Oceanic crust

As oceanic crust, oceanic crust also briefly referred to the oceanic portion of the earth's crust in the shell structure of the earth; it is thus part of the lithosphere. The oceanic crust is like the continental crust to a large part of silicon and oxygen, but has on the contrary to this a higher magnesium content, which is why they often abbreviates with Sima or SiMa (as opposed to SiAl for the continental crust ).

As the lithosphere of the continents is also the oceanic lithosphere in isostatic equilibrium with the asthenosphere of the upper mantle.

• 2.1 thickness
• 2.2 Age and origin
• 2.3 depth course on the ocean floor
• 2.4 Seismic properties
• 2.5 Structure and composition

Development process

The oceanic crust is constantly being rebuilt along the mid-ocean ridge, a process which is known as seafloor spreading, while it takes almost the same time also to the so-called ocean floor metamorphism, carried out by the mainly hydration of the crust (OH - ions are incorporated in minerals). The divergent plate boundaries follow, tears here the oceanic crust and upper mantle magma flows and forms forming new crust. The speed at which diverges the oceanic crust are known as spreading rate. This is different for different areas, and over time they may change. While this new crust cools and increases in thickness, it moves along with the already older crust like an assembly line away from their place of origin. At the plate boundary to a continental crust or a less dense oceanic crust (eg, in the western Pacific ) oceanic crust under the dive this from ( subduction ), a deep-water channel appears on the surface. This is possible because the cooling of the oceanic crust, the density of the crust increases and may even exceed the density of the underlying upper mantle. On descending the crustal material is converted, while retiring water caused, for example composite volcanoes on the overlying crust.

Cause

As an explanation for the motion ( plate tectonics ) of the oceanic crust, there are several models. A model sees the convection of the mantle (see also mantle convection ) as the cause of, wherein by means of friction, the earth's crust is moved. In another model, it is assumed that the oceanic crust is pulled apart at mid-ocean ridges due to gravity (back thrust, Eng. Ridge push) and pulled off to the subduction zones by the subducting crust ( Plattenzug, Eng. Slab pull). In this model, additional forces are present, there is a dispute about which of the forces plays the biggest role.

Properties

The very important for numerous processes an average density of the oceanic crust is 2.9 g/cm3.

Thickness

Normal oceanic crust has a thickness of 7 km ± 1 km to the Mohorovičić discontinuity, ie, is between 5 km and 8 km powerful. At Transform zones and at mid-ocean ridges with very high spread 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 over the center of a hot spots up to 20 km. At the places where islands or island arcs are, the thickness of the oceanic crust between 15 km and 30 km. Occasionally includes the oceanic crust also small pieces of continental crust, whereby the thickness can be greater than 30 km.

To take into account is still an average depth of the oceans of 3800 m.

Age and origin

Due to the peculiarity of the crust establishment and termination, the oceanic crust is on average just 80 million years old. In the oceans, there is no crust, which is older than 200 million years. Some of the oldest parts located in the Atlantic Ocean off North America and the Pacific east of the Mariana Trench. The Eastern Mediterranean is an exception here, the ocean floor is nearly 280 million years old. By special processes of mountain building, however, remnants of oceanic crust can get ashore ( autopsy ), so that these residues have a much higher age. These ophiolites mentioned deposits have also, apart from oceanic deep drilling (for example, the ODP of the Ocean Drilling Program ), the only way the structure of the oceanic crust in detail to observe. The oldest known ophiolites are 2.5, maybe even 3.8 billion years old ( see also Isua gneiss).

Seen Planetar counts oceanic crust to the secondary crusts, which are also on Mars or Venus. The crust was probably relatively early, a similar crust there was probably already within the first billion years of Earth's history. Prerequisite for the formation is only a pre-existing silicate mantle (which was probably already before 4.45 billion years, the case), which is partially melted.

Depth course on the ocean floor

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

For older parts of the crust, the corresponding curve is even flatter and the dependence of the depth of the age, by an exponential function of the type

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

The oceanic crust different bulges at the retreating apart ( divergent ) plate boundaries on strong, with a mid-ocean ridge is the only directly located at the plate boundary part. The entire bulge can span a range of several hundred kilometers to the right and left of the plate boundary, while the back itself is only a few kilometers wide. The size of the bulge corresponds not only to the different levels of seafloor spreading, but also leads to a change in global sea level over geologic time. Thus, a high spreading rate shows along with increased sea level and a lower rate with a lower sea level. For example, among other reasons, the sea level to 270 m higher than it is today located in the period between the late Jurassic and the Late Cretaceous.

Seismic properties

The speed of P- waves in oceanic crust is about 7 km / s and is therefore greater than the speed at continental crust of about 6 km / s The velocity of seismic waves is higher for a thinner and older (since colder ) crust. The speed of the S-wave is approximately 4 km / s

Structure and composition

The oceanic crust has due to its emergence in the mid-ocean ridges on a typically three-layer construction of igneous rocks, which is covered with increasing distance from an increasingly thicker layer of sediment. All three layers are mainly composed of basalt and gabbro, the associated plutonic rock. These rocks are compared to those of the continental crust poorer in silica (about 50 %) and consist mainly of the minerals diopside and plagioclase.

The top layer of oceanic crust consists of an approximately one kilometer thick package of pillow lava, which are penetrated by massive Doleritgängen ( dolerite is a special form of basalt). The courses are either steep or be horizontal ( sills ). The steeply angled transitions form the supply zones for the pillow lava as well as the warehouse aisles.

At depth, the gears are becoming more common, to the rock consists entirely of steep standing Doleritgängen. This second zone is about one to two kilometers deep, and is similar in cross-section a package provided upright cards, they will therefore be in English as indicated sheeted dike complex. The individual gears have a coarser crystallized inner zone, which is surrounded on both sides by finely crystalline to glassy material. The fine-grained zones arise from the fact that the glutflüssige material rapidly in the outer regions solidified upon forced upon by a cooler rock zone by cooling, so that no large crystals could form. In many cases, it can be observed that a rising transition has used the not yet completely solidified central zone of an older Ganges as ascent, so that the older gear was split. Each of the two halves is then formed fine-grained on one side and coarse on the other side.

The transition zone is underlain by coarse-grained gabbros. They originate from the magma chamber, which underlies the mid-ocean ridges and is fed by melt from the mantle. In the course of seafloor spreading the edges of the magma chamber are pushed apart, and solidifies the randliche material. Gabbrozone this has a thickness of two to five kilometers, depending on the spreading rate of the seabed. At a high propagation rate magma production is correspondingly large, so that the Gabbrozone has a higher thickness. The base of the Gabbrolage is often formed of banded gabbros and peridotites. They are caused by the decrease of early formed crystals that fall due to their high density in the magma chamber and form a sediment. The harness is probably due to the shearing motion between oceanic crust and the subjacent mantle.

Under Superimposed the three layers of the oceanic crust of the material of the upper mantle that has been affected by the melting processes that have led to the formation of the ascending magma. The original composition of the upper mantle is that of a Lherzoliths, a rock of the minerals olivine, enstatite and diopside. The Magmenbildung means that the lherzolite is extracted mainly the Diopsidanteil so that a mainly of olivine and enstatite existing rock ( Restit ) remains, the harzburgite.