The Sea Floor
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The Sea Floor |
Marine biology etextbook
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The deep sea floor is underlain by rocks that are of a very different composition from the rocks that make up the continental land masses. The seafloor or oceanic crust is
composed primarily of pillow lavas of basalt (an igneous rock composed primarily of pyroxene, olivine, and plagioclase feldspar) extruded near oceanic ridges. This surface pillow lava layer is underlain by a layer of sheeted basaltic dikes, and below that
are massive and layered gabbro (a coarse grained version of basalt), which rests on the upper mantle layer consisting of peridotite (an ultramafic igneous rock, mostly of the mineral olivine,
rare near the surface but thought to compose most of the earth's mantle) . Most of the basaltic surface layer, however, is covered by a fine layer of sediments, consisting primarily of terrestrial sources
(dust and volcanic ash blown into the oceans from the continents), oceanic sources (mostly the shells of planktonic organisms, but also some chemical precipitates), and cosmic sources (cosmic dust and meteorites).
Coarse grain sediments from land may be carried out to sea embedded in icebergs, then dropped to the sea floor a considerable distance from land when they melt, however, these are restricted to areas near large continental ice sheets,
presently Greenland and Antarctica.
Baslatic lava flows upward through the oceanic crust at the oceanic ridges then spread outward, constantly creating new seafloor which slowly moves outward . The sea floor near ridges is therefore younger than that further away from the ridges, and sediments also become thicker over sea floor that is further from the oceanic ridges. These ridges where new seafloor forms are also the edges of lithospheric plates, which may also contain continental land masses. As the sea floor spreads, the embedded continental land masses may move apart, a motion known as continental drift. Old seafloor is also constantly being destroyed at subduction zones in oceanic trenches, a process known as subduction, where seafloor slides under a continental landmass and is pushed deeper into the mantle where it may remelt. The amount of seafloor created is balanced worldwide by the amount of seafloor that is being destroyed by subduction (otherwise the increase in lithosphere would cause the earth to expand, which is obviously not occuring). A good example is the Mid-Atlantic ridge, where new seafloor is being created and spreading outward, pushing the Eurasian and African continents (and plates) further away from the North and South American continents (and plates). Athe same time, the Pacific ocean seafloor (and plate) is being subducted beneath adjacent plates on both its eastern and western sides, so the Pacific ocean is decreasing in width.
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