Misplaced Pages

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120-567: Cerro Guacha and the other volcanoes of that region are formed from the subduction of the Nazca Plate beneath the South American Plate . Above the subduction zone, the crust is chemically modified and generates large volumes of melts that form the local caldera systems of the APVC. Guacha is constructed over a basement of sediments. Two major ignimbrites, the 5.6-5.8 mya Guacha ignimbrite with

240-612: A c. 40 square kilometres (15 sq mi) carbonate platform with numerous fabrics of carbonate deposition. It is unclear what drives its formation as the climate at Pastos Grandes is similar to that of other salt lakes without such platforms but it may be a consequence of carbon dioxide degassing under the salar. At numerous points, calcite pisoliths are found at Pastos Grandes, usually associated with active or former springs. Rimstone dams and sinter terraces are also encountered close to inactive springs. All these cave formations encountered at Pastos Grandes are caused by

360-592: A Volcanic explosivity index of 8. The close succession of multiple large scale eruptions indicates that plutons feeding such eruptions are assembled over millions of years. The Guacha ignimbrite (including the Lowe Tara Ignimbrite, Chajnantor Tuff, Pampa Guayaques Tuff and possibly the Bonanza Ignimbrite) was first considered part of another ignimbrite named Atana Ignimbrite. It has a minimum volume of 1,300 cubic kilometres (310 cu mi) and covers

480-556: A dacite suite. Eruption products of Pastos Grandes are rich in potassium . Minerals encountered in the rock include amphibole , biotite , plagioclase , quartz and sanidine . The magmas underwent slow evolution in the 1,000,000 years preceding each eruption. Plutonic rocks linked to Pastos Grandes were erupted from the Chascon-Runtu Jarita vents 94,000 - 85,000 years ago. Three large ignimbrite-forming eruptions occurred at Pastos Grandes during its history. At first, it

600-500: 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,

720-590: A flattening of the subducting plate, occurred in the Oligocene 35-25 mya. Subsequently, renewed melt generation modified the overlying crust until major volcanism, associated with a "flare up" of ignimbritic volcanism occurred 10 mya. 100–250 kilometres (62–155 mi) beneath the local volcanic zone lies the Benioff zone of the subducting Nazca Plate . Recently a change in volcanic activity away from ignimbritic towards cone-forming volcanism has been observed. Guacha caldera

840-445: A lake basin north of Cerro Pastos Grandes, which is 10 kilometres (6.2 mi) wide and covers a surface area of about 100 square kilometres (39 sq mi) -120 square kilometres (46 sq mi) at an elevation of 4,400 metres (14,400 ft). It only covers a fraction of the area of Pastos Grandes caldera and is probably a remnant of a once-larger lake that filled the moat of the caldera. Earlier lacustrine episodes left

960-460: 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

1080-409: A layer of beige mud behind. This mud freezes during the winter months to a certain depth and cryoturbation has formed polygonal structures as well as large cracks in the crust on its surface. Surfaces of open water are concentrated on the eastern edge of the salt pan, in its very centre and isolated areas on the western side, these all form an intricated network of interconnected ponds especially in

1200-484: A million years later. Volcanic activity is linked to this fault zone and to the thermal maturation of the underlying crust. After 4 million years ago activity waned again in the Altiplano-Puna volcanic complex. The Guacha system was constructed over a timespan of 2 million years with a total volume of 3,400 cubic kilometres (820 cu mi). Eruptive activity occurred at regular intervals. Calculations indicate that

1320-457: 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

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1440-569: A minimum volume of 800 cubic kilometres (190 cu mi) and covers a surface area of at least 1,800 square kilometres (690 sq mi) in Chile and 2,300 square kilometres (890 sq mi) in Bolivia where it was at first not recognized. Some outflows are more than 200 metres (660 ft) thick. Several different dates have been determined on the basis of argon-argon dating , including 3.55±0.01 on biotite and 3.49±0.01 mya on sanidine , which

1560-460: 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

1680-520: A rim altitude of 5,250 metres (17,220 ft). Extended volcanic activity has generated two nested calderas, a number of lava domes and lava flows and a central resurgent dome. The caldera was discovered in 1978 thanks to Landsat imagery. It lies in Bolivia next to the Chilean frontier. The terrain is difficult to access being located at altitudes between 3,000–4,000 metres (9,800–13,100 ft). The caldera

1800-602: A summer wet climate, with most of the precipitation falling during a wet season in December–March. An estimate for the total precipitation is about 200 millimetres per year (7.9 in/year). That is, the climate is arid and evaporation rates can reach about 1,400 millimetres per year (55 in/year). Insolation is high and the temperatures can vary by as much as 15 °C (27 °F). During winter, they can drop as far as −25 °C (−13 °F). The lithium deposits have drawn attention of mining interests, with

