Misplaced Pages

#330669

84-524: By the late Cretaceous, Eurasia and the Americas had separated along the south Atlantic, and subduction on the west coast of the Americas had commenced, resulting in the Laramide orogeny , the early phase of growth of the modern Rocky Mountains . The Western Interior Seaway may be seen as a downwarping of the continental crust ahead of the growing Laramide/Rockies mountain chain. The earliest phase of

168-505: 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,

252-406: 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

336-460: 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

420-908: A margin of land temporarily rose above the water along the ancestral Transcontinental Arch , each time rejoining the separated, divergent land populations, allowing a temporary mixing of newer species before again separating the populations. At its largest, the Western Interior Seaway stretched from the Rockies east to the Appalachian Mountains , some 1,000 km (620 mi) wide. At its deepest, it may have been only 800 or 900 metres (2,600 or 3,000 ft) deep, shallow in terms of seas. Two great continental watersheds drained into it from east and west, diluting its waters and bringing resources in eroded silt that formed shifting delta systems along its low-lying coasts. There

504-596: A maxilla (BMNH 49014) from the Lower Cretaceous of Sussex, England and was originally thought to be a mandible that Louis Agassiz dubbed Pachyrhizodus in 1850, with Frederick Dixon creating the species name basalis for the specimen. Over the next few years, many Pachyrhizodus species would be named only from England until in 1872, Edward Drinker Cope described large remains from the Smoky Hill Chalk of Kansas of several new species. During

588-411: A meter in diameter. Entire schools of fish sometimes sought shelter within the shell of the giant Platyceramus . The shells of the genus are known for being composed of prismatic calcitic crystals that grew perpendicular to the surface, and fossils often retain a pearly luster. Subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere

672-457: 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

756-469: 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

840-475: 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

924-411: 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

SECTION 10

#1732765542331

1008-510: 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 ,

1092-464: Is Monument Rocks , an exposed chalk formation towering 70 feet (21 m) over the surrounding range land. The Western Interior Seaway is believed to have behaved similarly to a giant estuary in terms of water mass transport. Riverine inputs exited the seaway as coastal jets, while correspondingly drawing in water from the Tethys in the south and Boreal waters from the north. During the late Cretaceous,

1176-451: 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

1260-418: 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

1344-499: 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

1428-496: 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

1512-482: Is currently banned by international agreement. Furthermore, plate subduction zones are associated with very large megathrust earthquakes , making the effects of using any specific site for disposal unpredictable and possibly adverse to the safety of long-term disposal. Pachyrhizodus Pachyrhizodus is an extinct genus of ray-finned fish that lived during the Cretaceous to Paleocene in what

1596-444: 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

1680-614: 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

1764-477: 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

SECTION 20

#1732765542331

1848-494: Is now Europe, North America, South America, and Oceania. Many species are known, primarily from the Cretaceous of England and the midwestern United States. Pachyrhizodus fossils were first collected from Cambridgeshire, England in the 1840s and were very fragmentary, only a partial maxilla (SMB.9097) and were described as a species of Raphiosaurus in 1842 by Richard Owen . The type remains of Pachyrhizodus consisted of

1932-411: 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

2016-723: 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

2100-528: 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

2184-774: 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

2268-630: 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

2352-491: 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

2436-533: 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

2520-406: 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

2604-411: 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

Western Interior Seaway - Misplaced Pages Continue

2688-416: 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

2772-478: The 19th century, several complete and partial skeletons of Pachyrhizodus were collected from England, many of which belonging to P. basalis and P. subulidens . In 1899, Alban Stewart described the mandibles of another species, P. minimus , from Kansas and it is the most commonly discovered species of Pachyrhizodus . Since the 19th century, many complete skeletons and species have been described from many regions. P. caninus specifically has been discovered in

2856-506: 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

2940-588: 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

3024-712: The Dakotas and retreated south towards the Gulf of Mexico . This shrunken and final regressive phase is sometimes called the Pierre Seaway . During the early Paleocene , parts of the Western Interior Seaway still occupied areas of the Mississippi Embayment , submerging the site of present-day Memphis . Later transgression, however, was associated with the Cenozoic Tejas sequence , rather than with

