Mid-ocean ridge

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Course of the mid-ocean ridges on a world map
Physical world map including the relief of the ocean floors according to Heezen and Tharp hand-drawn by Heinrich C. Berann (1977). The course of the mid-ocean ridges can be clearly seen on it.

A mid-ocean ridge ( engl. Mid-oceanic ridge , abbreviated MOR ) is a volcanic active mountain range in the deep sea , extends along the seam of two diverging (divergent) lithospheric plates extends. Constantly creates new at the central axis of this mountain range oceanic crust , this process is seafloor spreading called (Seafloor Spreading). The ridges run through all of the ocean basins. The largest part of this system is the Mid-Atlantic Ridge , on which the Atlantic Ocean widens by two centimeters every year. Due to numerous transform disorders, the entire back system is divided into individual segments.

In addition to the subduction zones and hot spots, mid-ocean ridges are among the centers of magmatic activity on earth. One manifestation of this magmatism are the so-called black smokers .

Crust formation

Mid-ocean ridge ( petrological processes)
Mid-ocean ridge (chemical processes)

The lithospheric plates, which can be regarded as rigid, “float” on the tough sublithospheric earth's mantle , which slowly rises under the expansion zone in the course of mantle convection . During the ascent, the pressure and thus the melting point drop, so that the components of the mantle rock with the lowest melting points liquefy (so-called partial melting ). This begins at a depth of 10 to 70 kilometers, depending on the water content of the mantle and the spreading rate . The liquid part rises in cracks and rock pores and forms a magma chamber at a relatively shallow depth under the back.

Oceanic crust is made up of three layers with a similar, basic composition. The upper layer consists of basalt , which emerged from lava that penetrated from the magma chamber to the sea floor and solidified there quickly. Pillow lava is therefore typical for this layer . The middle layer consists of solidified magma that did not reach the sea floor but crystallized out relatively quickly in the form of ducts . The corresponding gangue rock is very similar to the basalt of the sea floor. Both are known as mid-ocean ridge basalt (MORB for short) due to a special geochemical signature that only basaltic rocks on mid-ocean ridges have . The lower layer consists of the deep rock gabbro , the mineral composition of which is identical to that of the MORB. It represents the slowly cooled and crystallized melt of the magma chamber. Below the crust joins the ultramafic lithospheric mantle. In the oceanic lithosphere, the higher part consists mostly of Harzburgite , the mantle rock that remains after the MOR magma has melted out.

The young crust of the mid-ocean ridges shows many crevices and fissures. In addition, it is still very hot at a certain distance from the sea floor, especially in the vicinity of the magma chamber. Sea water that penetrates deep into the crevices is heated up to 400 ° C to 500 ° C, so that a hydrothermal circulation occurs (see also ocean floor metamorphosis ). The water dissolves chemical compounds from the rock. When it emerges on the sea floor, the water cools down suddenly, causing the chemical compounds to precipitate out in the form of fine particles of sulfidic ore minerals (" black smoker ") and are deposited as ore sludge in the vicinity of the exit points. Such deposits, which arose in the more distant geological past and are now found on the continents as a result of mountain formation , form so-called volcanic-exhalative deposits . An example of this is the Cypriot copper ore, which has been mined since ancient times.

In some places, mid-ocean ridges rise so far that they pierce the surface of the water and form oceanic islands. Examples are the Azores and the island of Ascension in the Atlantic. A special case is Iceland , which is unusually large for an oceanic island . A combination of MOR volcanism and hot-spot volcanism is assumed to be the cause of the high magmatic activity there. In addition, at least the southeastern part of the island seems to be underlain by Greenland's old continental crust.

properties

Mid-ocean ridges show specific properties depending on the spread rate. A distinction is therefore made between backs with a high (> 65 mm / year, prime example: East Pacific back ), back with a low (<65 mm / year, prime example: Mid-Atlantic ridge ) and back with a very low (<20 mm / year) spread rate. fast-spreading ridges , slow-spreading ridges and ultraslow-spreading ridges ).

