Dorsal (geology)
See also: Dorsal
In Geology, a dorsal , or médio-oceanic wrinkle , is a border of divergence between two tectonic plates which deviates one from the other with oceanic creation of Lithosphère.
The term “dorsal” comes from the topography of this limit: the increase of magma and especially the more important temperature cause the formation of a relief by Isostasie. While moving away from the dorsal, the oceanic Lithosphère cools, becomes denser and its surface, the oceanic Croûte, deeper.
Morphological characteristics
The oceanic dorsals can be followed on more 60 000 km, its longest segment being the Atlantic dorsal , which separates the plates afro-European and American on approximately 7 000 km.
The dorsal is a chain of Montagne S underwater. Compared to the abyssal funds which surround it and which are inserted with more 4 000 m on average, the dorsals culminate with a depth of 2 700 m of average, is a positive unevenness about 1 300 Mr.
However the comparison between a dorsal and an assembly line shows its limits if one looks at the topography of the dorsals more closely: the unevenness is to be compared with the width, from 500 to 800 km (Atlantic Ocean) or of 1 000 with 1 500 km (Pacific Ocean). The slope is thus much weaker than in the case of the emerged mountainous solid masses. It also should be noticed that a dorsal is not a continuous line: it is made of a succession of many segments connected by transforming faults.
The axial line of this “mountain under the sea” can be dug of a narrow valley of a few kilometers called Rift, or trough fault. The rift is the place of an intense geological activity considered as a Oasis of life in oceans deep rather very low in biological activity fault of energy resources and chemical.
Petrographic characteristics
The axis of the dorsals is deprived of Sédiment ation. The more one moves away from the rift, the more the layer of sediments is thick. That is due to the phenomenon of extension and creation of the oceanic Lithosphère.
One finds geologically on the central axis of the Basalte S perfectly not-faded, i.e. very young speaking. They show a particular form in pillow lava (“cushions of Lave”) which is due to the effect of instantaneous Trempe of the lava which leaves a volcanic increase (for example with Hawaii or in Turkey). In lower part of these basalts in cushions are basalt seams, then Gabbro. This provision testifies to the installation of these rocks (which form the oceanic crust) after an increase to the magma on the level to the dorsal.
Heat flow, hydrothermalism, thermophilous life
The Asthénosphère, which forms a convexity on the level of the dorsal, thus evacuates the heat of the coat by movements of Convection and gone up magma.
A considerable thermal energy and quickly renewed is thus present, it is related to the circulation of in-depth water near the volcanic room. However, in spite of temperatures of 100 with 350°C, this water does not boil because of the high pressure at sea major. The heat of water in Geyser S in Iceland (placed on the Atlantic dorsal) is an example of this visible effect on the surface.
The circulation of water is done through the cracks and the faults. The warm water cools by going up rift, the cool water infiltrates in basalts and takes care in Ion S all while being heated. At the time of its increase, it leaves traces of Sulfure S (if it goes up in a fast way) or oxides of Fer and Manganèse (if it goes up in a slow way). Zones where the warm water spring constitute kinds of hydrothermal sources called the “black smokers” because they “blow” with high pressure a very hot water, charged with particles and Sulfure S dissolved which gives them a dark or black plume. These black smokers were discovered by chance in 1977 by the geologist John Corliss.
The immediate surroundings of these sources shelter a fauna and a flora adapted to these very particular ecological niches: absence of light of day, relatively weak lifespan of the geothermic source, strong variation in temperature, very strong chemical concentration in sulfur products, etc This ecosystem is thus independent of the Photosynthèse, but depends on another system called Chimiosynthèse. The flow of lava brings varied and suitable minerals for chemical reactions, in particular with bacteria Autotrophes specific to this environment (metabolism starting from the Soufre in particular).