Oceanic trenches are topographic depressions of the sea floor narrow in width, but long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are parallel to a volcanic island arc, about 200 km from a volcanic arc. Oceanic trenches extend 3 to 4 km below the level of the surrounding oceanic floor; the greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km2/yr. There are 50,000 km of convergent plate margins around the Pacific Ocean—the reason for the reference “Pacific-type” margin—but they are in the eastern Indian Ocean, with short convergent margin segments in the Atlantic Ocean and in the Mediterranean Sea.
Globally, there are over 50 major ocean trenches covering an area of 1.9 million km2 or about 0.5% of the oceans. Trenches that are infilled are known as "troughs" and sometimes they are buried and lack bathymetric expression, but the fundamental plate tectonics structures that these represent mean that the great name should be applied here; this applies to the Cascadia, southern Lesser Antilles, Calabrian trenches. Trenches along with volcanic arcs and zones of earthquakes that dip under the volcanic arc as as 700 km are diagnostic of convergent plate boundaries and their deeper manifestations, subduction zones. Trenches are related to but distinguished from continental collision zones, where continental crust enters a subduction zone; when buoyant continental crust enters a trench, subduction stops and the area becomes a zone of continental collision. Features analogous to trenches are associated with collisions zones, including sediment-filled foredeeps, such as those the Ganges River and Tigris-Euphrates rivers flow along.
Trenches were not defined until the late 1940s and 1950s. The bathymetry of the ocean was of little interest until the late 19th and early 20th centuries, when the Transatlantic telegraph cables on the seafloor between the continents were first laid; the elongated bathymetric expression of trenches was not recognized until well into the 20th century. The term "trench" does not appear in Hjort's classic oceanography book. Instead they applied the term "deep "for the deepest parts such as Challenger Deep. Experiences from World War I battlefields emblazoned the concept of a trench as an elongate depression defining an important boundary leading to the term “trench” being used to describe natural features in the early 1920s; the term was first used in a geologic context by Scofield two years after the war ended to describe a structurally controlled depression in the Rocky Mountains. Johnstone, in his 1923 textbook An Introduction to Oceanography, first used the term in its modern sense for any marked, elongate depression of the sea bottom.
During the 1920s and 1930s, Felix Andries Vening Meinesz developed a unique gravimeter that could measure gravity aboard a submarine and used it to measure gravity over trenches. His measurements revealed; the concept of downwelling at trenches was characterized by Griggs in 1939 as the tectogene hypothesis, for which he developed an analogue model using a pair of rotating drums. World War II in the Pacific led to great improvements of bathymetry in the western Pacific, the linear nature of these deeps became clear; the rapid growth of deep sea research efforts the widespread use of echosounders in the 1950s and 1960s confirmed the morphological utility of the term. Important trenches were identified and their greatest depths sonically plumbed; the early phase of trench exploration culminated in the 1960 descent of the Bathyscaphe Trieste, which set an unbeatable world record by diving to the bottom of the Challenger Deep. Following Robert S. Dietz’ and Harry Hess’ articulation of the seafloor spreading hypothesis in the early 1960s and the plate tectonic revolution in the late 1960s the term “trench“ has been redefined with plate tectonic as well as bathymetric connotations.
Trenches are centerpieces of the distinctive physiography of a convergent plate margin. Transects across trenches yield asymmetric profiles, with gentle outer slopes and a steeper inner slopes; this asymmetry is due to the fact that the outer slope is defined by the top of the downgoing plate, which must bend as it starts its descent. The great thickness of the lithosphere requires; as the subducting plate approaches the trench, it first bends upwards to form the outer trench swell descends to form the outer trench slope. The outer trench slope is disrupted by a set of sub-parallel normal faults that'staircase' the seafloor down to the trench; the plate boundary is defined by the trench axis itself. Beneath the inner trench wall, the two plates slide past each other along the subduction decollement, the seafloor intersection of which defines the trench location; the overriding plate contains a volcanic arc and forearc region. The volcanic arc is caused by physical and chemical interactions between the subducted plate at depth and asthenospheric mantle associated
2014 Iquique earthquake
The 2014 Iquique earthquake struck off the coast of Chile on 1 April, with a moment magnitude of 8.2, at 20:46 local time. The epicenter of the earthquake was 95 kilometres northwest of Iquique; the mainshock was preceded by a number of moderate to large shocks and was followed by a large number of moderate to large aftershocks, including a M7.7 event on 3 April. The megathrust earthquake triggered a tsunami of up to 2.11 metres that hit Iquique at 21:05 local time. Similar-sized tsunamis were reported to have hit the coasts of Pisagua and Arica. A number of mid-sized quakes struck the same area in the preceding weeks; these quakes and the main tremor are associated with the boundary of the Nazca Plate and the South American Plate. There was a cluster of earthquakes starting from the one occurring on March 16 with a magnitude of Mw 6.7, a large earthquake had been expected. The 8.2 earthquake was smaller than what was expected, with a rupture of 200 km in length instead of the expected 600 km rupture.
