oceanic crust | geology

oceanic crust, the outermost layer of Earth ’ mho lithosphere that is found under the oceans and formed at spreading centres on oceanic ridges, which occur at divergent home plate boundaries.

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oceanic crust is about 6 km ( 4 miles ) chummy. It is composed of several layers, not including the overlie sediment. The topmost layer, about 500 metres ( 1,650 feet ) thickly, includes lava made of basalt ( that is, rock material consisting largely of plagioclase [ feldspar ] and pyroxene ). oceanic crust differs from continental crust in respective ways : it is flimsy, dense, younger, and of different chemical composition. Like continental crust, however, oceanic crust is destroyed in subduction zones. The lava are by and large of two types : pillow lava and sheet flows. Pillow lavas appear to be shaped precisely as the diagnose implies—like large overstuffed pillows about 1 metre ( 3 feet ) in cross segment and 1 to respective metres long. They normally form small hills tens of metres high at the spread centres. sheet flows have the appearance of rumple bed sheets. They normally are thin ( entirely about 10 cm [ 4 inches ] thick ) and cover a broader area than pillow lava. There is testify that sheet flows are erupted at higher temperatures than those of the pillow kind. On the East Pacific Rise at 8° S latitude, a series of sheet flow eruptions ( possibly since the mid-1960s ) have covered more than 220 square kilometer ( 85 hearty miles ) of seafloor to an median depth of 70 metres ( 230 feet ).

Below the lava is a layer composed of feeder, or sheeted, dikes that measures more than 1 kilometer ( 0.6 nautical mile ) slurred. Dikes are fractures that serve as the plumb system for transporting magma ( dissolve rock corporeal ) to the seafloor to produce lava. They are about 1 metre ( 3 feet ) wide-eyed, subvertical, and elongate along the course of the spread concentrate where they formed, and they abut one another ’ randomness sides—hence the terminus sheeted. These dikes besides are of basaltic constitution. There are two layers below the dikes totaling about 4.5 km ( 3 miles ) in thickness. Both of these include gabbros, which are basically basalts with coarse mineral grains. These gabbro layers are thought to represent the magma chambers, or pockets of lava, that ultimately erupt on the seafloor. The upper gabbro layer is isotropic ( consistent ) in structure. In some places this layer includes pods of plagiogranite, a distinguish rock deep in silica than gabbro. The lower gabbro layer has a stratified structure and obviously represents the floor or sides of the magma bedroom. This layer social organization is called accumulate, meaning that the layers ( which measure up to respective metres thick ) solution from the deposit of minerals out of the fluent magma. The layers in the accumulate gabbro have less silica but are richer in iron and magnesium than the upper portions of the crust. Olivine, an iron-magnesium silicate, is a common mineral in the lower gabbro level .Crew members aboard a drilling ship inspecting a rock core during a scientific expedition that succeeded for the first time in drilling through the upper oceanic crust. Crew members aboard a drill transport inspecting a rock effect during a scientific excursion that succeeded for the inaugural prison term in drilling through the upper oceanic crust .JOI Alliance/IODP The oceanic crust lies atop Earth ’ s curtain, as does the continental crust. Mantle rock is composed by and large of peridotite, which consists chiefly of the mineral olivine with little amounts of pyroxene and amphibole.