Oceanic crust

Colors indicate the age of oceanic lithosphere, wherein red indicates the youngest age, and blue indicates the oldest age. The lines represent tectonic plates.

Oceanic crust is the uppermost layer of the oceanic portion of a tectonic plate. The crust overlies the solidified and uppermost layer of the mantle. The crust and the solid mantle layer together constitute oceanic lithosphere.

Oceanic crust is the result of erupted mantle material originating from below the plate, cooled and in most instances, modified chemically by seawater.[1] This occurs mostly at mid-ocean ridges, but also at scattered hotspots, and also in rare but powerful occurrences known as flood basalt eruptions. It 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 2.9 grams per cubic centimeter[2] as opposed to continental crust which has a density of about 2.7 grams per cubic centimeter.[3]

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 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.[4] Oceanic crust is significantly simpler than continental crust and generally can be divided in three layers. According to mineral physics experiments, at lower mantle pressures, oceanic crust becomes denser than the surrounding mantle.[5]

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.[10]

Life cycle

Oceanic crust is continuously being created at mid-ocean ridges. As plates diverge at these ridges, magma rises into the upper mantle and crust. As it moves away from the ridge, the lithosphere becomes cooler and denser, and sediment gradually builds on top of it. The youngest oceanic lithosphere is at the oceanic ridges, and it gets progressively older away from the ridges.[11]

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.[12]

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.[13] 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 are remnants of the much older Tethys ocean, at about 270 and up to 340 million years old.[14][15][16]

Magnetic anomalies

The oceanic crust displays an interesting 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[17]. 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, it aligns itself with the current position of the north magnetic pole of the Earth (which has reversed many times in its past) at the time of its cooling. 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. ↑ H. Elderfield (2006). The Oceans and Marine Geochemistry. Elsevier. pp. 182–. ISBN 978-0-08-045101-5.
  2. ↑ Rogers, N.; Blake, S.; Burton, K. An introduction to our dynamic planet. Cambridge University Press. p. 19. ISBN 978-0-521-49424-3. Retrieved January 2008. Check date values in: |access-date= (help)
  3. ↑ Cogley 1984
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ 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.
  7. ↑ D. R. Bowes (1989) The Encyclopedia of Igneous and Metamorphic Petrology, Van Nostrand Reinhold ISBN 0-442-20623-2
  8. ↑ 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.
  9. ↑ Jon Erickson (14 May 2014). Plate Tectonics: Unraveling the Mysteries of the Earth. Infobase Publishing. pp. 83–. ISBN 978-1-4381-0968-8.
  10. ↑ Clare P. Marshall, Rhodes W. Fairbridge (1999) Encyclopedia of Geochemistry, Kluwer Academic Publishers ISBN 0-412-75500-9
  11. ↑ "Understanding plate motions [This Dynamic Earth, USGS]". pubs.usgs.gov. Retrieved 2017-04-16.
  12. ↑ C.M.R. Fowler (2005) The Solid Earth (2nd Ed.), Cambridge University Press ISBN 0-521-89307-0
  13. ↑ Condie, K.C. 1997. Plate Tectonics and Crustal Evolution (4th Edition). 288 page, Butterworth-Heinemann Ltd.
  14. ↑ Müller, R. Dietmar (April 2008). "Age, spreading rates, and spreading asymmetry of the world's ocean crust". Geochemistry, Geophysics, Geosystems. American Geophysical Union. 9 (4).
  15. ↑ Benson, Emily (15 August 2016). "World’s oldest ocean crust dates back to ancient supercontinent". www.newscientist.com. New Scientist. Retrieved 11 September 2016.
  16. ↑ "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.
  17. ↑ 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. ISSN 2156-2202. doi:10.1029/JB073i006p02069.

References

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