Currently, the Gakkel Ridge in the Arctic Ocean is considered to be the ridge with the lowest known spreading rate (between 6 and 13 mm per year).

Discontinuities

extensive transform fault in the dorsal system between the Pacific and Antarctic plates
East Pacific Rise.jpg
Elevation model of the South Pacific. The circle marks an offset at the transition from the East Pacific to the Pacific-Antarctic Ridge of more than 1000 km.

Mid-ocean ridges are usually divided by active transform faults running transversely to the longitudinal axis of the ridge into a large number of mutually offset segments, each with a uniform spreading rate. Thus, on a MOR there are not only diverging plate boundaries along the longitudinal axis, but also conservative plate boundaries transverse to the longitudinal axis. In extreme cases, these transform sections can be more than 1000 kilometers long, for example in the South Pacific. The number of transform faults depends on the spreading rate of the back: it is higher on backs with a lower spreading rate. The transform faults on the Mid-Atlantic Ridge are only about 50 kilometers apart, while on the East Pacific Ridge they are several hundred kilometers apart.

So-called overlapping spreading centers (OSC for short) are also known from the rapidly expanding East Pacific Ridge . These are back segments that are offset from one another by a few kilometers across the longitudinal axis of the back and the ends of which overlap parallel to the longitudinal axis of the back. It is assumed that the OSC originated from normal transform offsets in that the spread ridges are propagated beyond the limiting transform disturbance. Microplates can finally emerge from the area between two overlapping ends. Examples of such plates are the Easter plate and the Juan Fernandez plate on the East Pacific Ridge.

topography

The topography of mid-ocean ridges differs depending on the spread rate. At a high rate, they are flat and rather evenly shaped. At a lower rate, the ridges are steep, fissured and a rift valley up to a few kilometers deep , the so-called central ditch , runs along the longitudinal axis of the ridge . Hundreds of very small seamounts , often only about 60 m high, have been observed within an 800 km long section of the central trench of the Mid-Atlantic Ridge . The two flanks of the back are often of different heights. At very low spreading rates, such as those experienced by the Gakkel ridge, there are no longer any large transform faults and a certain section of the ridge has in fact no magmatic activity. Apparently, no basaltic magmas have melted out of the upper mantle and have formed young ocean floors. Instead, the ocean floor is formed by peridotite , which appears to have risen from the upper mantle in a solid state ( amagmatic spreading ). Something similar was observed on the south-west Indian ridge .

Mid-ocean ridges as a habitat for the deep sea

Invertebrates (white crabs of the genus Kiwa and snails) on black smokers of the East Scotia Ridge (eastern border of the Scotia Plate ), Southwest Atlantic.

The magmatic activity on mid-ocean ridges is the cause of hydrothermal deep-sea springs, including so-called black and white smokers . They are created by seawater that penetrates through crevices into the crustal rock, which is still very hot at a greater depth, warms up there to well over a hundred degrees and then emerges again on the sea floor.

In some of the cooler sources, however, the heat comes mainly from the exothermic running Serpentinization of olivine into the ocean floor rocks (see Lost City ).

In this relatively hot but lightless environment, chemosynthesis ( chemotrophy ), i.e. the build-up of organic substances with an exergonic chemical conversion as an energy source , for example methane and hydrogen sulphide oxidation, is the basis of the food chain and not photosynthesis, as it is near the sea surface and on land with sunlight as an energy source.

One hypothesis about the origin of life on earth even assumes that the first ecosystems in the history of the earth were located at hydrothermal springs and that life spread from there.

research

With measuring methods such as seismics and geomagnetics , a ridge can be examined to a great depth.

The so-called MELT Experiment (Mantle Electromagnetic and Tomography Experiment) was started on the East Pacific Ridge in 1995, which examined the East Pacific Ridge between the Pacific Plate and the Nazca Plate . An asymmetrical area several hundred kilometers with partly liquid rock was found at a depth of up to 200 km. Below the Pacific plate, which has a speed more than twice as high as that of the Nazca plate, was the larger area with a width of 250 km compared to only 100 km below the eastern plate. The rate at which the plates move apart, 14.5 cm per year, was 10.1 cm per year on the Pacific and 4.5 cm per year on the Nazca plate.

