The Dorsal ocean Is a chain of underwater mountains that are between 1,000 and 2,000 kilometers wide. These formed along the zones of divergence of the tectonic plates, and they are extended over a length that exceeds 64,000 kilometers.
The ocean ridges are found in each ocean basin and appear to split the Earth. They rise from depths close to 5 kilometers to an essentially uniform depth of approximately 2.6 km and are more or less symmetrical in cross-section.
Worldwide distribution of ocean ridges.
Several of its peaks stand out on the surface of the water and form an island (as in the case of Iceland and the Azores).
Transformation faults are those along which lateral movement occurs. In some places, the crest of the ridge is displaced through the transformation faults within the fracture zones, and these faults can be followed by the flanks of the ridges. The flanks are marked by the sets of mountains and hills elongated and parallel to the tendency of the song.
Characteristics of oceanic ridges
A new oceanic crust and part of the upper mantle of the Earth, together with the crust, form the lithosphere, constituted in the marine centers that extend in these oceanic ridges. Due to this, certain geological features that are there, are considered unique.
Fresh basaltic lavas are exposed on the seafloor, on the ridges of the ridges. These lavas are progressively buried by the sediments while the bottom extends far from the site. The flow of heat outside the cortex is many times greater in the ridge than in other parts of the world.
Earthquakes are common along the ridges and in the transformation faults that join the segments of the compensation crest. Analysis of earthquakes occurring on the ridges indicates that the oceanic crust is under stress there.
On the other hand, the depths of the oceanic ridges correlate quite well with the age of the oceanic crust, specifically, it has been shown that the depth of the ocean is proportional to the square root of the age of the crust.
The theory that explains this relationship holds that the increase with age is due to the thermal contraction of the oceanic crust and the upper mantle, since they are carried away from the center of expansion of the seafloor in an oceanic plate.
Because such a tectonic plate is ultimately about 100 km thick, the contraction of only a small percentage predicts the relief of an ocean floor. It follows that the width of a ridge can be defined as twice the distance from the ridge to the point at which the plate has cooled to a constant thermal state.
Most of the cooling took place 70 million to 80 million years ago, at which time the ocean depth was about 5 to 5.5 km. Because this cooling is a function of age, slow-propagating ridges, such as the Middle Atlantic Ridge, are narrower than the more rapidly expanding dorsal ones, such as the Eastern Pacific ridge.
Likewise, a correlation has been found between the rates of global expansion and the transgression and regression of the oceanic waters to the continents. About 100 million years ago, during the early Cretaceous period, when global expansion rates were uniformly high, ocean ridges occupied comparatively more of the ocean basins, causing ocean waters to flow to continents.
In addition to the width of the ridge, another feature seems to be a function of the dispersion rate. Global expansion rates range from 10 mm per year to 160 mm per year. Ocean ridges can be classified as slow (up to 50 mm per year), intermediate (up to 90 mm per year) and fast (up to 160 mm per year).
Finally, slow-propagating ridges are characterized by a crack in the ridge (known as rift or rift valleys) and are controlled by faults. It is typically 1.4 km deep and 20-40 km wide.
The ridges that expand faster do not have rift valleys. At intermediate levels, peak regions are maximum widths, with occasional valleys delimited by faults, to no more than 200 meters. At fast speeds, an axial high is present at the crest. The torn ridges of slow propagation have an irregular topography on their flanks, whereas the ridges that propagate faster have much smoother flanks.
Worldwide Distribution
Oceanic extension centers are found in all ocean basins. In the Arctic Ocean, a low-velocity expansion center is located near the eastern side of the Eurasian basin. You can continue to the south, offset by the transformation faults in Iceland.
Iceland has been created by a hot spot located directly beneath an oceanic expansion center. The ridge leading to the south of Iceland is called the Reykjanes ridge, and although it extends to 20 mm per year or less, it lacks cracks. This is thought to be the result of the influence of the hot spot.
Atlantic Ocean
The Atlantic Cordillera extends from southern Iceland to the extreme South Atlantic Ocean, about 60 ° S latitude. It became known in a rudimentary way during the nineteenth century. In 1855, Mateo Fontaine Maury of the United States Navy prepared an Atlantic charter and identified it as a shallow"middle ground." During the 1950s, American oceanographers Bruce Heezen and Maurice Ewing proposed that it was a continuous mountain range.
In the North Atlantic the dorsal one extends slowly and shows a crack and mountainous flanks. In the South Atlantic the propagation rates are between slow and intermediate, and cracks are generally absent, since they occur only near transformation faults.
Indian Ocean
A very slow oceanic ridge, the southwestern ridge of India, divides the ocean between Africa and Antarctica. It joins the Indian ridges of the Middle-Indian and southeast to the east of Madagascar.
