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37-499: The oceanic crust is moving away from the East Pacific Rise to either side. Near Easter Island the rate is over 150 mm (6 in) per year which is the fastest in the world. However, on the northern end, it is much slower at only roughly 60 mm ( 2 + 1 ⁄ 2  in) per year. On the eastern side of the rise, the eastward-moving Cocos and Nazca plates meet the westward moving South American plate and

74-413: A chemosynthetic ecosystem rather than one using photosynthesis . Oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates . It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust , composed of troctolite , gabbro and ultramafic cumulates . The crust overlies the rigid uppermost layer of

111-404: A combination of factors favorable to ore formation. A key factor for magma saturation and volatile formation is the sulphide saturation in the original magma. High solubility and high concentration of sulphur in magma lead to high sulphide saturation and could be an important factor in formation of big ore deposits. This saturated sulphide in melt can enrich the concentration of metals in

148-455: A complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are thrust onto and preserved on the continents), comparisons of the seismic structure of the oceanic crust with laboratory determinations of seismic velocities in known rock types, and samples recovered from

185-407: A density of about 2.7 grams per cubic centimeter. The crust uppermost is the result of the cooling of magma derived from mantle material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas. As

222-467: A liquid and a solid phase from a specific initial chemical solution. Depending on the initial chemical composition of the liquid, the melt is going to generate different minerals. The initial fluid can form crystals (solid phase) by cooling down and by adding a certain water's concentration. In subduction zones , more specific in magmatic arcs, it is possible to transport water into the Earth's mantle , as

259-405: A mechanism that extracts the interstitial liquid from the already crystallised solids. There is an increase in the solid portion of the magma chamber with decreasing temperature. This implies that the permeability lowers with temperature. This also halts convection in the system, and the progressive accumulation of crystals increases the efficiency of expulsion of melt from the underlying parts of

296-420: Is magma that contains a significant amount of crystals (up to 50% of the volume) suspended in the liquid phase (melt). As the crystal fraction makes up less than half of the volume , there is no rigid large-scale three-dimensional network as in solids . As such, their rheological behavior mirrors that of absolute liquids. Within a single crystal mush, there is grading to a higher solid fraction towards

333-410: Is continuously being created at mid-ocean ridges. As continental plates diverge at these ridges, magma rises into the upper mantle and crust. As the continental plates move away from the ridge, the newly formed rocks cool and start to erode with sediment gradually building up on top of them. The youngest oceanic rocks are at the oceanic ridges, and they get progressively older away from the ridges. As

370-449: Is in the west Pacific and north-west Atlantic  — both are about up to 180-200 million years old. However, parts of the eastern Mediterranean Sea could be remnants of the much older Tethys Ocean , at about 270 and up to 340 million years old. The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the basalt . A symmetrical pattern of positive and negative magnetic lines emanates from

407-455: Is the concentration of silica in the magma, which leads to the differentiation of magma. At the end of the crystallization is possible to crystallize quartz, but only when the melt contains a high concentration of SiO 2 , which is the main component of the mineral. The rapid increase in the crystal content over a short temperature interval generates ideal rheological conditions for melt extraction. The buoyant, lighter magmas extracted from

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444-425: Is very high, the system starts behaving like a solid within the timescales of applied stress in the system ( Maxwell time ). Since magma comprise different compositional fractions, it may undergo different processes like melt extraction, phase separation, water and gas enrichment, and injection of magma from deeper magma chambers (typically within the lower crust (geology) ). All these may directly or indirectly cause

481-556: The Azores and Iceland . Prior to the Neoproterozoic Era 1000 Ma ago the world's oceanic crust was more mafic than present-days'. The more mafic nature of the crust meant that higher amounts of water molecules ( OH ) could be stored the altered parts of the crust. At subduction zones this mafic crust was prone to metamorphose into greenschist instead of blueschist at ordinary blueschist facies . Oceanic crust

518-712: The North American plate and are being subducted under them. The belt of volcanos along the Andes and the arc of volcanoes through Central America and Mexico are the direct results of this collision. Due east of the Baja California peninsula , the Rise is sometimes referred to as the Gulf of California Rift Zone . In this area, newly formed oceanic crust is intermingled with rifted continental crust originating from

555-414: The chamber due to loading. These mechanisms contribute to the decoupling of behaviour between crystals and liquid, enabling the liquid to percolate upwards. This extraction mechanism, however, operates in an optimal interval of crystal fraction. If there is a low crystal fraction, convection still operates in the system, thus halting crystal settling and liquid extraction. However, if the crystal fraction

592-400: The eruption events. One of the factors that can initiate magma eruption is phase separation of the liquid and crystal components of the crystal mush. As the magma develops over time and the crystal content of the magma increases, phase separation is taking place and the liquid phase of the magma is pushed up, driven by its buoyancy as a result of its lower density. Volcanoes , as valves of

629-424: The mantle . The crust and the rigid upper mantle layer together constitute oceanic lithosphere . Oceanic crust is primarily composed of mafic rocks, or sima , which is rich in iron and magnesium. It is thinner than continental crust , or sial , generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 grams per cubic centimeter as opposed to continental crust which has

666-647: The East Pacific Rise have oblique spreading, such as the Nazca–Pacific plate boundary between 29°S and 32°S. This is seafloor spreading that is not orthogonal to the nearest ridge segment. The southern extension of the East Pacific Rise (the PAR) merges with the Southeast Indian Ridge at the Macquarie triple junction south of New Zealand . The southern stretch of the East Pacific Rise is also one of

