Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. 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 solidified and uppermost layer of the mantle. The crust and the solid 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 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 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 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 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 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, the Azores and Iceland.
Prior to the Neoproterozoic Era 1000 Ma ago as 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 is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges.
As 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−1 half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses 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 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 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 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 are 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 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.
The African Plate is a major tectonic plate straddling the equator as well as the prime meridian. It includes much of the continent of Africa, as well as oceanic crust which lies between the continent and various surrounding ocean ridges. Between 60 million years ago and 10 million years ago, the Somali Plate began rifting from the African Plate along the East African Rift. Since the continent of Africa consists of crust from both the African and the Somali plates, some literature refers to the African Plate as the Nubian Plate to distinguish it from the continent as a whole.Basement (geology)
In geology, basement and crystalline basement are the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin. In the same way, the sediments or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover".Chemosynthesis
In biochemistry, chemosynthesis is the biological conversion of one or more carbon-containing molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic compounds (e.g., hydrogen gas, hydrogen sulfide) or methane as a source of energy, rather than sunlight, as in photosynthesis. Chemoautotrophs, organisms that obtain carbon through chemosynthesis, are phylogenetically diverse, but also groups that include conspicuous or biogeochemically-important taxa include the sulfur-oxidizing gamma and epsilon proteobacteria, the Aquificae, the methanogenic archaea and the neutrophilic iron-oxidizing bacteria.
Many microorganisms in dark regions of the oceans use chemosynthesis to produce biomass from single carbon molecules. Two categories can be distinguished. In the rare sites at which hydrogen molecules (H2) are available, the energy available from the reaction between CO2 and H2 (leading to production of methane, CH4) can be large enough to drive the production of biomass. Alternatively, in most oceanic environments, energy for chemosynthesis derives from reactions in which substances such as hydrogen sulfide or ammonia are oxidized. This may occur with or without the presence of oxygen.
Many chemosynthetic microorganisms are consumed by other organisms in the ocean, and symbiotic associations between chemosynthesizers and respiring heterotrophs are quite common. Large populations of animals can be supported by chemosynthetic secondary production at hydrothermal vents, methane clathrates, cold seeps, whale falls, and isolated cave water.
It has been hypothesized that chemosynthesis may support life below the surface of Mars, Jupiter's moon Europa, and other planets. Chemosynthesis may have also been the first type of metabolism that evolved on Earth, leading the way for cellular respiration and photosynthesis to develop later.Continental crust
Continental crust is the layer of igneous, sedimentary, and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in silicates and aluminium minerals and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate minerals and is denser. Changes in seismic wave velocities have shown that at a certain depth (the Conrad discontinuity), there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.
The continental crust consists of various layers, with a bulk composition that is intermediate (SiO2 wt% = 60.6). The average density of continental crust is about 2.83 g/cm3, less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3. Continental crust is also less dense than oceanic crust, whose density is about 2.9 g/cm3. At 25 to 70 km, continental crust is considerably thicker than oceanic crust, which has an average thickness of around 7–10 km. About 40% of Earth's surface area and about 70% of the volume of the Earth's crust is continental crust.Most continental crust is dry land above sea level. However, 94% of the Zealandia continental crust region is submerged beneath the Pacific Ocean, with New Zealand constituting 93% of the above-water portion.Convergent boundary
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually 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, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and 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 the formation of newer crust, it cools, thins, and becomes denser. Subduction initiates when this dense crust converges with the less dense crust. The force of gravity helps drive the subducting slab into the mantle. Evidence supports that the force of gravity will increase plate velocity. As the relatively 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, generally at depths of 65 – 130 km.Some lithospheric plates consist of both continental and 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 likely that the plate may break along the boundary of continental and oceanic crust. Seismic tomography reveals pieces of lithosphere that have broken off during convergence.Depression (geology)
In geology, a depression is a landform sunken or depressed below the surrounding area. Depressions form by various mechanisms.
Blowout: a depression created by wind erosion typically in either a partially vegetated sand dune ecosystem or dry soils (such as a post-glacial loess environment).
Glacial valley: a depression carved by erosion by a glacier.
River valley: a depression carved by fluvial erosion by a river.
Area of subsidence caused by the collapse of an underlying structure such as sinkholes in karst terrain.
Sink: an endorheic depression generally containing a persistent or intermittent (seasonal) lake, a salt flat (playa) or dry lake, or an ephemeral lake.Collapse-related:
Sinkhole: a depression formed as a result of the collapse of rocks lying above a hollow. This is common in karst regions.
Kettle: a shallow, sediment-filled body of water formed by melting glacial remnants in terminal moraines.