1920-533: A surface area of at least 5,800 square kilometres (2,200 sq mi). Several different dates have been determined on the basis of argon-argon dating , including 5.81±0.01 on biotite and 5.65±0.01 mya on sanidine , which is the preferred age. Various samples are separated by distances of up to 130 kilometres (81 mi), making this ignimbrite among the most widespread in the Andes. One stream spreads 60 kilometres (37 mi) northwards past Uturunku volcano along

2040-409: A volume of 1,300 cubic kilometres (310 cu mi) and the 3.5-3.6 mya Tara ignimbrite with a volume of 800 cubic kilometres (190 cu mi) were erupted from Cerro Guacha. More recent activity occurred 1.7 mya and formed a smaller ignimbrite with a volume of 10 cubic kilometres (2.4 cu mi). The larger caldera has dimensions of 60 by 40 kilometres (37 mi × 25 mi) with

2160-510: 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 ,

2280-418: 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

2400-633: Is 4.2 mya old. After research indicated that it was different from another ignimbrite named Atana, it was originally linked to the Guacha caldera but Salisbury et al. in 2011 linked the Tara ignimbrite to Guacha instead. Another ignimbrite associated with Guacha is the Guataquina Ignimbrite named after Paso de Guataquina. It covers an area of 2,300 square kilometres (890 sq mi) and has an approximate volume of 70 cubic kilometres (17 cu mi). It

2520-499: 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

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2640-465: Is among the largest known. Volcanic structures are aligned along the eastern moat of this structure, which is filled by lacustrine deposits and welded ignimbrites. Another eastern collapse was generated by the Tara Ignimbrite eruption, with dimensions of 30 by 15 kilometres (18.6 mi × 9.3 mi). The margins of the caldera-graben structure are about 5,250 metres (17,220 ft) high while

2760-461: 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 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

2880-488: 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

3000-481: Is consistent with the climate of the Guacha region displaying long-term aridity for the last 10 mya as well as with the scarcity of pronounced geothermal systems in the APVC which are essentially limited to the El Tatio and Sol de Manana fields. Guacha has been the source of eruptions with volumes of more than 450 cubic kilometres (110 cu mi) dense rock equivalents . These eruptions in Guacha's case have

3120-418: 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. Pastos Grandes 21°45′S 67°50′W  /  21.750°S 67.833°W  / -21.750; -67.833 Pastos Grandes

3240-403: Is driven by the temperature difference between the slab and the surrounding asthenosphere, as 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

3360-444: 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

3480-437: Is flanked by lava domes on the north-northwestern, southwestern and southeastern side. The activity of Pastos Grandes may be associated with the ongoing development of a pluton underneath the caldera. Major regional faults running through the region have influenced the shape of the calderas, giving them an elliptic shape which is also evident at Pastos Grandes. Pastos Grandes has erupted calc-alkaline rocks which define

3600-593: 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

3720-491: Is known for having formed the Piedras de Dali hoodoos , named like that by tourists because of their surreal landscape. It has a volume of 10 cubic kilometres (2.4 cu mi) and it was apparently erupted at the hinge of the Guacha caldera. It has been argon-argon dated at 1.72±0.01 mya , making it the youngest Guacha caldera volcanite. The Puripicar ignimbrite has a volume of 1,500 cubic kilometres (360 cu mi) and

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3840-477: 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

3960-521: Is mostly tectonic in origin. The Guacha Ignimbrite is rhyodacite and rich in crystals. The Chajnantor lava dome contains sanidine while Rio Guacha of dacitic composition contains amphibole and pyroxene . The Tara ignimbrite has a composition intermediary to that of these two domes, being andesitic - rhyolithic . The Guacha Ignimbrite contains 62-65% SiO 2 , Puripicar 67-68% and the Tara Ignimbrite 63%. Plagioclase and quartz are found in all ignimbrites. Geological considerations indicate that

4080-573: Is named after Cerro Guacha, a feature named as such by local topographic maps. Later research by the Geological Service of Bolivia indicated the presence of three welded tuffs . Paleogene red beds and Ordovician sediments form the basement of the caldera. Cerro Guacha is part of the Altiplano-Puna volcanic complex , an area of extensive ignimbrite volcanism in the Central Andes between

4200-513: Is not very diverse, probably due to their relative youth and the harsh and often highly variable climates of the past in the region. Pastos Grandes is one of many endorheic lakes that cover the region. The neighbouring Altiplano was formerly covered by lakes as well during the Pleistocene . After they dried up, the Salar de Uyuni and Salar de Coipasa were left behind. The area of Pastos Grandes has

