# Oceanic crust

Transition from oceanic and continental crust on a passive continental margin. Representation greatly simplified

The oceanic earth crust , also called oceanic crust for short , is the oceanic portion of the earth's crust in the shell structure of the earth ; it is part of the lithosphere . The oceanic crust, like the continental crust, consists largely of silicon and oxygen , but in contrast to this has a higher magnesium content , which is why some authors use the simplistic abbreviation Sima or SiMa (in contrast to SiAl for the continental crust).

Like the lithosphere of the continents, the oceanic lithosphere is in isostatic equilibrium with the asthenosphere of the upper mantle .

## Development process

The oceanic earth's crust is constantly being rebuilt along the mid-ocean ridges , a process known as ocean floor spreading , with so-called ocean floor metamorphosis occurring almost at the same time , through which the crust is hydrated (OH ions are built into minerals). Following the diverging plate boundaries, the oceanic crust rips open, magma from the upper mantle flows in and forms new crust. The speed at which the oceanic crust diverges is known as the spreading rate . This is different for different areas, and it can also change over time. While this new crust cools down and increases in thickness, it moves away from its place of origin together with the older crust like a conveyor belt. At the plate boundary to a continental earth crust or a less dense oceanic earth crust (e.g. in the western Pacific), the oceanic earth crust dips below this ( subduction ), a deep sea channel appears on the surface. This is possible because as the oceanic crust formed from igneous material cools, the density of the crust increases and can even exceed the density of the upper mantle below. When it sinks, the crust material is transformed , and the water that separates out causes stratovolcanoes on the earth's crust above.

### root cause

There are several models to explain the movement ( plate tectonics ) of the oceanic crust. One model sees the convection of the earth's mantle (see also mantle convection ) as the cause, whereby the earth's crust is moved by means of friction. In a further model to (back thrust, assumes that the oceanic crust is pulled apart at the mid-ocean ridges due to gravity engl. Ridge push ) and is pulled down to the subduction zones by submerging crust (Plattenzug, engl. Slab pull ). There are other forces in this model, although it is disputed which of the forces plays the greatest role.

## properties

The average density of the oceanic crust, which is very important for the surface shape of the solid earth body and plate tectonic processes , is usually given in textbooks with values ​​of 2.9 to 3.0 g / cm 3 .

### thickness

Normal oceanic earth's crust has a thickness of 7 km ± 1 km up to the Mohorovičić discontinuity , so it is between 6 km and 8 km thick . At transform zones and at mid-ocean ridges with particularly high spreading rates, the thickness increases significantly due to the high magma production. In the vicinity of hot spots, the thickness is about 11 km, it can be up to 20 km above the center of a hot spot. In the places where islands or island arcs are located, the thickness of the oceanic crust is between 15 km and 30 km. Occasionally the oceanic crust also includes small pieces of continental crust, which can then be more than 30 km thick.

For the height and depth values ​​of the corresponding surface forms, the average depth of the oceans is approx. 3700 m. * to be considered.

* The values ​​in the oceanography textbooks are not uniform here. Stewart (2008) stated 3400 m, Garrison & Ellis (2014) 3800 m and Pinet (2009) “4 kilometers”.

### Age and origin

World map showing the age of the oceanic crust. The red areas mark the youngest crustal sections along the mid-ocean ridges, the blue and purple areas the oldest crustal sections, some of which are located near the subduction zones of the western Pacific and some on passive continental margins. The ocean floor of the eastern Mediterranean formed in the Permian is marked in pink .

The oceanic lithosphere found today as the sea floor when deep drilling in the world's oceans has emerged continuously over the last 200 million years ( Jurassic , Cretaceous and Cenozoic ). Meanwhile, there were relapses with increased spreading rates . The oceanic lithosphere under the world's seas is nowhere older than Jura . Some of the oldest parts are in the Atlantic Ocean off the east coast of North America and in the Pacific east of the Mariana Trench . The average age of today's oceanic lithosphere is 80 million years. The disappearance of oceanic crust due to immersion at convergent plate boundaries is caused by the fact that the oceanic crust is less differentiated and therefore heavier than the continental crust ( subduction ).

The eastern Mediterranean is an exception. There is an ocean floor that is almost 280 million years old ( Permian ).