Thermokarst hollow: caused by volume loss of the ground as the result of permafrost thawing.Impact-related:
Impact crater: a depression created by an impact such as a meteorite crater.Sedimentary-related:
Sedimentary basin: in sedimentology, an area thickly filled with sediment in which the weight of the sediment further depresses the floor of the basin.Structural or tectonic-related:
Structural basin: a syncline-like depression; a region of tectonic downwarping as a result of isostasy (the Hawaiian Trough is an example) or subduction (such as the Chilean Central Valley).
Graben or rift valley: fallen and typically linear depressions or basins created by rifting in a region under tensional tectonic forces.
Pull-apart basin caused by offset in a strike slip or transform fault (example: the Dead Sea area).
Oceanic trench: a deep linear depression on the ocean floor. Oceanic trenches are caused by subduction (when one tectonic plate is pushed underneath another) of oceanic crust beneath either oceanic crust or continental crust.
A basin formed by an ice sheet: an area depressed by the weight of the ice sheet resulting in post-glacial rebound after the ice melts (the area adjacent to the ice sheet may be pulled down to create a peripheral depression.)Volcanism-related:
Caldera: a volcanic depression resulting from collapse following a volcanic eruption.
Pit crater: a volcanic depression smaller than a caldera formed by a sinking, or caving in, of the ground surface lying over a void.
Maar: a depression resulting from phreatomagmatic eruption or diatreme explosion.Farmakas
Farmakas (Greek: Φαρμακάς) is a village in the Nicosia District of Cyprus, located around 5 km east of Palaichori Oreinis.
It is a fragment of a fully developed oceanic crust, consisting of plutonic, intrusive and volcanic rocks and chemical sediments. The stratigraphic completeness of the ophiolite makes it unique. It was created during the complex process of sea-floor spreading and formation of oceanic crust and was emerged and placed in its present position through complicated tectonic processes related to the collision of the Eurasian plate to the north and the African plate to the south.
The Troodos Ophiolite has a very significant role for the water budget of the island. Most of the rocks, especially the gabbros and the sheeted dykes are good aquifers due to fracturing. The perennial rivers running radially are feeding the main aquifers in the periphery of the Troodos and the plains.Gabbro
Gabbro ( ) is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is chemically equivalent to rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term "gabbro" may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic".Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute. The Laramide orogeny occurred in a series of pulses, with quiescent phases intervening. The major feature that was created by this orogeny was deep-seated, thick-skinned deformation, with evidence of this orogeny found from Canada to northern Mexico, with the easternmost extent of the mountain-building represented by the Black Hills of South Dakota. The phenomenon is named for the Laramie Mountains of eastern Wyoming. The Laramide orogeny is sometimes confused with the Sevier orogeny, which partially overlapped in time and space.
The orogeny is commonly attributed to events off the west coast of North America, where the Kula and Farallon Plates were sliding under the North American plate. Most hypotheses propose that oceanic crust was undergoing flat-slab subduction, i.e., with a shallow subduction angle, and as a consequence, no magmatism occurred in the central west of the continent, and the underlying oceanic lithosphere actually caused drag on the root of the overlying continental lithosphere. One cause for shallow subduction may have been an increased rate of plate convergence. Another proposed cause was subduction of thickened oceanic crust.
Magmatism associated with subduction occurred not near the plate edges (as in the volcanic arc of the Andes, for example), but far to the east, called the Coast Range Arc. Geologists call such a lack of volcanic activity near a subduction zone a magmatic gap. This particular gap may have occurred because the subducted slab was in contact with relatively cool continental lithosphere, not hotter asthenosphere. One result of shallow angle of subduction and the drag that it caused was a broad belt of mountains, some of which were the progenitors of the Rocky Mountains. Part of the proto-Rocky Mountains would be later modified by extension to become the Basin and Range Province.List of tectonic plates
This is a list of tectonic plates on the Earth's surface. Tectonic plates are pieces of Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). The composition of the two types of crust differs markedly, with mafic basaltic rocks dominating oceanic crust, while continental crust consists principally of lower-density felsic granitic rocks.Mid-ocean ridge
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of ~ 2,600 meters (8,500 ft) and rises about two kilometers above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a divergent plate boundary. The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin. The production of new seafloor and oceanic lithosphere results from mantle upwelling in response to plate separation. The melt rises as magma at the linear weakness in the oceanic crust, and emerges as lava, creating new crust and lithosphere upon cooling. The Mid-Atlantic Ridge is a spreading center that bisects the North and South Atlantic basins; hence the origin of the name 'mid-ocean ridge'. Most oceanic spreading centers are not in the middle of their hosting ocean basis but regardless, are called mid-ocean ridges. Mid-ocean ridges around the globe are linked by plate tectonic boundaries and the outline of the ridges across the ocean floor appears similar to the seam of a baseball. The mid-ocean ridge system thus is the longest mountain range on Earth, reaching about 65,000 km (40,000 mi).Obduction
Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate.Obduction occurs where a fragment of continental crust is caught in a subduction zone with resulting overthrusting of oceanic mafic and ultramafic rocks from the mantle onto the continental crust. Obduction often occurs where a small tectonic plate is caught between two larger plates, with the crust (both island arc and oceanic) welding onto an adjacent continent as a new terrane. When two continental plates collide, obduction of the oceanic crust between them is often a part of the resulting orogeny.Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.Ophiolite
An ophiolite is a section of the Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
The Greek word ὄφις, ophis (snake) is found in the name of ophiolites, because the superficial texture of some of them. Serpentinite especially evokes a snakeskin. The suffix lite from the Greek lithos means "stone". Some ophiolites have a green color. The origin of these rocks, present in many mountainous massifs, remained uncertain until the advent of plate tectonics.