4320-411: 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

4440-689: 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

4560-467: Is part of the Altiplano-Puna volcanic complex (APVC), an igneous province in the central Andes covering a surface area of 70,000 square kilometres (27,000 sq mi). Here on an average altitude of 4,000 metres (13,000 ft) between 10 and 1 mya roughly 10,000 cubic kilometres (2,400 cu mi) of ignimbrites were erupted. Gravitic research indicates the presence of a low density area centered beneath Guacha. The magmatic body underpinning

4680-404: 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 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

4800-476: Is the name of a caldera and its crater lake in Bolivia . The caldera is part of the Altiplano-Puna volcanic complex , a large ignimbrite province that is part of the Central Volcanic Zone of the Andes. Pastos Grandes has erupted a number of ignimbrites through its history, some of which exceeded a volume of 1,000 cubic kilometres (240 cu mi). After the ignimbrite phase, the lava domes of

4920-566: Is the preferred age. The Chajnantor lavas and the Rio Guacha dome in the caldera have been K-Ar dated at 3.67±0.13 and 3.61±0.02 mya respectively. This ignimbrite ponded inside the Guacha caldera, and one particularly thick layer (>200 metres (660 ft)) is found beneath Zapaleri stratovolcano. This ignimbrite was formerly known as Upper Tara. Geological considerations indicate that this ignimbrite formed from pre-existent melts and an influx of andesitic magma. The Puripica Chico ignimbrite

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5040-561: The Altiplano and the Atacama and associated with the Central Volcanic Zone of the Andes. Several large caldera complexes are found within this area, formed by crustal magma chambers generated by magmas derived from the melting of deep crustal layers. Present day activity is limited to geothermal phenomena in El Tatio , Sol de Manana and Guacha, with recent activity encompassing the extrusion of Quaternary lava domes and flows. Deformation in

5160-519: 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

5280-610: The Cerro Chascon-Runtu Jarita complex were erupted close to the caldera and along faults . The caldera is the site of a few lakes, some of which are fed by hot springs . A number of minerals, including lithium , are dissolved in the lakes. Pastos Grandes lies in the Sud Lipez Region of Bolivia . Geographically the area is part of the Altiplano , a high plateau bordered by the Cordillera Occidental and

5400-754: 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

5520-569: The Cordillera Oriental . The Altiplano contains two large salt pans , the Salar de Uyuni and Salar de Coipasa . The specific area of Pastos Grandes is remote and poorly accessible, the existence of the caldera was first established by satellite imagery. The region has been heavily affected by volcanism , including large ignimbrites and stratovolcanoes extending into Chile . Volcanic rocks include andesite , dacite and rhyodacite with

5640-621: 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

5760-692: The Quetena valley until Suni K'ira . Some ash deposits in the northern Chilean Coast Range are linked to the Guacha eruption. The Guacha ignimbrite was also known as Lower Tara at first. The later Tara ignimbrite (including the Upper Tara Ignimbrite, the Filo Delgado Ignimbrite and the Pampa Tortoral Tuff) forms the western dome of the Guacha caldera and spreads mostly north and southeast, between Argentina , Bolivia and Chile . It has

5880-653: The South American Plate in the Peru-Chile Trench . This process has formed three main volcanic zones at the Andes, the Northern Volcanic Zone , the Central Volcanic Zone and the Southern Volcanic Zone . Pastos Grandes is part of the Central Volcanic Zone along with about 50 volcanoes with recent activity and other ignimbrite generating volcanic centres. This ignimbritic volcanism began in

6000-484: 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

6120-533: 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

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6240-406: 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

6360-411: 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

6480-416: 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

6600-402: The 40 by 25 kilometres (25 mi × 16 mi) smaller Pastos Grandes caldera. The caldera is about 35 by 40 kilometres (22 mi × 25 mi) wide and had a maximum depth of 400 metres (1,300 ft). Cerro Pastos Grandes is 5,802 metres (19,035 ft) high and shows traces of a sector collapse . It might be a 500–1,200 metres (1,600–3,900 ft) high resurgent dome and

6720-513: The APVC is centered beneath Guacha. Guacha caldera is also closely linked to the neighbouring La Pacana caldera. The Guacha caldera forms a structure with the neighbouring Cerro Panizos , Coranzulí and Vilama calderas associated with a fault named the Lípez lineament. Activity along this lineament commenced with the Abra Granada volcanic complex 10 mya ago and dramatically increased more than

