Oceanic crust

Last updated
The colours indicate the age of oceanic crust, wherein lighter indicates younger age, and darker indicates older age. The lines represent tectonic plate boundaries. Global Map of Oceanic Plate Age by Fabio Crameri.png
The colours indicate the age of oceanic crust, wherein lighter indicates younger age, and darker indicates older age. The lines represent tectonic plate boundaries.
Continental and oceanic crust on the Earth's upper mantle Continental and oceanic crust.png
Continental and oceanic crust on the Earth's upper mantle

Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. [1] [2] The crust overlies the rigid uppermost layer of the mantle. The crust and the rigid upper mantle layer together constitute oceanic lithosphere.

Contents

Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick; however, it is denser, having a mean density of about 3.0 grams per cubic centimeter as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter. [3]

The crust uppermost is the result of the cooling of magma derived from mantle material below the plate. The magma is injected into the spreading center, which consists mainly of a partly solidified crystal mush derived from earlier injections, forming magma lenses that are the source of the sheeted dikes that feed the overlying pillow lavas. [4] As the lavas cool they are, in most instances, modified chemically by seawater. [5] These eruptions occur mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. But most magma crystallises at depth, within the lower oceanic crust. There, newly intruded magma can mix and react with pre-existing crystal mush and rocks. [6]

Composition

Although a complete section of oceanic crust has not yet been drilled, geologists have several pieces of evidence that help them understand the ocean floor. Estimations of composition are based on analyses of ophiolites (sections of oceanic crust that are thrust onto and preserved on 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 submersibles, dredging (especially from ridge crests and fracture zones) and drilling. [7] Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. [8] According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle. [9]

Geochemistry

The most voluminous volcanic rocks of the ocean floor are the mid-oceanic ridge basalts, which are derived from low-potassium tholeiitic magmas. These rocks have low concentrations of large ion lithophile elements (LILE), light rare earth elements (LREE), volatile elements and other highly incompatible elements. There can be found basalts 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. [15]

Prior to the Neoproterozoic Era 1000 Ma ago the world's oceanic crust was more mafic than present-days'. The more mafic nature of the crust meant that higher amounts of water molecules (OH) could be stored the altered parts of the crust. At subduction zones this mafic crust was prone to metamorphose into greenschist instead of blueschist at ordinary blueschist facies. [16]

Life cycle

Oceanic crust is continuously being created at mid-ocean ridges. As continental plates diverge at these ridges, magma rises into the upper mantle and crust. As the continental plates move away from the ridge, the newly formed rocks cool and start to erode with sediment gradually building up on top of them. The youngest oceanic rocks are at the oceanic ridges, and they get progressively older away from the ridges. [17]

As the mantle rises it cools and melts, as the pressure decreases and it crosses the solidus. The amount of melt produced depends only on the temperature of the mantle as it rises. Hence most oceanic crust is the same thickness (7±1 km). Very slow spreading ridges (<1 cm·yr−1 half-rate) produce thinner crust (4–5 km thick) as the mantle has a chance to cool on upwelling and so it crosses the solidus and melts at lesser depth, thereby producing less melt and thinner crust. An example of this is the Gakkel Ridge under the Arctic Ocean. Thicker than average crust is found above plumes as the mantle is hotter and hence it crosses the solidus and melts at a greater depth, creating more melt and a thicker crust. An example of this is Iceland which has crust of thickness ~20 km. [18]

The age of the oceanic crust can be used to estimate the (thermal) thickness of the lithosphere, where young oceanic crust has not had enough time to cool the mantle beneath it, while older oceanic crust has thicker mantle lithosphere beneath it. [19] The oceanic lithosphere subducts at what are known as convergent boundaries. 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 than 200 million years old. [20] The process of super-continent formation and destruction via repeated cycles of creation and destruction of oceanic crust is known as the Wilson Cycle.

The oldest large-scale oceanic crust is in the west Pacific and north-west Atlantic — both are about up to 180-200 million years old. However, parts of the eastern Mediterranean Sea could be remnants of the much older Tethys Ocean, at about 270 and up to 340 million years old. [21] [22] [23]

Magnetic anomalies

The oceanic crust displays a pattern of magnetic lines, parallel to the ocean ridges, frozen in the basalt. A symmetrical pattern of positive and negative magnetic lines emanates from the mid-ocean ridge. [24] New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock, its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth. New magma then forces the older cooled magma away from the ridge. This process results in parallel sections of oceanic crust of alternating magnetic polarity.