3108-411: 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

3192-527: The Western Interior Seaway went through multiple periods of anoxia , when the bottom water was devoid of oxygen and the water column was stratified. At the end of the Cretaceous, continued Laramide uplift hoisted the sandbanks (sandstone) and muddy brackish lagoons (shale), thick sequences of silt and sandstone still seen today as the Laramie Formation , while low-lying basins between them gradually subsided. The Western Interior Seaway divided across

3276-472: 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,

3360-523: 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

3444-435: 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

Western Interior Seaway - Misplaced Pages Continue

3528-579: 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

3612-597: 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

3696-609: 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

3780-408: 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,

3864-448: 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

3948-446: 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

4032-440: 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

4116-455: 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

4200-420: 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

4284-430: 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

SECTION 50

#1732765542331

4368-574: 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

4452-415: 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

4536-484: 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

4620-463: 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

4704-538: The massive 4-to-5-metre (13 to 16 ft) long Xiphactinus , larger than any modern bony fish . Other sea life included invertebrates such as mollusks , ammonites , squid-like belemnites , and plankton including coccolithophores that secreted the chalky platelets that give the Cretaceous its name, foraminiferans and radiolarians . The seaway was home to early birds, including the flightless Hesperornis that had stout legs for swimming through water and tiny wings used for marine steering rather than flight; and

4788-492: 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

4872-429: 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

4956-412: 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

5040-399: 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

5124-510: 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

SECTION 60

#1732765542331

5208-484: The oxygen-poor bottom mud of the seaway. These left abundant fossils in the Kiowa , Greenhorn , Niobrara , Mancos , and Pierre formations. There is great variety in the shells and the many distinct species have been dated and can be used to identify specific beds in those rock formations of the seaway. Many species can easily fit in the palm of the hand, while some like Inoceramus (Haploscapha) grandis could be well over

5292-404: 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

5376-590: 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

5460-520: 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

5544-479: The previous event responsible for the seaway. The Western Interior Seaway was a shallow sea, filled with abundant marine life. Interior seaway denizens included predatory marine reptiles such as plesiosaurs , and mosasaurs . Other marine life included sharks such as Squalicorax , Cretoxyrhina , and the giant shellfish-eating Ptychodus mortoni (believed to be 10 metres (33 ft) long); and advanced bony fish including Pachyrhizodus , Enchodus , and

5628-444: 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

5712-802: The seaway began in the mid-Cretaceous when an arm of the Arctic Ocean transgressed south over western North America; this formed the Mowry Sea, so named for the Mowry Shale , an organic-rich rock formation . In the south, the Gulf of Mexico was originally an extension of the Tethys Ocean . In time, the southern embayment merged with the Mowry Sea in the late Cretaceous, forming a completed seaway, creating isolated environments for land animals and plants. Relative sea levels fell multiple times , as

5796-436: 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

5880-450: 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,

5964-453: 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

6048-447: 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

6132-481: 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

6216-526: 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

6300-509: 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

6384-451: 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

6468-454: 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

6552-400: The tern-like Ichthyornis , an early avian with a toothy beak. Ichthyornis shared the sky with large pterosaurs such as Nyctosaurus and Pteranodon . Pteranodon fossils are very common; it was probably a major participant in the surface ecosystem, though it was found in only the southern reaches of the seaway. Inoceramids (oyster-like bivalve molluscs) were well-adapted to life in

6636-439: 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

6720-444: 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

6804-400: 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

6888-628: 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

6972-543: Was little sedimentation on the eastern shores of the seaway; the western boundary, however, consisted of a thick clastic wedge eroded eastward from the Sevier orogenic belt . The western shore was thus highly variable, depending on variations in sea level and sediment supply. Widespread carbonate deposition suggests that the seaway was warm and tropical, with abundant calcareous planktonic algae . Remnants of these deposits are found in northwest Kansas. A prominent example

7056-582: 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

#330669