Mid-Atlantic Ridge

A bathymetric map of the Mid-Atlantic Ridge (shown in light blue in the middle of the Atlantic Ocean)

The Mid-Atlantic Ridge is a mountain range running mostly underwater and generally north-south through the middle of the Atlantic Ocean. If it were all above water, the ridge would be commonly regarded, at 16,000 km, as the world's longest near-continuous mountain span under one name. Its northern extent lies at approximately 87°N (about 330 km south of the North Pole); in the south it stretches to subantarctic Bouvet Island before turning east to become the Atlantic-Indian Ridge. Evidence has been discovered that some of the largest whales that frequent the Atlantic migrate along the ridge.

Part of the Earth's mid-oceanic ridge system, which reaches throughout all the world's oceans, the Mid-Atlantic Ridge was discovered in the 1950s by American geologists whose work led in the next decade to the theories of seafloor spreading, continental drift, and plate tectonics. The studies done of the areas below the surface of the seas greatly illuminated the scientific understanding of what was more clearly visible on dry land.

Discovery

A ridge under the northern Atlantic Ocean was first inferred by Matthew Fontaine Maury in 1853, based on soundings by the USS Dolphin. The existence of the ridge and its extension into the South Atlantic was confirmed during the expedition of HMS Challenger in 1872. A team of scientists on board, led by Charles Wyville Thomson, discovered a large rise in the middle of the Atlantic while investigating the future location for a transatlantic telegraph cable. The existence of such a ridge was confirmed by sonar in 1925 and was found to extend around Cape Agulhas into the Indian Ocean by the German Meteor expedition.

In the 1950s, mapping of the Earth's ocean floors by Marie Tharp, Bruce Heezen, Maurice Ewing, and others revealed that the Mid-Atlantic Ridge had a strange bathymetry of valleys and ridges, with its central valley being seismologically active and the epicenter of many earthquakes. Ewing, Heezen and Tharp discovered that the ridge is part of a Template:Convert/LoffAonDbSon long essentially continuous system of mid-ocean ridges on the floors of all the Earth's oceans. The discovery of this worldwide ridge system led to the theory of seafloor spreading and general acceptance of Alfred Wegener's theory of continental drift and expansion in the modified form of plate tectonics. The ridge is a feature whose contribution to the breakup of the supercontinent of Pangaea, in the period from about 200 to 160 million years ago, is considered in the modelling of such breakup in modern tectonic theory, where subduction and mantle plumes mechanisms are hypothesised to be primary, although historically this was contentious.

Interoceanic Ridge System

The ridge is now known to be an oceanic rift separating the North American Plate from the Eurasian Plate in the North Atlantic, and the South American Plate from the African Plate in the South Atlantic. It actually sits on top of the "mid-Atlantic rise," a linear bulge running the length of the ocean with the ridge resting on the highest point of the bulge, which is thought to be caused by upward convective forces in the Earth's mantle.

The Mid-Atlantic Ridge is an example of what is called a slow-spreading mid-oceanic ridge system, which typically is broad with a deep, central rift valley. (Fast-spreading systems are more typical of the Pacific Ocean, where deep trenches are more common than in the Atlantic.) New seabed is continually being formed from the spreading ridge, which creates relatively shallow areas in the middle of the Atlantic above the extremely flat abyssal plains that begin at the margins of the continental shelves.

Near the Equator, the ridge bends sharply in an S-curve and is dissected into the North Atlantic Ridge and South Atlantic Ridge by the Romanche Trench, a narrow submarine ravine with a maximum depth of 7,758 m, one of the deepest points in the Atlantic Ocean. To the north is a giant canyon hundreds of miles long and about 20 miles wide named the Charlie-Gibbs Fracture Zone. Situated straight west of Ireland and southeast of Greenland, the fracture zone cuts through the Mid-Atlantic Ridge and connects the two halves of the North Atlantic. Great volumes of water pour from the west side of the ridge thru Charlie-Gibbs to the east side, with weaker return flows occurring elsewhere along the mountain range.