Due to special processes during mountain formation , however, remnants of the oceanic crust can land on land ( autopsy ), so that these remnants are much older. This ophiolite occurrences mentioned also offer, apart from oceanic deep drilling (for example, the Ocean Drilling Program , ODP), the only way the structure of the oceanic crust in detail to watch. The oldest known ophiolites are 2.5, possibly even 3.8 billion years old (see also Isua gneiss ).

From a planetary perspective, oceanic crust is one of the secondary crusts that also exist on Mars and Venus . The crust probably formed relatively early, a similar crust probably already existed within the first billion years of Earth's history. The only requirement for the formation is an already existing silicate earth mantle (which was probably already the case 4.45 billion years ago), which is partially melted .

### Depth on the ocean floor

The surface of the oceanic crust is identical to the ocean floors beneath the deep-sea sediments . After the magma rises to the ocean floor on a mid-ocean ridge, it begins to cool. This increases the density of the rock and thus also the depth of the sea. With the help of bathymetry , a depth profile can be measured up to an age of around 70 million years, which corresponds to such an assumption. The result is a simplified function (Sclater formula) for the ocean depth, which only depends on the depth of the mid-ocean ridge (≈2.5 km) and the elapsed time:

${\ displaystyle {\ text {Sea depth in km}} = 2 {,} 5 + 0 {,} 35 \ cdot {\ sqrt {\ text {Age of the crust in million years}}}}$

For older parts of the crust, the associated curve becomes even flatter and the dependence of the depth on age can be determined by an exponential function of the type

${\ displaystyle Te ^ {- k \ cdot {\ text {(age in million years)}}}}$

can be approximated with two positive constants T and k. The actual course is usually disturbed, for example by the influence of hot spots .

At the diverging plate boundaries, the oceanic crust bulges to different degrees, whereby a mid-ocean ridge is only the part located directly at the plate boundary. The complete bulge can encompass an area of ​​several hundred kilometers to the right and left of the plate boundary, while the ridge itself is only a few kilometers wide. The size of the bulge not only corresponds to the different levels of ocean floor spread , but also leads to a change in the global sea ​​level height over geological time periods. This shows a high spread rate together with an increased sea level and a lower rate with a lower sea level. For example, in the period between the late Jurassic and the late Cretaceous , this is one of the reasons why the sea level was 270 m higher than it is today.

### Seismic properties

The speed of the P waves in the oceanic crust is about 7 km / s and is thus greater than the speed of about 6 km / s in the continental crust. The speed of the seismic waves is higher with a thinner and older (since colder) crust. The speed of the S waves is around 4 km / s.

### Structure and composition

Due to its formation on the mid-ocean ridges, the oceanic earth's crust has a typically three-layer structure of igneous rocks , which is covered by an increasingly thick layer of sediment as the distance increases. All three layers consist mainly of basalt and gabbro , the associated deep rock . Compared to those of the continental crust, these rocks are poorer in silicon dioxide (approx. 50%) and consist mainly of the minerals diopside and plagioclase .

The uppermost layer of the oceanic crust consists of a one kilometer thick package of pillow lava interspersed with massive dolerite tunnels (dolerite is a special form of basalt). The aisles are either steep or horizontal ( storage aisles ). The steep corridors form the feed zones for the pillow lavas as well as for the storage corridors.

The tunnels become more and more frequent towards the depths until the rock consists exclusively of steep dolerite tunnels. This second zone is about one to two kilometers thick and resembles a packet of upright maps in cross-section, which is why it is called the sheeted dike complex in English . The individual corridors have a coarsely crystallized inner zone, which is surrounded on both sides by finely crystalline to glass-like material. The fine-grained zones are due to the fact that the glowing liquid material quickly solidified when it penetrated through a cooler rock zone in the outer areas, so that no large crystals could form. In many cases it can be observed that an ascending corridor used the not yet completely solidified central zone of an older corridor as a path of ascent, so that the older corridor was split up. Each of the two halves is then fine-grained on one side and coarse-grained on the other side.

The corridor zone is underlain by coarse-grain gabbros. They come from the magma chamber, which lies beneath the mid-ocean ridges and is fed by melts from the Earth's mantle. As the ocean floor spreads, the edges of the magma chamber are pushed apart and the material at the edge solidifies. This gabbro zone is two to five kilometers thick, depending on the rate of expansion of the seabed. If the rate of spread is high, the magma production is correspondingly large, so that the gabbro zone has a greater thickness. The base of the gabbrolage is often formed by banded gabbros and peridotites . They are caused by the sinking of crystals that formed early and, due to their high density, sink in the magma chamber and form a sediment. The banding is believed to be due to the shear movement between the oceanic crust and the underlying mantle.