The earthquake was felt in Chile and Bolivia. The intensity reached intensity VIII in Chile. Four men died of heart attacks and one woman was crushed to death when a wall collapsed. A loader died of the injuries afterwards. Around 80,000 were displaced by the event. Electricity and water services were interrupted in the regions of Arica y Parinacota and Tarapacá. During the aftermath of the earthquake, 293 prisoners escaped from a women's prison in Iquique when a wall collapsed. Many returned voluntarily a short time while Chilean soldiers searched for the rest. According to the Peruvian emergency services, nine people were injured, seven households have been affected, one temple has collapsed and electricity outages in the affected regions of Tacna and Arequipa occurred, which were restored later. There were several significant aftershocks above 6.0 magnitude and many more of lower magnitude over subsequent days. Such large earthquakes can have effects far away other than tsunamis. A megathrust quake can shake the entire earth, but causes stronger movement and strain on the entire associated oceanic plate, beyond the few hundred kilometer rupture zone.
Though too far to be an aftershock, a 6.0 quake on a thin protruding wedge of the Nazca Plate was reported off Panama within 12 hours of the main shock. Under advice from the Pacific Tsunami Warning Center, tsunami warnings were issued for the Latin American Pacific coastlines of Chile and Ecuador shortly after the earthquake occurred. Chile was subsequently hit by a tsunami of 2.11 m in its northern territories. The tsunami warning was canceled for all countries except Chile and Peru within a few hours of the earthquake; the tsunami warning was canceled for both Peru at around 4:58 UTC on 2 April. Hawaii was under a tsunami advisory for over 13 hours. On April 3 local time, tsunamis were observed in Japan; the tsunami reached 60 centimetres high in Iwate Prefecture, Japan. 1868 Arica earthquake 1877 Iquique earthquake 2010 Chile earthquake List of earthquakes in 2014 List of earthquakes in Chile Lay, Thorne. "The 1 April 2014 Iquique, Chile, 8.1 earthquake rupture sequence". Geophysical Research Letters.
41: 3818–3825. Bibcode:2014GeoRL..41.3818L. Doi:10.1002/2014GL060238. M8.2 - 95km NW of Iquique, Chile – United States Geological Survey Tsunami alert after 8.2 quake strikes off Chile – BBC News 5 Dead After Powerful Quake Strikes Off Chile's Coast – ABC News Powerful earthquake strikes off Chile – CNN Tsunami Animation: Iquique, Chile, 1 April 2014 – National Tsunami Warning Center The International Seismological Centre has a bibliography and/or authoritative data for this event
Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced to sink due to gravity into the mantle. Regions where this process occurs are known as subduction zones. Rates of subduction are in centimeters per year, with the average rate of convergence being two to eight centimeters per year along most plate boundaries. Plates include continental crust. Stable subduction zones involve the oceanic lithosphere of one plate sliding beneath the continental or oceanic lithosphere of another plate due to the higher density of the oceanic lithosphere; that is, the subducted lithosphere is always oceanic while the overriding lithosphere may or may not be oceanic. Subduction zones are sites that have a high rate of volcanism and earthquakes. Furthermore, subduction zones develop belts of deformation and metamorphism in the subducting crust, whose exhumation is part of orogeny and leads to mountain building in addition to collisional thickening.
