The layer 2 and layer 3 phases are easily recognized on modern record sections from oceanic crust (Fig. All rocks in Earth’s crust are constantly being recycled through the rock cycle. Oceanic crust is found between layers of the ocean and beneath the ocean’s surface. At mid-ocean ridges, three crustal horizons are found where contrasts in elastic properties are sufficiently large that the horizons can be mapped with reflection techniques. Unlike the continental crust, the oceanic crust is continually recycled by … Spreading rate has long been recognized as a fundamental variable governing crustal accretion at ridges, with first-order differences observed in a wide range of ridge properties. As the magma cools, it forms plutonic rocks called gabbro. Oceanic crust is formed as a result of decompression melting in the mantle at relatively shallow depths below the mid-ocean ridges, as the mantle rises in passive response to plate separation. Oceanic crust consists almost exclusively of extrusive basalt and its intrusive equivalents. The Oceanic crust Is the part of the earth's crust that is covered by the oceans. 2), suggesting a transition from crustal velocities of around 6.8–7.2  km/s to upper mantle velocities of around 8  km/s over a depth interval that is short compared to seismic wavelengths (which are typically a few hundred meters for a large airgun source). But the continental crust consists of two continuous zones—an upper zone of rocks that are less dense and are composed of silicates of aluminum, sodium, potassium, and calcium—felsic rocks—and a lower zone of denser, mafic rock. An ESP record section from oceanic crust in the North Atlantic (reprinted from Mithal, R., and Mutter, J. C. (1989). The major seismic divisions of the oceanic crust are also marked. Microbes have been found living deep inside crust at the bottom of the sea. Much of the geochemical variability that is observed in MORB probably occurs within the crystal–liquid mush zone and thin magma lens that underlie the ridge crest. The youngest of the Hawaiian lavas — at Kilauea Volcano on the island of Hawaii — is just a few hours old (or less!) The boundary between the two zones corresponds roughly to the oceanic Layer 2 to Layer 3 boundaries or the contact between sheeted dike complex and gabbro in ophiolite complexes. Peter T. Harris, in Seafloor Geomorphology as Benthic Habitat (Second Edition), 2020. Life Cycle of the Oceanic Crust. False ... Curving chains of volcanic islands and seamounts, almost always found parallel to the concave edges of trenches are called _____ _____. Layer 2 is commonly identified with extrusive basaltic lavas and a sheeted dike complex, whereas layer 3 is identified with intrusive gabbroic rocks. Seafloor spreading rates vary from about 10-20 mm/yr at slow spreading ridges, such as along the mid Atlantic ridge, to about 200 mm/yr at fast spreading ridges, such as in the southern Pacific Ocean. A magnetic-reversal time scale is also shown. All the present oceanic crust is young, not older than Jurassic. The entire thickness of the oceanic crust has not been sampled in situ and therefore the bulk composition has been estimated based on investigations of ophiolites (fragments of oceanic and back-arc crust that have been thrust up on to 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 dredging, drilling, submersibles, and remotely operated vehicles. Basaltic oceanic crust contains more than three elements. However, most of those rocks originated as either granite or basalt. The oldest oceanic crust is around 280 Ma in the eastern Mediterranean, and the oldest parts of the open ocean are around 180 Ma on either side of the north Atlantic. The basalts of the oceanic crust, referred to as mid-ocean ridge basalts (MORB), are dominantly tholeiitic and are, on average, depleted in incompatible trace elements compared to basalts erupted in other tectonic environments. 2017. New oceanic crust (and part of Earth’s upper mantle, which, together with the crust, makes up the lithosphere) is formed at seafloor spreading centres at these crests of the oceanic ridges. The oceanic crust is thin, relatively young and uncomplicated compared to the continental crust, and chemically magnesium-rich compared to continental material. W.M. Of course, the reason for this is that all sea floor older than that has been either subducted or pushed up to become part of the continental crust. White, E.M. Klein, in Treatise on Geochemistry (Second Edition), 2014. Oceanic crust - Oceanic crust - Marine magnetic anomalies: Marine magnetic anomalies were first discovered off the coast of the western United States in the late 1950s and completely baffled scientists. Along with the continental crust, the oceanic crust separates the earth's surface from the mantle, the inner layer of earth containing viscous and hot materials. This, combined with the young age of the oceanic crust, explains why few craters are likely to be found in the oceans. Physical Geology by Steven Earle is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. The Moho marks the seismic boundary between plutonic rocks that are gabbroic in composition and those that are mostly ultramafic but may have formed by crystal accumulation in the crust. Component is dominated by mafic rocks is commonly identified with intrusive gabbroic what are found at the oceanic crust pushed away from a ocean! 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