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Age of oceanic crust
Age of oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. The lithosphere (IPA, from the Greek λίθος for "rocky" + σφαίρα for "sphere" is the solid outermost shell of a rocky Planet. An ocean (from Greek, ''Okeanos'' (Oceanus) is a major body of saline water, and a principal component of the Hydrosphere. Oceanic crust is primarily composed of mafic rocks, or sima. Mafic is an adjective describing a Silicate mineral or rock that is rich in magnesium and iron the term was derived by contracting "magnesium" and "ferric" Sima is the name for the lower layer of the Earth's crust. This layer is made of rocks rich in silicates and Magnesium minerals It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is more dense, having a mean density of about 3. Sial or SiAl is also the name for the upper layer of the Earth's crust, which is also known as the Continental crust because 3 grams per cubic centimeter. For other uses of the words gram or gramme see Gram (disambiguation. A cubic centimetre or cubic centimeter (symbol cm3 —the abbreviation cc, though widely used is deprecated is a commonly used unit of Volume

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Composition

Although a complete section of oceanic crust has not yet been drilled, there are estimations of composition based on analyses of ophiolites, comparison of 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. An Ophiolite is a section of the Earth's Oceanic crust and the underlying upper mantle that has been uplifted or emplaced to be exposed within Continental A submersible, or Bathyscaphe, is a type of Underwater vessel with limited mobility which is typically transported to its area of operation by a surface Oceanic crust is significantly simpler than continental crust and generally it can be divided in three layers. The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed

Geochemistry

The most voluminous eruptive rocks composing ocean floor are mid-ocean ridge basalts (MORBs), which are derived from low-potassium tholeiitic magmas. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 These rocks have low concentrations of large ion litophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements (Th, U, Nb, Ta and Pb). Incompatible element is a term used in Petrology and Geochemistry. There can be found MORBs 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. HotSpot is the primary Java Virtual Machine for desktops and servers produced by Sun Microsystems. The Azores ( Açores ɐˈsoɾɨʃ or) is a Portuguese Archipelago in the Atlantic Ocean, about 1500 km (950  mi) from Iceland, officially the Republic of Iceland ( ( Ísland or Lýðveldið Ísland ( [2]

Life cycle

Oceanic lithosphere generally does not survive longer than 200 million years. It is continuously being created at mid-ocean ridges. At these ridges, magma rises into the upper mantle and crust, as the plates diverge. Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet 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 is progressively older away from the ridges.

The oceanic lithosphere subducts at what are known as convergent boundaries. In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other In Plate tectonics, a convergent boundary – also known as a convergent plate boundary or a destructive plate boundary – is an actively deforming region 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 200 million years old. Plate tectonics is the study of these processes. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere

Magnetic lines

Main article: Seafloor spreading

The oceanic crust displays an interesting pattern of parallel magnetic lines, parallel to the ocean ridges, frozen in the basalt. Seafloor spreading occurs at Mid-ocean ridges where new Oceanic crust is formed through volcanic activity and then gradually moves away from the ridge In the 1950’s, scientists mapped the magnetic field generated by rocks on the ocean floor. They noticed a symmetrical pattern of positive and negative magnetic lines as they moved along the ocean floor, and the line of symmetry was at the mid ocean ridge. The fact that the anomalies were symmetrical at the mid-ocean ridge was explained by the hypothesis that new rock was being formed by magma at the mid-ocean ridges, and the ocean floor was spreading out from this point. When the magma cooled to form rock, it aligned itself with the current position of the north magnetic pole of the Earth (which has reversed many times in its past) at the time of its cooling. A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged New magma forced the older cooled magma away from the ridge. Approximately half of the new rock was formed on one side of the ridge and half on the other.

References

  1. ^ D. R. Bowes (1989) The Encyclopedia of Igneous and Metamorphic Petrology, Van Nostrand Reinhold ISBN: 0-442-20623-2
  2. ^ Clare P. Marshall, Rhodes W. Fairbridge (1999) Encyclopedia of Geochemistry, Kluwer Academic Publishers ISBN: 0-412-75500-9

See also

The continental crust is the layer of granitic, Sedimentary, and Metamorphic rocks which form the Continents and the areas of shallow seabed The Mohorovičić discontinuity, usually referred to as the Moho, is the boundary between the Earth 's crust and the mantle. A mid-ocean ridge or mid-oceanic ridge is an underwater Mountain range typically having a valley known as a Rift running along its axis formed by
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