Subduction zones are sites of gravitational sinking of Earth's lithosphere. Subduction zones exist at convergent plate boundaries where one plate of oceanic lithosphere converges with another plate; the descending slab, the subducting plate, is over-ridden by the leading edge of the other plate. The slab sinks at an angle of twenty-five to forty-five degrees to Earth's surface; this sinking is driven by the temperature difference between the subducting oceanic lithosphere and the surrounding mantle asthenosphere, as the colder oceanic lithosphere has, on average, a greater density. At a depth of greater than 60 kilometers, the basalt of the oceanic crust is converted to a metamorphic rock called eclogite. At that point, the density of the oceanic crust provides additional negative buoyancy, it is at subduction zones that Earth's lithosphere, oceanic crust and continental crust, sedimentary layers and some trapped water are recycled into the deep mantle. Earth is so far the only planet. Subduction is the driving force behind plate tectonics, without it, plate tectonics could not occur.
Oceanic subduction zones dive down into the mantle beneath 55,000 kilometers of convergent plate margins equal to the cumulative 60,000 kilometers of mid-ocean ridges. Subduction zones burrow but are imperfectly camouflaged, geophysics and geochemistry can be used to study them. Not the shallowest portions of subduction zones are known best. Subduction zones are asymmetric for the first several hundred kilometers of their descent, they start to go down at oceanic trenches. Their descents are marked by inclined zones of earthquakes that dip away from the trench beneath the volcanoes and extend down to the 660-kilometer discontinuity. Subduction zones are defined by the inclined array of earthquakes known as the Wadati–Benioff zone after the two scientists who first identified this distinctive aspect. Subduction zone earthquakes occur at greater depths than elsewhere on Earth; the subducting basalt and sediment are rich in hydrous minerals and clays. Additionally, large quantities of water are introduced into cracks and fractures created as 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, hot and more buoyant than the surrounding rock, rises into the overlying mantle where it lowers the pressure in the mantle rock to the point of actual melting, generating magma; the magmas, in turn, rise. The mantle-derived magmas can continue to rise to Earth's surface, resulting in a volcanic eruption; the chemical composition of the erupting lava depends upon the degree to which the mantle-derived basalt interacts with Earth's crust and/or undergoes fractional crystallization. Above subduction zones, volcanoes exist in long chains called volcanic arcs. Volcanoes that exist along arcs tend to produce dangerous eruptions because they are rich in water and tend to be explosive. Krakatoa, Nevado del Ruiz, Mount Vesuvius are all examples of arc volcanoes. Arcs are known to be associated with precious metals such as gold and copper believed to be carried by water and concentrated in and around their host volcanoes in rock called "ore".
Although the process of subduction as it occurs today is well understood, its origin remains a matter of discussion and continuing study. Subduction initiation can occur spontaneously if denser oceanic lithosphere is able to founder and sink beneath adjacent oceanic or continental lithosphere. Both models can yield self-sustaining subduction zones, as oceanic crust is metamorphosed at great depth and becomes denser than the surrounding mantle rocks. Results from numerical models favor induced subduction initiation for most modern subduction zones, supported by geologic studies, but other analogue modeling shows the possibility of spontaneous subduction from inherent density differences between two plates at passiv
1906 Valparaíso earthquake
The 1906 Valparaíso earthquake hit Valparaíso, Chile, on August 16 at 19:55 local time. Its epicenter was offshore from the Valparaíso Region, its intensity was estimated at magnitude 8.2 Mw. Much of Valparaíso was destroyed; the earthquake was felt from Peru to Puerto Montt. Reports said. A tsunami was generated; the earthquake killed a reported 3,882 people. The record of previous seismic activity includes major earthquakes in 1647, 1730 and 1822; the 1906 disaster was predicted by Captain Arturo Middleton, Chief of the Chilean Army Meteorological Office, in a letter, published in El Mercurio, one week before it occurred. Admiral Luis Gómez Carreño ordered the shooting of at least 15 people, who were caught looting after the earthquake. A Board for Reconstruction was formed some weeks after the earthquake; the Seismological Service of Chile was created. Chile lies above a convergent plate boundary, an area where the Nazca Plate under the Pacific Ocean is subducted or moved beneath the South American Plate.