Underlying the three layers of the oceanic crust are material from the upper mantle that has been altered by the melting processes that led to the formation of the rising magma. The original composition of the upper mantle is that of a lherzolite , a rock made up of the minerals olivine , enstatite and diopside. The magma formation leads to the fact that the Lherzolite mainly the diopside portion is withdrawn, so that a rock consisting mainly of olivine and enstatite ( restite ) remains, the Harzburgite .

## Exploration

The oceanic crust can be investigated by drilling through the use of probes and by extracting drill cores .

Seismics allow conclusions to be drawn from the propagation of pressure and shear waves on the stratification of materials with different wave propagation properties and the geometry of the cohesion.

For example, the Japanese research vessel Chikyū is expected to begin drilling through the 6–7 km thick oceanic crust by 2030 and for the first time in human history to drill into the upper mantle. A suitable location is currently being sought.

## Individual evidence

1. Frisch, Wolfgang; Meschede, Martin: Plate tectonics - continent shift and mountain formation. Verlag Darmstadt Wissenschaftliche Buchgesellschaft, 2005. P. 101
2. ^ Wolfgang Frisch, Martin Meschede, Ronald Blakey: Plate Tectonics - Continental Drift and Mountain Building. Springer Verlag 2010.
3. subduction min-web.de,
4. ^ University of Michigan: Plate Driving Forces and Tectonic Stress
5. Lee Karp-Boss, Emmanuel Boss, Herman Weller, James Loftin, Jennifer Albright: Teaching Physical Concepts in Oceanography. The Oceanography Society, 2007 ( PDF ), p. 7
6. Frisch & Meschede 2005 , p. 71
7. ^ Robert S. White, Dan McKenzie, R. Keith O'Nions: Oceanic Crustal Thickness From Seismic Measurements and Rare Earth Element Inversions . In: Journal of Geophysical Research . 97 (B13), 1992, pp. 19683-19715 , doi : 10.1029 / 92JB01749 .
8. ^ Robert H. Stewart: Introduction To Physical Oceanography. Texas A&M University, 2008 ( PDF ), p. 25
9. Tom Garrison, Robert Ellis: Oceanography: An Invitation to Marine Science. 9th edition. Cengage Learning, Belmont (CA) 2014, ISBN 978-1-305-10516-4 , p. 111
10. ^ Paul R. Pinet: Invitation to oceanography. 5th edition. Jones and Bartlett Publishers, Sudbury (MA) 2009, ISBN 978-0-7637-5993-3 , p. 355
11. Harald Furnes, Minik Rosing , Yildirim Dilek, Maarten de Wit, Isua supracrustal belt (Greenland) —A vestige of a 3.8 Ga suprasubduction zone ophiolite, and the implications for Archean geology, Lithos, Volume 113, Issues 1-2, November 2009 , Pages 115-132, , doi : 10.1016 / j.lithos.2009.03.043 .
12. ^ Frisch & Meschede: Plate tectonics. Darmstadt, 2011. 4th edition.
13. Claude J. Allègre, Gérard Manhès, Christa Göpel, The age of the Earth, Geochimica et Cosmochimica Acta, Volume 59, Issue 8, April 1995, Pages 1445-1456, , doi : 10.1016 / 0016-7037 ( 95) 00054-4 .
14. ^ S. Ross Taylor, Scott McLennan, Planetary Crusts: Their Composition, Origin and Evolution. Cambridge Planetary Science, 2009. ISBN 0-521-84186-0 . Page 22 f., 208
15. David T. Sandwell: Simple formula for the ocean depth as a function of time (equation 38) ( Memento of the original from February 21, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.4 MB)
16. Results of the MELT experiment , whoi.edu
17. H. Seyfried, R. Leinfelder: Sea Level Fluctuations - Causes, Consequences, Interactions ( Memento of the original from April 28, 2007 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
18. jamstecchannel: Scientific Deep Sea Drilling and Coring Technology youtube.com, video 2:54 p.m. , November 12, 2013, accessed in February 2018. - Animation: drilling methods, extraction and analysis of drill cores.
19. https://edition.cnn.com/2017/04/07/asia/japan-drill-mantle/index.html