See also

Notes

  1. ↑ Gillis et al (2014). Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204-208
  2. ↑ Pirajno F. (2013). Ore Deposits and Mantle Plumes. Springer. p. 11. ISBN   9789401725026.
  3. ↑ Rogers, N., ed. (2008). An Introduction to Our Dynamic Planet. Cambridge University Press and The Open University. p. 19. ISBN   978-0-521-49424-3.
  4. ↑ Sinton J.M.; Detrick R.S. (1992). "Mid‐ocean ridge magma chambers". Journal of Geophysical Research. 97 (B1): 197–216. Bibcode:1992JGR....97..197S. doi:10.1029/91JB02508.
  5. ↑ H. Elderfield (2006). The Oceans and Marine Geochemistry. Elsevier. pp. 182–. ISBN   978-0-08-045101-5.
  6. ↑ Lissenberg, C. J., MacLeod, C. J., Horward, K. A., and Godard, M. (2013). Pervasive reactive melt migration through fast-spreading lower oceanic crust (Hess Deep, equatorial Pacific Ocean). Earth Planet. Sci. Lett. 361, 436–447. doi : 10.1016/j.epsl.2012.11.012
  7. ↑ Kodaira, S., Noguchi, N., Takahashi, N., Ishizuka, O., & Kaneda, Y. (2010). Evolution from fore‐arc oceanic crust to island arc crust: A seismic study along the Izu‐Bonin fore arc. Journal of Geophysical Research: Solid Earth,115(B9), N/a.
  8. ↑ Hansteen, Thor H; Troll, Valentin R (2003-02-14). "Oxygen isotope composition of xenoliths from the oceanic crust and volcanic edifice beneath Gran Canaria (Canary Islands): consequences for crustal contamination of ascending magmas". Chemical Geology. 193 (3): 181–193. Bibcode:2003ChGeo.193..181H. doi:10.1016/S0009-2541(02)00325-X. ISSN   0009-2541.
  9. ↑ Li, M., & McNamara, A. (2013). The difficulty for subducted oceanic crust to accumulate at the Earth's core‐mantle boundary. Journal of Geophysical Research: Solid Earth,118(4), 1807-1816.
  10. ↑ Peter Laznicka (2 September 2010). Giant Metallic Deposits: Future Sources of Industrial Metals. Springer Science & Business Media. pp. 82–. ISBN   978-3-642-12405-1.
  11. ↑ D. R. Bowes (1989) The Encyclopedia of Igneous and Metamorphic Petrology, Van Nostrand Reinhold ISBN   0-442-20623-2
  12. ↑ Yildirim Dilek (1 January 2000). Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean Drilling Program. Geological Society of America. pp. 506–. ISBN   978-0-8137-2349-5.
  13. ↑ Gillis et al (2014). Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204-208
  14. ↑ Jon Erickson (14 May 2014). Plate Tectonics: Unraveling the Mysteries of the Earth. Infobase Publishing. pp. 83–. ISBN   978-1-4381-0968-8.
  15. ↑ Clare P. Marshall, Rhodes W. Fairbridge (1999) Encyclopedia of Geochemistry, Kluwer Academic Publishers ISBN   0-412-75500-9
  16. ↑ Palin, Richard M.; White, Richard W. (2016). "Emergence of blueschists on Earth linked to secular changes in oceanic crust composition". Nature Geoscience. 9 (1): 60. Bibcode:2016NatGe...9...60P. doi:10.1038/ngeo2605. S2CID   130847333.
  17. ↑ "Understanding plate motions [This Dynamic Earth, USGS]". pubs.usgs.gov. Retrieved 2017-04-16.
  18. ↑ C.M.R. Fowler (2005) The Solid Earth (2nd Ed.), Cambridge University Press ISBN   0-521-89307-0
  19. ↑ McKenzie, Dan; Jackson, James; Priestley, Keith (May 2005). "Thermal structure of oceanic and continental lithosphere". Earth and Planetary Science Letters. 233 (3–4): 337–349. doi:10.1016/j.epsl.2005.02.005.
  20. ↑ Condie, K.C. 1997. Plate Tectonics and Crustal Evolution (4th Edition). 288 page, Butterworth-Heinemann Ltd.
  21. ↑ Müller, R. Dietmar (April 2008). "Age, spreading rates, and spreading asymmetry of the world's ocean crust". Geochemistry, Geophysics, Geosystems. 9 (4): Q04006. Bibcode:2008GGG.....9.4006M. doi: 10.1029/2007GC001743 . S2CID   15960331.
  22. ↑ Benson, Emily (15 August 2016). "World's oldest ocean crust dates back to ancient supercontinent". www.newscientist.com. New Scientist . Retrieved 11 September 2016.
  23. ↑ "Researcher uncovers 340 million year-old oceanic crust in the Mediterranean Sea using magnetic data". www.sciencedaily.com. Science Daily. 15 August 2016. Retrieved 11 September 2016.
  24. ↑ Pitman, W. C.; Herron, E. M.; Heirtzler, J. R. (1968-03-15). "Magnetic anomalies in the Pacific and sea floor spreading". Journal of Geophysical Research. 73 (6): 2069–2085. Bibcode:1968JGR....73.2069P. doi:10.1029/JB073i006p02069. ISSN   2156-2202.

Related Research Articles

<span class="mw-page-title-main">Plate tectonics</span> Movement of Earths lithosphere

Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s.