In the region around Valparaiso, the rate of convergence is about 70 mm/yr. As these two plates converge, it drives the Nazca plate with massive movements called megathrust earthquakes; the 1906 event was one of many large earthquakes in Chile along this plate boundary. Earthquakes can originate at the plate interface itself or within either the subducting or overriding plates. Citing the conjunction of Neptune with the moon, Captain Arturo Middleton, Chief of the Chilean Army's Meteorological Office, predicted the earthquake in a letter published in the Valparaíso newspaper El Mercurio on August 6. Captain Middleton was criticized in the following days, was described as "ignorant and obscurantist." On August 16, 1906, at 19:55 local time, while most Chileans were dining, a subterraneous sound was heard, before it ended, the first tremor occurred, lasting about four minutes. The second tremor occurred at 20:06 and, was much more violent. There were numerous aftershocks: at least 56 of them occurred during the first 24 hours after the beginning tremors.
The magnitude of the earthquake has been estimated to be 8.4 ML, 8.2 Mw or Ms = 8.2–8.3. The energy release has been re-evaluated with an estimated seismic moment of 2.8 x 1028, equivalent to a magnitude of 8.26 Mw . The rupture length of the earthquake has been estimated at about 200 km with a focal depth of about 40 km; the focal mechanism has been assessed using contemporary seismograph records from five stations, which were published soon after the earthquake. The data suggest that the earthquake was along the subduction interface. Modelling of a tsunami using these source parameters shows that this earthquake was the origin of the transpacific tsunamis recorded that same day in Hawaii and Japan, rather than the contemporaneous 1906 Aleutian Islands earthquake; the 30-minute time gap between the Aleutian and Chilean earthquakes is thought to be coincidental, with no causal link between the two. The earthquake caused damage from Illapel to Talca. There were several destructive fires in El Almendral, Mercado Cardonal, Teatro de la Victoria, the Intendencia, the Maritime Government in Sotomayor Square and the Fiscal Dock at the port.
The earthquake was felt in Santiago, the capital of Chile. The newspaper El Mercurio reported in its August 17 edition that "the earthquake was produced in a violent way since its beginning, provoked an indescribable panic through all the four thousands of inhabitants of Santiago and an unprecedented terror in the last years. Two or three-story buildings the most solid ones such as the National Congress, were swinging like a vessel in the sea; the shakings were so strong that many people thought the earth was going to open itself in deep and long strips."According to the University of Chile, there were 3,882 deaths. The earthquake left more than 20,000 injured. On August 19, Admiral Luis Gómez Carreño was appointed Plaza Port Chief. Gómez ordered the distribution of water and food, removal of corpses and demolition of buildings in risk of collapse, from a tent in Plaza de La Victoria. Adm. Gómez ordered the shooting of at least 15 people. Despite the state of the city, authorities organized themselves into relief groups.
Firefighters from other cities of Chile, including Santiago, Concepción and Talcahuano, moved to Valparaíso to help the local Fire Bureau. Physician José Grossi worked to counteract the plagues. On August 25, President Germán Riesco and President-Elect Pedro Montt arrived at Valparaíso, they horseback to survey the magnitude of the disaster. Some weeks after the earthquake, a Board for Reconstruction was formed, using money received from other countries. In 1906, the Seismological Service of Chile was created, its first chief executive was Fernand de Montessus de Ballore. The effects of this historic seismic event in the Valpariso rupture zone would be studied and measurable in the context of further activity in this vicinity. List of earthquakes in 1906 List of earthquakes in Chile Seismicity of the Chilean coast Das, Shamita. Earthquake Source Mechanics. Washington, D. C.: American Geophysical Union. ISBN 9780875904054. La catástrofe del 16 de agosto de 1906 en la República de Chile. Santiago, Chile: Imprenta y Litografía Barcelona.
TERREMOTO DE 1906 in Valparaíso Fire Bureau's website The International Sei
Bolivia the Plurinational State of Bolivia is a landlocked country located in western-central South America. The capital is Sucre; the largest city and principal industrial center is Santa Cruz de la Sierra, located on the Llanos Orientales a flat region in the east of Bolivia. The sovereign state of Bolivia is a constitutionally unitary state, divided into nine departments, its geography varies from the peaks of the Andes in the West, to the Eastern Lowlands, situated within the Amazon Basin. It is bordered to the north and east by Brazil, to the southeast by Paraguay, to the south by Argentina, to the southwest by Chile, to the northwest by Peru. One-third of the country is within the Andean mountain range. With 1,098,581 km2 of area, Bolivia is the fifth largest country in South America, the 27th largest in the world and the largest landlocked country in the Southern Hemisphere; the country's population, estimated at 11 million, is multiethnic, including Amerindians, Europeans and Africans.
