Mid-ocean ridge
A mid-ocean ridge (german mid - oceanic ridge ) is a mountain range in the ocean as a hot seam between two diverging plates, see seafloor spreading. The largest of these mountains is the Mid-Atlantic Ridge. There, the Atlantic Ocean becomes wider every year to two centimeters. The back pull through all the oceans, offset laterally in numerous transform faults.
The hard plates float on the mantle tough that rises slowly under the spreading zone and also apart soaked. During the ascent the pressure and thus the melting point decreases, so that the shell material partially liquefied. This begins at a depth of 10 to 70 kilometers, depending on the water content of the mantle and the spreading rate. The liquid portion is released through pores and forms a shallow magma chamber under the back. The low density of the magma rises back to ( isostasy ). From the magma produced new oceanic crust. While the crust moves away from the place of origin, it cools slowly, mainly by deeply penetrating sea water. It heats up and comes loaded with minerals out again (see black smoke ).
Crusting
New crust is formed in three layers with a similar, basic composition. In the middle layer of the magma filled cracks. Here gangue produced ( transitions from diabase ). Often magma penetrates to the water and solidifies as pillow lava. In both cases, the magma cools rapidly and forms the volcanic rock basalt (mid ocean ridge basalt, MORB ). In addition to the central fissure zone, the crust on its underside grows very slowly into the magma chamber and forms the plutonic rock gabbro.
Seawater penetrates into the earth's crust and it comes to a hydrothermal circulation (see also ocean floor metamorphism ), the water is heated to 400 ° C to 500 ° C and minerals triggers the oceanic crust. On the way back, the water cools and sulfidic ore slurry are deposited, which hydrothermal deposits occur. An example of this is the Cyprus copper ore that has been mined since ancient times. Furthermore, the back also pierce the water surface, such as Iceland, the Azores and Ascension Island in the Atlantic Ocean have emerged.
Properties
Mid-ocean ridge show specific characteristics depending on the spreading rate. Therefore, a distinction it in quickly spreading back and slowly spreading ridge. Some scientists suspect that it additionally is a separate type of very slow spreading ridge.
Currently, the Gakkel Ridge is known as the back with the lowest known spreading rate ( 6-13 mm per year).
Transform faults
A mid-ocean ridge is divided active Transform disorders in a plurality of mutually shifted segments each having a uniform spreading rate. Thus, there is transverse to a divergent plate boundary, the location of the back, a conservative plate boundary. Starting from small transform faults they may even be in extreme cases over a hundred kilometers long. In the Pacific, it comes in addition to the appearance of the "overlapping spreading zones", ie back segments that are displaced parallel to each other and partially overlap, with microplates arise with diameters ranging from 100 km to 200 km.
The frequency of transform faults is dependent on the spreading rate of the back: at a low rate, their number increases. So, these are, for example, in the Atlantic Ocean about 50 kilometers apart, while they have in the Pacific at a distance of several hundred kilometers, as this is much more active.
Topography
The topography of mid-ocean ridge differs depending on the spreading rate. At a high rate, they are formed flat and more uniform. At a lower rate of the back are steep towering, jagged and along the back runs a deep to a few kilometers grave breach. On the Mid-Atlantic Ridge hundreds of very small, often only about 60 m high seamounts have been observed on a 800 km long section. The two sides of the back are often vary. At very low spreading rates are found no major transform faults more and it has been observed that on the back not only magma from the earth's core, rise, but also solid stone blocks.
Nature
At mid-ocean ridges form fields deep-sea hydrothermal sources, including the so-called black and white smokers, a private biotope with many, mostly living only in these types of environments.
The thermal springs occur in the young oceanic crust by the absorption of heat from the still-hot crust, and later by the exothermic chemical process of serpentinization. Here penetrate the superficial crust cooled sea water through cracks and under absorption of large amounts of water, olivine from the mantle to serpentinite around.
In this hot, lightless environment is the chemosynthesis, ie the structure of organic substances with an exergonic chemical substance implementation as an energy source, such as methane and hydrogen sulfide oxidation, the base of the food chain, and not as at the surface photosynthesis with sunlight as an energy source.
Research
With measurement methods such as seismic and geomagnetic a back can be studied to great depths.
Thus, on the East Pacific Rise in 1995, the so-called MELT experiment ( Mantle Electromagnetic and Tomography Experiment) began, which examined the back between the Pacific Plate and the Nazca Plate. Here, a few hundred kilometers, asymmetric area with partially molten rock was found at a depth of up to 200 km. Below the Pacific Plate, which has more than twice the speed over the Nazca plate, the larger area was with a width of 250 km compared to 100 km below the eastern plate. The plates move is preoccupied with, 14.5 cm per year from the speed accounted for 10.1 cm per year on the Pacific and 4.5 cm per year on the Nazca plate.