smaller fishing communities, such as Kenai , Soldotna , Ninilchick, Anchor Point and Homer . Many residents of

15006-582: Was once hotter, but not that subduction conditions were hotter. Previously, the lack of pre-Neoproterozoic blueschist was thought to indicate a different type of subduction. Both lines of evidence refute previous conceptions of modern-style subduction having been initiated in the Neoproterozoic Era 1.0 Ga ago. Harry Hammond Hess , who during World War II served in the United States Navy Reserve and became fascinated in

15129-583: Was the destination for most marine traffic in upper Cook Inlet. Approximately 400,000 people live within the Cook Inlet watershed. Cook inlet, along with the Kenai Peninsula, the Kenai Mountains, the Chugach Mountains, and Kodiak Island , formed approximately 65 million years ago, just after the extinction of the dinosaurs. Like most of Alaska's mountains, they consist mainly of sedimentary rock and metamorphic rock that were deposited on

#704295