The racial and social segregation that arose from Spanish colonialism has continued to the modern era. Spanish is the official and predominant language, although 36 indigenous languages have official status, of which the most spoken are Guarani and Quechua languages. Before Spanish colonization, the Andean region of Bolivia was part of the Inca Empire, while the northern and eastern lowlands were inhabited by independent tribes. Spanish conquistadors arriving from Cuzco and Asunción took control of the region in the 16th century. During the Spanish colonial period Bolivia was administered by the Royal Audiencia of Charcas. Spain built its empire in large part upon the silver, extracted from Bolivia's mines. After the first call for independence in 1809, 16 years of war followed before the establishment of the Republic, named for Simón Bolívar. Over the course of the 19th and early 20th century Bolivia lost control of several peripheral territories to neighboring countries including the seizure of its coastline by Chile in 1879.
Bolivia remained politically stable until 1971, when Hugo Banzer led a coup d'état which replaced the socialist government of Juan José Torres with a military dictatorship headed by Banzer. Banzer's regime cracked down on leftist and socialist opposition and other forms of dissent, resulting in the torture and deaths of a number of Bolivian citizens. Banzer was ousted in 1978 and returned as the democratically elected president of Bolivia from 1997 to 2001. Modern Bolivia is a charter member of the UN, IMF, NAM, OAS, ACTO, Bank of the South, ALBA and USAN. For over a decade Bolivia has had one of the highest economic growth rates in Latin America, it is a developing country, with a medium ranking in the Human Development Index, a poverty level of 38.6%, one of the lowest crime rates in Latin America. Its main economic activities include agriculture, fishing and manufacturing goods such as textiles, refined metals, refined petroleum. Bolivia is rich in minerals, including tin and lithium. Bolivia is named after Simón Bolívar, a Venezuelan leader in the Spanish American wars of independence.
The leader of Venezuela, Antonio José de Sucre, had been given the option by Bolívar to either unite Charcas with the newly formed Republic of Peru, to unite with the United Provinces of Rio de la Plata, or to formally declare its independence from Spain as a wholly independent state. Sucre opted to create a brand new state and on 6 August 1825, with local support, named it in honor of Simón Bolívar; the original name was Republic of Bolívar. Some days congressman Manuel Martín Cruz proposed: "If from Romulus comes Rome from Bolívar comes Bolivia"; the name was approved by the Republic on 3 October 1825. In 2009, a new constitution changed the country's official name to "Plurinational State of Bolivia" in recognition of the multi-ethnic nature of the country and the enhanced position of Bolivia's indigenous peoples under the new constitution; the region now known as Bolivia had been occupied for over 2,500 years. However, present-day Aymara associate themselves with the ancient civilization of the Tiwanaku culture which had its capital at Tiwanaku, in Western Bolivia.
The capital city of Tiwanaku dates from as early as 1500 BC when it was a small, agriculturally based village. The community grew to urban proportions between AD 600 and AD 800, becoming an important regional power in the southern Andes. According to early estimates, the city covered 6.5 square kilometers at its maximum extent and had between 15,000 and 30,000 inhabitants. In 1996 satellite imaging was used to map the extent of fossilized suka kollus across the three primary valleys of Tiwanaku, arriving at population-carrying capacity estimates of anywhere between 285,000 and 1,482,000 people. Around AD 400, Tiwanaku went from being a locally dominant force to a predatory state. Tiwanaku expanded its reaches into the Yungas and brought its culture and way of life to many other cultures in Peru and Chile. Tiwanaku was not a violent culture in many respects. In order to expand its reach, Tiwanaku exercised great political astuteness, creating colonies, fostering trade agree
Convergent boundaries are areas on Earth where two or more lithospheric plates collide. One plate slides beneath the other causing a process known as subduction; the subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, orogenesis, destruction of lithosphere, deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, continental-continental lithosphere; the geologic features related to convergent boundaries vary depending on crust types. Plate tectonics is driven by convection cells in the mantle. Convection cells are the result of heat generated by radioactive decay of elements in the mantle escaping to the surface and the return of cool materials from the surface to the mantle; these convection cells bring hot mantle material to the surface along spreading centers creating new crust. As this new crust is pushed away from the spreading center by formation of newer crust, it cools and becomes denser.