6840-501: 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

6960-584: 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

7080-688: The Andes prevents moisture from the Amazon from reaching the Altiplano area. The area is also too far north for the precipitation associated with the Westerlies to reach Guacha. This arid climate may go back to the Mesozoic and was enhanced by geographical and orogenic changes during the Cenozoic . Oxygen isotope analysis indicates that the Guacha caldera ignimbrites have had little influence from meteoric waters. This

7200-420: The Guacha ignimbrite was stored at a depth of 5–9.2 kilometres (3.1–5.7 mi) and the Tara ignimbrite at a depth of 5.3–6.4 kilometres (3.3–4.0 mi). Zircon temperatures are 716 °C (1,321 °F), 784 °C (1,443 °F) and 705 °C (1,301 °F) for Guacha, Tara and Chajnantor respectively. The climate of the Central Andes is characterized by extreme aridity. The eastern mountain chain of

7320-456: The Guacha system was supplied by magmas at a rate of 0.007–0.018 cubic kilometres per year (5.3 × 10–0.000137 cu mi/Ms). Located at a high altitude in an area of long term arid climate has preserved old volcanic deposits over time. Thus, unlike in other areas of the world such as the Himalayas where water erosion governs the landscape the morphology of the Altiplano-Puna volcanic complex

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7440-405: The area occurs beneath Uturuncu volcano north of the Guacha centre. A westward-facing semicircular scarp (60 by 40 kilometres (37 mi × 25 mi)) contains subvertically banded Guacha ignimbrite layers rich in lithic clasts and is the presumable vent of the Guacha ignimbrite. The resulting caldera formed like a trapdoor and with a volume of 1,200 cubic kilometres (290 cu mi)

7560-472: 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,

7680-510: 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 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

7800-458: The caldera floors are about 1,000 metres (3,300 ft) lower. Probably dacitic lava domes are found on the northern caldera rim, with the caldera floor possibly containing lava flows. The caldera contains a resurgent dome , the western part of it is formed by the Tara ignimbrite while the eastern is part of the Guacha ignimbrite. This dome was cut by the Tara collapse, exposing 700 metres (2,300 ft) of Guacha ignimbrites. The resurgent dome in

7920-408: The caldera rises about 1.1 kilometres (0.68 mi) above the caldera floor. A second resurgence episode occurred inside the Tara caldera. The caldera is filled up to 1 kilometre (0.62 mi) thick with ignimbrites. Three lava domes, roughly coeval with the Tara ignimbrite, are constructed on the northern side of the resurgent dome. The western dome is named Chajnantor and is the most silica -rich of

8040-406: 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 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

8160-435: 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

8280-506: 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, 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

8400-611: 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

8520-597: 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

8640-609: 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

8760-448: 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

8880-440: 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

9000-455: 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

9120-406: The domes. Rio Guacha in the middle is more dacitic . The Puripica Chico lavas on the western side of the caldera are not associated with a collapse. Dark coloured lava flows are found to the southwest of the caldera. Some geothermal activity occurs within the caldera. Laudrum et al. suggested that the heat from Guacha and Pastos Grandes may be transferred to the El Tatio geothermal system to

9240-619: The forearc may include an accretionary wedge of sediments scraped off the subducting slab and accreted to 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

9360-420: 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

9480-457: The former dominating in the Chilean stratovolcanoes and the latter in the ignimbrites. The dry regional climate means that there is little erosion and that volcanic centres are well conserved. The surface covered by volcanic rocks amounts to about 300,000 square kilometres (120,000 sq mi). Volcanic activity in the region is the consequence of the subduction of the Nazca Plate beneath

9600-430: 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

9720-453: The lakes include amphipods , elmids and leeches in freshwater and by Cricotopus in saltwater. Additional animals are Euplanaria dorotocephala , Chironomidae , Corixidae , Cyclopoida , Ephydridae , Harpacticoida , Orchestidae , Ostracoda and Tipulidae species. Similar but different animal species have been found in other local lakes, indicating that they are largely separate systems. The animal flora of such Altiplano lakes

9840-402: The late Miocene and formed a large field known as the Altiplano-Puna volcanic complex , a large volcanic province which clusters around the tripoint between Argentina , Bolivia and Chile. Pastos Grandes is a nested caldera which underwent repeated collapse in the past, most likely along defined sectors of its rim. It has been subdivided into two calderas, a larger Chuhuila caldera and

9960-518: The latter seems to rise from the ring fault of Pastos Grandes. but is apparently unrelated to the caldera. Cerro Chascon-Runtu Jarita is less than 100,000 years old according to argon-argon dating . This and ongoing geothermal manifestations suggest that volcanic activity may still occur at Pastos Grandes. Finally, Pastos Grandes and Cerro Guacha may be the heat source for the El Tatio geothermal field west of Pastos Grandes. At an elevation of 4,430 metres (14,530 ft), Pastos Grandes contains

