Is the oceanic or continental crust older?
oceanic crust, forms the subsurface of the deep sea areas. The oceanic crust differs fundamentally from the continental crust in structure, composition and genesis. Structure and structure are derived from the process of ocean floor spreading (plate tectonics). From a morphological point of view, three main regions can be distinguished: the deep-sea regions, the mid-ocean ridges and the peripheral areas with the transition to the continent. The shelf areas, e.g. the North Sea and Baltic Sea with shallow water depths, belong to the continental shelf and thus to the continental crust. The submarine seamounts must be seen as morphological and genetic anomalies.
Apart from a more or less thick sedimentary cover, the oceanic crust is made up of basalts and gabbros. Due to this mafic composition, the mean SiO is2-Share only approx. 50 percent by weight. In addition, in contrast to the continental crust, it is only about 200 million years old. Formerly older oceanic earth crusts have been subduced (subduction) in the course of the plate tectonic processes. Under a mean water cover of 4.5 km, the oceanic crust has a thickness of 5-10 km. It is threefold. Under a thin layer of a few hundred meters of pelagic sediments (the thickness of the sediments increases from the oceanic ridge to the deep sea basin), the crystalline rocks follow the actual oceanic crust. The upper half kilometer of the oceanic basement consists of basaltic lavas, followed by the approximately 1 km thick zone with basaltic intrusions. The lower part of the oceanic crust, with a thickness of a few kilometers, is built up by gabbros. The lower limit of the oceanic crust forms, similar to the continental area, the Mohorovi
ic´ discontinuity (short: Moho). Here the seismic speed increases from 6.8 km / s to over 8.0 km / s. The peridotic mantle of the earth is referred to as layer 4. Under the mid-ocean ridges, the Moho discontinuity loses contrast as melts rise from greater depths here and penetrate the oceanic crust. The magnetic stripe patterns are characteristic of the oceanic crust (paleomagnetism). These stripe patterns are formed in the course of penetration and cooling and the associated thermoremanent magnetization of the mafic rocks on the flanks of the mid-ocean ridges. The changing polarity of the magnetic anomalies comes about by reversing the earth's magnetic field.
From a geothermal point of view, the oceanic crust shows a characteristic behavior. With increasing distance from the axis of the back towards the continental margin, the heat flux density increases from 150-200 mW / m2 to approx. 40 mW / m2 as the oceanic crust cools on its way from the still hot mid-ocean ridge to the cold continental margin. Depending on the geotectonic position, the transition from the oceanic to the continental crust is different. At the passive continental margin there is a thinning of the oceanic crust and a thickening of the continental crust. This transition is very often associated with thick sediment deposits. Oil and gas deposits can form in these sediments. The transition on the active continental margins is completely different. Here the oceanic crust and the underlying oceanic lithosphere submerge as a lower plate under the lithosphere of the continental upper plate. This superposition process leads to an oceanic Moho discontinuity being superimposed by a continental Moho discontinuity in the convergence zone. [PG]
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