The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean, slowly spreading at a rate of 2 to 5 centimeters 0. In contrast, the East Pacific Rise spreads fast, at rates of 6 to 16 centimeters 3 to 6 inches per year. Due to the fast spreading rates, there is no rift valley in the Pacific, just a smooth volcanic summit with a crack along the crest that is much smaller than the Atlantic rift valley.
Despite being such prominent feature on our planet, much of the mid-ocean ridge system remains a mystery. While we have mapped about half of the global mid-ocean ridge in high resolution, less than one percent of the mid-ocean ridge has been explored in detail using submersibles or remotely operated vehicles. The greater the offset across a transform fault offset being measured by the age of the old crust next to the nodal basin , the deeper the nodal basin and associated fracture zone valley.
The detailed shape of the Mid-Oceanic Ridge is not just a simple alternation of transform faults and spreading centers. Interspersed between transform faults are lesser irregularities ranging from ridge axis discontinuities significant offset of the axis but without transverse fault development down to minor departures from linearity.
The smaller the irregularity, the more ephemeral the feature. When the offset is small, the tips of the offset spreading center may propagate past each other, forming so-called overlapping spreading centers. This is an unstable configuration and results in deformation of the crust between the overlappers. When one center continues to propagate at the expense of the offset spreading center, the result is a propagating rift.
Such rifts tend to propagate away from the regional topographic bulges. Smaller-scale irregularities of the ridge axis are probably related to local volcanic centers developed several tens of kilometers to a hundred kilometers apart along the axis. Surrounding such vents are deposits of hydrothermal minerals rich in metals, as well as exotic animal communities including, in some vent fields, giant tubeworms and clams.
Sulfate-reducing bacteria, rather than photosynthesizing plants, are the base of the food chain at the vents. Life on Earth may have originated in hydrothermal vent environments similar to those presently found along the axis of the Mid-Oceanic Ridge. See also: Hydrothermal vent ; Volcano. If a particular section of the Mid-Oceanic Ridge becomes inactive—through cessation of sea-floor spreading—it is referred to as an extinct or abandoned spreading axis or center. This may happen on the scale of an entire plate boundary, such as 36 million years ago, when spreading ceased along the Mid-Labrador Sea Ridge between Greenland and North America; as a result, the entire Greenland plate became part of the North American plate.
More commonly, only short sections of accreting plate boundary are abandoned as a section of boundary jumps to a new location inside preexisting crust. Propagating rifts create a special category of extinct spreading center known as failed rifts. As rifts propagate continuously, the extinct spreading center is abandoned continuously and therefore varies in age along its length.
After a section of Mid-Oceanic Ridge becomes extinct, its regional topographic high gradually decays as a result of cooling and thermal contraction. However, the shorter-wavelength topography does not decay, and the site of an extinct ridge will generally still be recognizable by its median rift valley, symmetric magnetic anomaly pattern, and other features.
The properties of the crust and upper mantle below the Mid-Oceanic Ridge have been inferred mostly from seismic waves generated by earthquakes and by artificial sources such as explosions and air guns.
None of these drill holes have penetrated to the Moho seismic M-Discontinuity , the boundary between the crust and upper mantle. However, it is believed that rock assemblages known as ophiolites, exposed in ancient mountain belts, represent remnants of former oceanic crust and uppermost mantle that are tectonically separated from the main down-going slab and thrust into the upper continental crust. At least in terms of physical properties such as seismic-wave velocity, the ophiolites resemble the modern oceanic crust as determined by geophysical measurements.
Although magma molten rock erupts occasionally onto the sea floor at the axis of the Mid-Oceanic Ridge, seismic methods have failed to detect a continuous crustal magma chamber, at least below slow-spreading ridges.
However, this hot mantle is partially molten at depths of between about 10 and 50 km 6 and 30 mi below the axis of the Mid-Oceanic Ridge. Seismic tomography indicates that the anomalously warm, rising mantle below the Mid-Oceanic Ridge probably extends to depths of several hundred kilometers. See also: Magma ; Marine geology. To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 10, highly qualified scientists and 46 Nobel Prize winners. Paired lines on either side of the axis show the amount of crust generated in the last 10 million years at the current opening rates.
Dotted lines show the volcanic arcs, which are lines of volcanoes and volcanic islands formed from magma rising from the subducted plate. Width and related effects The height and thermal contraction rate of the ridge crest are relatively independent of the rate of sea-floor spreading; thus, the width and regional slopes of the Mid-Oceanic Ridge depend primarily on the rate of plate separation spreading rate.
Hot spots Although the Mid-Oceanic Ridge exhibits little systematic depth variation along much of its length, there are several bulges swells of shallower sea floor.
Local topography The average or regional depth and shape of the Mid-Oceanic Ridge is modulated by more local topographic relief, some of which equals or exceeds the regional ridge in amplitude just as the local relief represented by Pikes Peak exceeds the regional topographic high of the Rocky Mountains [ Figs. See also: Fault and fault structures ; Rift valley Fig.
At a fast-spreading ridge opening at up to 15 cm a year the crust is smoother, with flat lavas flowing from fissures, than at a slow-spreading ridge about 2 cm a year , where the median valley contains a chain of small volcanoes linked by fissure eruptions. From: mid-ocean ridge in A Dictionary of Earth Sciences ». Subjects: Science and technology — Earth Sciences and Geography.
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