Subduction initiates. The force of gravity helps drive the subducting slab into the mantle. Evidence supports; as the cool subducting slab sinks deeper into the mantle, it is heated causing dehydration of hydrous minerals. This releases water into the hotter asthenosphere, which leads to partial melting of asthenosphere and volcanism. Both dehydration and partial melting occurs along the 1000 °C isotherm at depths of 65 – 130 km; some lithospheric plates consist of both oceanic lithosphere. In some instances, initial convergence with another plate will destroy oceanic lithosphere, leading to convergence of two continental plates. Neither continental plate will subduct, it is that the plate may break along the boundary of continental and oceanic crust. Seismic tomography reveals pieces of lithosphere. Subduction zones are areas where one lithospheric plate slides beneath another at a convergent boundary due to lithospheric density differences; these can vary. Subduction zones are marked by an abundance of earthquakes, the result of internal deformation of the plate, convergence with the opposing plate, bending at the oceanic trench.
Earthquakes have been detected to a depth of 670 km. The cold and dense subducting plates are pulled into mantle and help drive mantle convection. In collisions between two oceanic plates, the cooler, more dense oceanic lithosphere sinks beneath warmer, less dense oceanic lithosphere; as the slab sinks deeper into the mantle, it releases water from dehydration of hydrous minerals in the oceanic crust. This water causes partial melting. Partial melt will travel up through the asthenosphere reach the surface, form volcanic island arcs; when oceanic lithosphere and continental lithosphere collide, the dense oceanic lithosphere slides beneath the less dense continental lithosphere. An accretionary wedge forms on the continental crust as deep-sea sediments and oceanic crust are scraped from the oceanic plate. Volcanic arcs form on continental lithosphere as the result of partial melting due to dehydration of the hydrous minerals of the subducting slab; some lithospheric plates consist of both oceanic crust.
Subduction initiates as oceanic lithosphere slides beneath continental crust. As the oceanic lithosphere subducts to greater depths, the attached continental crust is pulled closer to the subduction zone. Once the continental lithosphere reaches the subduction zone, subduction processes are altered as continental lithosphere is more buoyant and resists subduction beneath other continental lithosphere. A small portion of the continental crust may be subducted until the slab breaks, allowing the oceanic lithosphere to continue subducting, hot asthenosphere to rise and fill the void, rebound of the continental lithosphere. Evidence of this continental rebound include ultrahigh pressure metamorphic rocks which form at depths of 90 – 125 km that are exposed at the surface. Oceanic crust contains hydrated minerals such as the amphibole group. During subduction, oceanic lithosphere is heated and metamorphosed causing dehydration of these hydrous minerals contained within basalts, releasing water into the asthenosphere.
The release of water into the asthenosphere leads to partial melting. Partial melting allows the rise of more buoyant, hot material and can lead to volcanism at the surface and emplacement of plutons in the subsurface; this processes which generate magma are not understood. Where these magmas reach the surface they create volcanic arcs. Volcanic arcs can form as arcs on continental crust. Three series of volcanic rocks form arcs, calc-alkaline, alkaline which are rare. Back arc basins form behind a volcanic arc and are associated with extensional tectonics and high heat flow being home to seafloor spreading centers; these spreading centers are like mid ocean ridges, though the magma composition of back arc basins is more varied and contains a higher water content than mid ocean ridge magmas. Back arc basins are characterized by thin, hot lithosphere. Opening of back arc basins are still being studied but it’s possible that movement of hot asthenosphere into lithosphere causes extension. Oceanic trenches are narrow topographic lows.