See also

literature

  • Roger Searle: Mid-Ocean Ridges. Cambridge University Press. Cambridge (UK) 2013, ISBN 978-1-107-01752-8
  • Wolfgang Frisch, Martin Meschede: Plate tectonics. Continent shift and mountain formation. 5th updated edition. Primus Verlag, Darmstadt 2013, ISBN 978-3-86312-366-6

swell

  1. ^ Trond H. Torsvik, Hans EF Amundsen, Reidar G. Trønnes, Pavel V. Doubrovine, Carmen Gaina, Nick J. Kusznir, Bernhard Steinberger, Fernando Corfu, Lewis D. Ashwal, William L. Griffin, Stephanie C. Werner, Bjørn Jamtveit: Continental crust beneath southeast Iceland. Proceedings of the National Academy of Science of the United States of America. Vol. 112, No. 15, 2015, E1818 – E1827, doi: 10.1073 / pnas.1423099112 .
  2. ^ Philip Kearey, Keith A. Klepeis, Frederick J. Vine: Global Tectonics. 3rd Edition, Wiley-Blackwell, Chichester 2009, ISBN 978-1-4051-0777-8 , p. 122 ff.
  3. ↑ Jun Korenaga, Richard N. Hey: Recent dueling propagation history at the fastest spreading center, the East Pacific Rise, 26 ° -32 ° S. Journal of Geophysical Research: Solid Earth. Vol. 101, No. B8, 1996, 18023-18041, doi: 10.1029 / 96JB00176 .
  4. ^ Richard N. Hey: Propagating rifts and microplates at mid-ocean ridges. In Richard C. Selley, L. Robin M. Cocks, Ian R. Plimer: Encyclopedia of Geology. Volume 5. Academic Press (Elsevier), Amsterdam a. a. 2005, pp. 396-405, ISBN 0-12-636385-4 .
  5. ^ Searle: Mid-ocean ridges. 2013 (see literature ), p. 87 ff.
  6. ↑ Deborah K. Smith, Johnson R. Cann: Hundreds of small volcanoes on the median valley floor of the Mid-Atlantic Ridge at 24-30 ° N. Nature. Vol. 348, 1990, 152-155, doi: 10.1038 / 348152a0 .
  7. ↑ Deborah K. Smith, Johnson R. Cann: The role of seamount volcanism in crustal construction at the Mid-Atlantic Ridge (24 ° -30 ° N). Journal of Geophysical Research: Solid Earth. Vol. 97, No. B2, 1992, 1645-1658, doi: 10.1029 / 91JB02507 .
  8. ↑ PJ Michael, CH Langmuir, HJB Dick, JE Snow, SL Goldstein, DW Graham, K. Lehnert, G. Kurras, W. Jokat, R. Mühe, HN Edmonds: Magmatic and amagmatic seafloor generation at the ultraslow-spreading Gakkel ridge , Arctic Ocean. Nature. Vol. 423, 956-961, doi: 10.1038 / nature01704 .
  9. ↑ Mathilde Cannat, Daniel Sauter, Véronique Mendel, Etienne Ruellan, Kyoko Okino, Javier Escartin, Violaine Combier, Mohamad Baala: Modes of seafloor generation at a melt-poor ultraslow-spreading ridge. Geology. Vol. 34, No. 7, 1992, 605-608, doi: 10.1130 / G22486.1 .
  10. ^ Entire section after William Martin: Hydrothermal springs and the origin of life. Everything has a beginning, including evolution. Biology in our time. Vol. 39, No. 3 (special issue evolution research), 2009, 166–174, doi: 10.1002 / biuz.200910391 .
  11. ↑ Results of the MELT experiment , whoi.edu
  12. ↑ The Big MELT , whoi.edu

Web links

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