Their great significance relates to their occurrence within mountain belts such as the Alps and the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction. This insight was one of the founding pillars of plate tectonics, and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts.Partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid.
Partial melting occurs where the solidus and liquidus temperatures are different. For single minerals this can happen when they exhibit solid solution, for example in olivines between iron and magnesium. In rocks made up of several different minerals, some will melt at lower temperatures than others.
Partial melting is an important process in geology with respect to the chemical differentiation of crustal rocks. On the Earth, partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle and oceanic crust at subduction zones creates continental crust. In all these places partial melting is often associated with volcanism, although some melts do not make it to the surface. Partial melts are thought to play an important role in enriching old parts of the continental. In conclusion partial melting is not all that important and can be forgotten about. lithosphere in incompatible elements. Partial melts produced at depth move upwards due to the compaction of the surrounding matrix.Pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava under water, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one metre in diameter. They form the upper part of Layer 2 of normal oceanic crust.Seafloor spreading
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.Sedimentary basin
Sedimentary basins are regions of Earth of long-term subsidence creating accommodation space for infilling by sediments. The subsidence can result from a variety of causes that include: the thinning of underlying crust, sedimentary, volcanic, and tectonic loading, and changes in the thickness or density of adjacent lithosphere. Sedimentary basins occur in diverse geological settings usually associated with plate tectonic activity. Basins are classified structurally in various ways, with a primary classifications distinguishing among basins formed in various plate tectonic regime (divergent, convergent, transform, intraplate), the proximity of the basin to the active plate margins, and whether oceanic, continental or transitional crust underlies the basin. Basins formed in different plate tectonic regimes vary in their preservation potential. On oceanic crust, basins are likely to be subducted, while marginal continental basins may be partially preserved, and intracratonic basins have a high probability of preservation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification. A number of basins formed in extensional settings can undergo inversion which has accounted for a number of the economically viable oil reserves on earth which were formerly basins.Subduction
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 typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.Plates include both oceanic crust and 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 usually 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 also leads to mountain building in addition to collisional thickening.Volcanic arc
A volcanic arc is a chain of volcanoes formed above a subducting plate,
positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally, volcanic arcs result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench. The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the mantle. As the oceanic plate is subducted, it is subjected to greater and greater pressures with increasing depth. This pressure squeezes water out of the plate and introduces it to the mantle. Here the mantle melts and forms magma at depth under the overriding plate. The magma ascends to form an arc of volcanoes parallel to the subduction zone.
These should not be confused with hotspot volcanic chains, where volcanoes often form one after another in the middle of a tectonic plate, as the plate moves over the hotspot, and so the volcanoes progress in age from one end of the chain to the other. The Hawaiian Islands form a typical hotspot chain; the older islands (tens of millions of years old) to the northwest are smaller and more lush than the recently created (400,000 years ago) Hawaii island itself, which is more rocky. Hotspot volcanoes are also known as "intra-plate" volcanoes, and the islands they create are known as Volcanic Ocean Islands. Volcanic arcs do not generally exhibit such a simple age-pattern.
There are two types of volcanic arcs:
oceanic arcs form when oceanic crust subducts beneath other oceanic crust on an adjacent plate, creating a volcanic island arc. (Not all island arcs are volcanic island arcs.)
continental arcs form when oceanic crust subducts beneath continental crust on an adjacent plate, creating an arc-shaped mountain belt.In some situations, a single subduction zone may show both aspects along its length, as part of a plate subducts beneath a continent and part beneath adjacent oceanic crust.
Volcanoes are present in almost any mountain belt, but this does not make it a volcanic arc. Often there are isolated, but impressively huge volcanoes in a mountain belt. For instance, Vesuvius and the Etna volcanoes in Italy are part of separate but different kinds of mountainous volcanic ensembles.
The active front of a volcanic arc is the belt where volcanism develops at a given time. Active fronts may move over time (millions of years), changing their distance from the oceanic trench as well as their width.
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