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

<span class="mw-page-title-main">Ophiolite</span> Uplifted and exposed oceanic crust

An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks.

<span class="mw-page-title-main">Divergent boundary</span> Linear feature that exists between two tectonic plates that are moving away from each other

In plate tectonics, a divergent boundary or divergent plate boundary is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts, which eventually become rift valleys. Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges.

<span class="mw-page-title-main">Andesite</span> Type of volcanic rock

Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

<span class="mw-page-title-main">Earth's mantle</span> A layer of silicate rock between Earths crust and its outer core

Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×1024 kg (8.84×1024 lb) and thus makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometers (1,800 mi) making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.

<span class="mw-page-title-main">Earth's crust</span> Earths outer shell of rock

Earth's crust is its thick outer shell of rock, referring to less than 1% of the planet's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates whose motion allows heat to escape the interior of the Earth into space.

<span class="mw-page-title-main">Mid-ocean ridge</span> Basaltic underwater mountain system formed by plate tectonic spreading

A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about 2,600 meters (8,500 ft) and rises about 2,000 meters (6,600 ft) above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a divergent plate boundary. The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin.

<span class="mw-page-title-main">Rock cycle</span> Transitional concept of geologic time

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

<span class="mw-page-title-main">Passive margin</span> Transition between oceanic and continental lithosphere that is not an active plate margin

A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting forms new ocean basins. Eventually the continental rift forms a mid-ocean ridge and the locus of extension moves away from the continent-ocean boundary. The transition between the continental and oceanic lithosphere that was originally formed by rifting is known as a passive margin.

<span class="mw-page-title-main">Magmatism</span> Emplacement of magma on the outer layers of a terrestrial planet, which solidifies as igneous rocks

Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of magma or lava. Volcanism is the surface expression of magmatism.

<span class="mw-page-title-main">Cobb hotspot</span> Volcanic hotspot in the Pacific Ocean

The Cobb hotspot is a marine volcanic hotspot at, which is 460 km (290 mi) west of Oregon and Washington, North America, in the Pacific Ocean. Over geologic time, the Earth's surface has migrated with respect to the hotspot through plate tectonics, creating the Cobb–Eickelberg Seamount chain. The hotspot is currently collocated with the Juan de Fuca Ridge.

Volcanic passive margins (VPM) and non-volcanic passive margins are the two forms of transitional crust that lie beneath passive continental margins that occur on Earth as the result of the formation of ocean basins via continental rifting. Initiation of igneous processes associated with volcanic passive margins occurs before and/or during the rifting process depending on the cause of rifting. There are two accepted models for VPM formation: hotspots/mantle plumes and slab pull. Both result in large, quick lava flows over a relatively short period of geologic time. VPM's progress further as cooling and subsidence begins as the margins give way to formation of normal oceanic crust from the widening rifts.

<span class="mw-page-title-main">Igneous rock</span> Rock formed through the cooling and solidification of magma or lava

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava.

<span class="mw-page-title-main">Subduction zone metamorphism</span> Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the Earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets produced by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types formed by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process generates and alters water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

The lower oceanic crust is the lower part of the oceanic crust and represents the major part of it. It is generally located 4–8 km below the ocean floor and the major lithologies are mafic which derive from melts rising from the Earth's mantle. This part of the oceanic crust is an important zone for processes such as melt accumulation and melt modification. And the recycling of this part of the oceanic crust, together with the upper mantle has been suggested as a significant source component for tholeiitic magmas in Hawaiian volcanoes. Although the lower oceanic crust builds the link between the mantle and the MORB, and can't be neglected for the understanding of MORB evolution, the complex processes operating in this zone remain unclear and there is an ongoing debate in Earth Sciences about this. It is 6KM long.

<span class="mw-page-title-main">Earth's crustal evolution</span>

Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface.

<span class="mw-page-title-main">Chile Ridge</span> Submarine oceanic ridge in the Pacific Ocean

The Chile Ridge, also known as the Chile Rise, is a submarine oceanic ridge formed by the divergent plate boundary between the Nazca Plate and the Antarctic Plate. It extends from the triple junction of the Nazca, Pacific, and Antarctic plates to the Southern coast of Chile. The Chile Ridge is easy to recognize on the map, as the ridge is divided into several segmented fracture zones which are perpendicular to the ridge segments, showing an orthogonal shape toward the spreading direction. The total length of the ridge segments is about 550–600 km.

Magmatism along strike-slip faults is the process of rock melting, magma ascent and emplacement, associated with the tectonics and geometry of various strike-slip settings, most commonly occurring along transform boundaries at mid-ocean ridge spreading centres and at strike-slip systems parallel to oblique subduction zones. Strike-slip faults have a direct effect on magmatism. They can either induce magmatism, act as a conduit to magmatism and magmatic flow, or block magmatic flow. In contrast, magmatism can also directly impact on strike-slip faults by determining fault formation, propagation and slip. Both magma and strike-slip faults coexist and affect one another.

References

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.