703-868: The North American plate. Near Easter Island , the East Pacific Rise meets the Chile Rise at the Easter Island and Juan Fernandez microplates, trending off to the east where it subducts under the South American plate at the Peru–Chile Trench along the coast of southern Chile . This portion of the Rise has been referred to as the Cape Adare-Easter Island Ridge, Albatross Cordillera, Easter Island Cordillera, Easter Island Rise, and Easter Island Swell. Parts of

740-407: The content of the liquid phase, and consequently, the pressure inside the chamber, which is concurrently released as a flux of lava onto the earth surface. The “crystal mush” is a leading and most promising model of magma bodies, that supported by findings ( ignimbrites ) on the surface, although there are some controversial aspects. Magmas containing volatiles are stable at high pressures in

777-537: The crystal mush can ascend through the crust and form plutonic complexes. The crystal mush model presents a view of plutons as crystal graveyards. This implies that the crystals are separated from the silicate liquid where they were crystallised. This model contrasts with the view of intrusive magma bodies as failed eruptions. Upon cooling, a crystal mush may experience different Igneous differentiation processes, such as crystal fractionation, mixing, melting. To create an accumulation of crystals, there has to be

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814-402: The deep crust; when this mixture of magma and volatiles rises though the crust the pressure decreases and the volatiles start exsolving from the magma. This leads to oversaturation of volatiles in magma. Also crystallization of dry minerals within the magma and crystal mush will progressively increase the fluid concentration of the melt, possibly leading to saturation. These fluids, lighter than

851-477: The denser oceanic plate subducts under the other – continental or younger oceanic – plate. Water is a key factor for this geochemical process and has a significant impact on the geotherm of the subducting plate. It causes partial melting of the crust, which will then generate a chamber of liquid phase that will later be crystallized and generate minerals. The source of water stays in minerals that contain H 2 O in their chemical compositions. Another key factor

888-416: The emission of gas increases. This process is expressed by a high fraction of bubbles that drive the liquid phase toward the earth surface. In addition, the higher the content of dissolved water and other gases, the more violent the eruption will be. The last and the most trivial cause for magma eruption is an injection of fresh magma from lower parts of the crust into the issued magma chamber, which increases

925-488: The existence of crystal mushes rather than fully liquid magmatic bodies. Crystal mushes can have a wide range of mineral and chemical compositions , from mafic ( SiO 2 -poor, MgO -rich) to felsic (SiO 2 -rich, MgO-poor). Crystal mushes form at various depths in the Earth's crust . They result from fractional crystallization of a fluid. Fractional crystallization is a physical and chemical process that generates

962-486: The fastest-spreading divergent boundaries on Earth, peaking at 79.3 mm (3.12 in)/year. Along the East Pacific Rise the hydrothermal vents called black smokers were first discovered by the RISE project in 1979, and have since been extensively studied. These vents are forming volcanogenic massive sulfide ore deposits on the ocean floor. Many unique deep-water creatures have been found with vents, that subsist in

999-412: The lavas cool they are, in most instances, modified chemically by seawater. These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. But most magma crystallises at depth, within the lower oceanic crust . There, newly intruded magma can mix and react with pre-existing crystal mush and rocks. Although

1036-448: The magma they were once in, exsolve and rise up to even shallower crust; potentially forming ore deposits . If these volatiles are sufficiently concentrated to form ore minerals and if they are trapped by the surrounding host rocks in the continental crust within a narrow enough space, they can form economically valuable ore deposits. Some magmatic chambers are also more predisposed to form large ore deposits, due to regional setting and

1073-406: The mantle rises it cools and melts, as the pressure decreases and it crosses the solidus . The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses

1110-482: The margins of the pluton , while the liquid fraction increases towards the uppermost portions, forming a liquid lens at the top. Furthermore, depending on depth of placement crystal mushes are likely to contain a larger portion of crystals at greater depth in the crust than at shallower depth, as melting occurs from the adiabatic decompression of the magma as it rises, this is particularly the case for mid-ocean ridges . Seismic investigation offers strong evidence for

1147-460: The mid-ocean ridge. New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock, its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth. New magma then forces the older cooled magma away from the ridge. This process results in parallel sections of oceanic crust of alternating magnetic polarity. Crystal mush A crystal mush

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1184-438: The mid-oceanic ridge basalts, which are derived from low- potassium tholeiitic magmas . These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements . There can be found basalts enriched with incompatible elements, but they are rare and associated with mid-ocean ridge hot spots such as surroundings of Galapagos Islands ,

1221-413: The ocean floor by submersibles , dredging (especially from ridge crests and fracture zones ) and drilling. Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle. The most voluminous volcanic rocks of the ocean floor are

1258-508: The oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it. The oceanic lithosphere subducts at what are known as convergent boundaries . These boundaries can exist between oceanic lithosphere on one plate and oceanic lithosphere on another, or between oceanic lithosphere on one plate and continental lithosphere on another. In

1295-436: The open system, provide the path for gas release and magma eruption. The amount of dissolved gases may be a further factor that controls the eruption event. The deeper the magma chamber is located, the higher is the amount of gas that can be dissolved in the magma (high pressure conditions), especially in andesitic and rhyolitic magmas. As phase separation occurs and the liquid fraction increases along with decreasing pressure,

1332-485: The second situation, the oceanic lithosphere always subducts because the continental lithosphere is less dense. The subduction process consumes older oceanic lithosphere, so oceanic crust is seldom more than 200 million years old. The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson Cycle . The oldest large-scale oceanic crust

1369-538: The solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is the Gakkel Ridge under the Arctic Ocean . Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this is Iceland which has crust of thickness ~20 km. The age of

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