The Carlsberg ridge lies at the northern end of the Middle-Indian ridge. Continue north to join the propagation centers in the Gulf of Aden and the Red Sea. The dispersion is very slow at this point, but approaches the intermediate rates in the Carlsberg and Middle-Indian ridges.
The south-east corner of India extends to intermediate rates. It continues from the western Indian Ocean in a southeasterly direction, dividing the ocean between Australia and Antarctica. The sloping ridges and steep mountain ridges are characteristic of the southwest Indian ridge.
The Middle-Indian ridge has fewer features of this type, and the South-Eastern ridge has a smoother topography. The latter also shows distinct asymmetrical seabed that extend to southern Australia. The analysis of magnetic anomalies shows that the rates on opposite sides of the diffusion center have been uneven many times in the last 50 million or 60 million years.
Pacific Ocean
From one point, the Pacific-Antarctic range can be traced halfway between New Zealand and Antarctica, to the northeast to where it joins the East Pacific Levant, outside the margin of South America. The former is propagated at intermediate to high rates.
The East Pacific Rise extends from this site north to the Gulf of California, where it joins the Pacific-North America plate boundary transformation zone.
At some distance from Chile and Peru, the East Pacific Rise is currently expanding at fast speeds of 159 mm per year or even more.
Rates fall to about 60 millimeters per year at the mouth of the Gulf of California. The crest of the ridge shows a low topographic elevation along its length rather than a crack.
The East Pacific Rise was first detected during the 1870s Challenge Expedition. It was described in its crude form during the 1950s and 1960s by oceanographers, including Heezen, Ewing, and Henry W. Menard. During the 1980s, Kenneth C. Macdonald, Paul J. Fox and Peter F. discovered that the main center of propagation appeared to be disrupted and displaced a few kilometers to one side at various locations along the East Pacific Rise.
However, the ends of the compensation separation centers overlap each other by several kilometers. These were identified as a new type of geological feature of oceanic extension centers and were designated as overlapping propagation centers. It is believed that such centers result from interruptions of the supply of magma to the dorsal along its length and define a fundamental segmentation thereof on a scale of tens to hundreds of kilometers.
Many smaller extension centers branch off from the main or are located behind arches of the island. In the western Pacific, propagation centers are found on the Fiji plateau between the New Hebrides and Fiji islands and in the Woodlark basin between New Guinea and the Solomon Islands.
A number of propagation centers and transformation faults are found between the East Pacific and South America Rise near 40 ° to 50 ° S latitude. The Scotia Sea between South America and the Antarctic Peninsula contains a diffusion center. The Galápagos propagation center extends east-west between the East Pacific and South American Levant near the equator. Three short propagation centers are located a few hundred kilometers from the Pacific Northwest coast.
In a careful study of the history of the Galapagos and the propagation centers of Juan de Fuca, North American geophysicist Richard N. Hey developed the idea of the propagating crack. In this phenomenon, a branch of an expansion center that ends in a transformation failure, is extended at the expense of the propagation center through the fault.
The crack velocity and the fault propagate from one to five times, and create chevron patterns in magnetic anomalies. The grain of the topography of the seabed resembles the wake of a ship.
Hydrothermal vents
Hydrothermal vents are localized discharges of hot sea water. They result from the cold sea water that seeps down into the warm oceanic crust through the cracks zone and returns to the seafloor in a pipe-like flow on the axis of the neovolcanic zone.
Hot waters often carry zinc sulfide minerals, iron and copper leached from the bark. The output of these hot waters probably account for 20 percent of the Earth's heat loss.
There are exotic biological communities around the hydrothermal vents. These ecosystems are totally independent of the energy of the sun. They do not depend on photosynthesis, but on the chemosynthesis by sulfur-fixing bacteria. Sulphide minerals precipitated in the neovolcanic zone can accumulate in substantial amounts and are sometimes buried by lava flows at a later time.
Magma Cameras
Magma chambers have been detected under the ridge crest of the East Pacific by seismic experiments. The principle underlying the experiments is that partially molten rock decreases the path of seismic waves and also strongly reflects them.
The depth to the top of the cameras is about 2 km under the seabed. The width is more difficult to determine, but is probably 1 to 4 km. Its thickness seems to be around 2 to 6 kilometers based on studies of ophiolites.
The chambers have been mapped along the trend of the ridge between 9 ° and 13 ° N latitude. The upper part is relatively continuous but is apparently interrupted by shifting of transformation faults and overlapping of extension centers.
References
- José F. Vigil, Wikimedia Commons (2016). Retrieved from: future-sciences.com.
- Jean Francheteau (2016). Dorsales of the sea. Encyclopedia Britannica. Retrieved from: universalis.fr.
- John P. Rafferty (2009). Retrieved from: oceanexplorer.noaa.gov.
- Adam Augustyn (2008). Retrieved from: divediscover.whoi.edu.