10080-574: 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

10200-484: 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

10320-463: 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

10440-484: 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

10560-402: 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 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

10680-467: 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 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

10800-399: 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

10920-404: 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

11040-582: 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

11160-603: The precipitation of calcite from oversaturated waters at the surface. What drives the loss of carbon dioxide and thus the oversaturation is not clear but may involve photosynthesis by algae. Algae and diatoms grow within the open waters in Pastos Grandes, the diatoms being represented by oligohaline species such as some Fragilaria and Navicularia species. Different water surfaces are dominated by different diatom species, distinctions that are only partly mediated by different salinities. Animal species found within

11280-520: 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

11400-444: 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

11520-418: The salt pan has been considered a potential site for lithium and potassium mining. Salt contents range 144–371 grams per litre (0.0052–0.0134 lb/cu in). The salt chemistry is strongly influenced by the climate; the precipitation of mirabilite due to cold and evaporation of water cause changes in the composition of the waters. Unique among most other salars of the Andes, Pastos Grandes features

11640-432: The salt pan; the longest flow through the southeastern parts of the catchment. The entire drainage basin of the lake has a surface area of 655 square kilometres (253 sq mi) -660 square kilometres (250 sq mi) and is delimited to the west and east by rhyolitic ridges. Apart from surface streams, springs contribute to the water budget of Pastos Grandes. Hot springs are active or were recently active on

11760-436: 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

11880-450: 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,

12000-453: 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

12120-447: 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

12240-481: 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

12360-399: The subducted plate and in the overlying mantle wedge. This type of melting selectively concentrates volatiles and transports them into 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

12480-434: 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 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,

12600-526: 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

12720-500: 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

12840-446: The subduction zone, there is often an outer trench high or outer trench swell . Here the plate shallows slightly before plunging downwards, as 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,

12960-451: 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

13080-454: 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

13200-439: 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

13320-444: 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

13440-658: The west. Guacha is part of a volcanic complex in the back-arc region of the Andes in Bolivia. The Central Andes are underlaid by the Paleoproterozoic - Paleozoic Arequipa-Antofalla terrane . The Central Andes started to form 70 mya . Previously, the area was formed from a Paleozoic marine basin with some early volcanics. Since the Jurassic , subduction has been occurring on the western margin of present-day South America , resulting in variable amounts of volcanic activity. A short interruption of volcanism, associated with

13560-413: The western half of the salt pan. One of these open water surfaces on the western side of the lake basin is known as Laguna Caliente, while another square-shaped lake in the southern part of the caldera is known as Laguna Khara. Sometimes after heavy precipitation, these open water surfaces can join into a ring lake around the centre. Intermittent streams drain the catchment of Pastos Grandes and reach

13680-448: The western side of the salt pan and bear names such as La Salsa, La Rumba and El Ojo Verde, where temperatures of 20–75 °C (68–167 °F) have been measured. On the western shore, colder springs predominate. The heat appears to originate from a 200–250 °C (392–482 °F) hot reservoir. Salts found within the salt pan include gypsum , halite and ulexite . The brines are rich in boron , lithium and sodium chloride ,

13800-446: Was also attributed to Pastos Grandes, although it originated in a centre northeast of the Pastos Grandes caldera known as Cerro Juvina. These ignimbrites crop out on the outside of the Pastos Grandes caldera, where they extend to distances of 50 kilometres (31 mi), but also cover parts of the caldera. Given the volumes involved, at least some of the eruptions are classified as 8 on the volcanic explosivity index . Pastos Grandes

13920-455: Was assumed that large eruptions first occurred 8.1 million years ago, a second 5.6 million years and a third 2.3 million years ago. However, it is not clear which of any eruption formed the caldera. A number of ignimbrites has been attributed to Pastos Grandes, some of them may be different names for the same ignimbrite: The 6.1 million years old Carcote ignimbrite may also have originated here. The 5.22 ± 0.02 million years old Alota ignimbrite

14040-607: 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

14160-409: Was later interpreted to be a combination of the Guacha, Tara and non-Guacha Atana ignimbrites. Subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere 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

14280-563: 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

14400-432: Was volcanically active for a long time, more than many other Altiplano-Puna volcanic complex centres. Later more recent volcanic centres formed within the caldera, the youngest of these centres are relatively recent Such recent centres close to Pastos Grandes are Cerro Chao and Cerro Chascon-Runtu Jarita complex . The former of which lies on a lineament that appears to coincide with the caldera rim of Pastos Grandes, and

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