The Humboldt Current called the Peru Current, is a cold, low-salinity ocean current that flows north along the western coast of South America. It is an eastern boundary current flowing in the direction of the equator, extends 500–1,000 km offshore; the Humboldt Current is named after the Prussian naturalist Alexander von Humboldt. In 1846, von Humboldt reported measurements of the cold-water current in his book Cosmos; the current extends from the southern Chile to northern Peru where cold, upwelled waters intersect warm tropical waters to form the Equatorial Front. Sea surface temperatures off the coast of Peru, around 5th parallel south, reach temperatures as low as 16 °C; this is uncharacteristic of tropical waters, as most other regions have temperatures measuring above 25 °C. Upwelling brings nutrients to the surface, which support phytoplankton and increase biological productivity; the Humboldt Current is a productive ecosystem. It is the most productive eastern boundary current system, it accounts for 18-20% of the total worldwide marine fish catch.
The species are pelagic: sardines and jack mackerel. The system's high productivity supports other important fishery resources as well as marine mammals and seabirds. Periodically, the upwelling that drives the system's productivity is disrupted by the El Niño-Southern Oscillation event with large social and economical impacts; the Humboldt has a considerable cooling influence on the climate of Chile and Ecuador. It is largely responsible for the aridity of Atacama Desert in northern Chile and coastal areas of Peru and of the aridity of southern Ecuador. Marine air is cooled by the current and thus; the trade winds are the primary drivers of the Humboldt Current circulation. Variability in this system is driven by latitudinal shifts between the Intertropical Convergent Zone and the trade winds in the north. Shifts within the South Pacific High at mid-latitudes, as well as cyclonic storms and movement of the Southern Westerlies southward contribute to system changes. Atmospheric variability off central Chile is enhanced by the aggravation of coastal low pressure systems trapped between the marine boundary layer and the coastal mountains.
This is prominent poleward from 27th parallel south to 42nd parallel south. The Humboldt current, occupying the upper ocean, flows equatorward carrying fresh, cold Sub-Antarctic surface water northward, along the outskirts of the subtropical gyre; the main flow of the current veers offshore in southern Peru, as a weaker limb continues to flow equatorward. Around 18th parallel south the fresh, cold waters begin to mix with the warm, high salinity Subtropical Surface waters; this collision causes partial subductions. Within this region, the equatorial undercurrent flows eastward along the equator, feeding the Peru-Chile undercurrent that moves poleward. Off the coast of central Chile, there is a coastal transition zone, characterized by high eddy kinetic energy; this energy forms mesoscale eddies. The CTZ has three distinct regions within its boundaries: high chlorophyll-a concentrations in wide regions off the coast of Peru, high chlorophyll-a concentrations in wide regions off the coast of Chile, high chlorophyll-a concentrations in narrow regions off the coast of northern Chile.
High chlorophyll-a concentrations are found within 50 km of the coast. The limb of the HCS that veers off the coast of Peru creates a decrease in ventilation within the system; this lack of ventilation is the primary driver of an intense oxygen minimum zone, formed in the sub-surface to intermediate depths. In the north, the EUC ventilates the OMZ, in the south the PCU advects low oxygen waters southward towards northern Chile; this OMZ is the fourth largest permeant hypoxic zone in the world's oceans. It occupies an area about 2.18 ± 0.66 × 106 km3. The core of this zone is centered off Peru, creating a shallow upper boundary that reaches from about 100 m down to 600 m. Another factor contributing to the OMZ is sinking and decay of primary productive resources; the OMZ forces many organisms to stay near the surface where nutrients and oxygen are obtainable. The presence of a shallow OMZ restricts the migration of zooplankton within the water column. Between 0 and 600 m, many species of zooplankton occupy this space within the OMZ.
This allows for a substantial exchange of carbon between the euphotic layer and the OMZ. 75% of the total zooplankton biomass move in and out of the OMZ. The OMZ serves as a refuge for organisms that can live in hypoxic conditions. Coastal upwelling is the main factor contributing to the high biological productivity of the Humboldt current. Upwelling within the current is not uniform across the entire system. Three notable upwelling subsystems are produced by this current: seasonal upwelling in Chile only during the spring and summer, because of the displacement of the subtropical center of high pressure during the period January–March, upwelling "shadow", less productive, but still large in northern Chile and Southern Peru, productive year-round upwelling in Peru; the upwelling shadow identified between 35°S and 15°S is caused by the oligotrophic subtropical gyre impinging on the coast. This creates a narrow, but productive, upwelling zone. Due to the upwelling zones within the Humboldt current, biological diversity is high.
The Humboldt Current is considered a Class I productive (>300 gC/