Map showing the Mid-Atlantic Ridge splitting Iceland and separating the North American and Eurasian Plates. The red triangles show locations of some of Iceland's active volcanoes

The northernmost portion of the ridge passes under and beyond Iceland, where it is actually at the floor of the Arctic Ocean rather than the Atlantic. The straddling of the ridge by Iceland and its many volcanoes makes for a natural scientific laboratory for the study of the geophysical processes that otherwise occurs mostly underwater. One discovery is that Iceland itself is rifting at the rate of a few centimeters per year.

Running almost perpendicular to the Mid-Atlantic Ridge and nearly parallel to one another are scores of fracture zones extending out to great distances along the ocean floor. Such zones, phenomena of the entire interoceanic ridge system, bisect the ridge and split it into distinct segments. Though the process of their formation is not clearly understood, wherever less volcanic material is extruded from the ridge tends to become the location of a chasm. As the seafloor spreads away from the ridge, the chasms—or fracture zones—continually extend outward and still remain visible because they are too deep for sediment to fill them.

Geology

Pangaea's separation (animated)

The Mid-Atlantic Ridge's divergent boundary first formed in the Triassic period, in the time of the supercontinent Pangaea. Essentially, the ridge is the remnant of that continent when it pulled apart from its jigsaw arrangement of proto-Africa and proto-Europe on one side and the pre-Americas on the other.

The entire worldwide mid-oceanic ridge rests in areas where tectonic plates are pulling apart. The pulling motion creates cracks in the ocean floor that are called rift zones. As the plates pull apart, magma rises to fill the spaces. Heat from the magma then causes the crust on either side of the rifts to expand, which forms the ridges. These ridges run along a divergent boundary, which occurs where the plates are moving away from one another. Iceland, which is splitting along the spreading edge between the North American and Eurasian Plates, is an example.

Basaltic rocks of the Mid-Atlantic Ridge observed by the Hercules ROV during the 2005 Lost City Expedition

The ridge sits atop a geologic feature known as the Mid-Atlantic Rise, which is a progressive bulge that runs the length of the Atlantic Ocean, with the ridge resting on the highest point of this linear bulge. This bulge is thought to be caused by upward convective forces in the asthenosphere pushing the oceanic crust and lithosphere. This divergent boundary first formed in the Triassic period, when a series of three-armed grabens coalesced on the supercontinent Pangaea to form the ridge. Usually, only two arms of any given three-armed graben become part of a divergent plate boundary. The failed arms are called aulacogens, and the aulacogens of the Mid-Atlantic Ridge eventually became many of the large river valleys seen along the Americas and Africa (including the Mississippi River, Amazon River and Niger River). The Fundy Basin on the Atlantic coast of North America between New Brunswick and Nova Scotia in Canada is evidence of the ancestral Mid-Atlantic Ridge.

Islands and Seamounts

The highest summits of the Mid-Atlantic Ridge extend above the ocean's surface to form islands. The peaks that fall short of the ocean waves are called seamounts, of which there are many along the ridge.

The islands are, from north to south, with their respective national sovereignty, highest point, elevation in meters, and location:

Island	National Sovereignty	Highest Point	Elevation	Location
Jan Mayen	Norwegian	Beerenberg	2,277 m	east of Greenland (in the Arctic Ocean)
Iceland	Icelandic	Hvannadalshnúkur	2,119 m	in Vatnajökull glacier
Azores	Portuguese	Ponta do Pico	2,351 m	west of the Iberian Peninsula
Saint Peter and Paul Rocks	Brazilian	Southwest Rock	23 m	between Brazil and Liberia
Ascension Island	British	Green Mountain	858 m	between Brazil and Angola
Saint Helena	British	Diana's Peak	820 m	between Brazil and Angola
Tristan da Cunha	British	Queen Mary's Peak	2,062 m	between Argentina and South Africa
Gough Island	British	Edinburgh Peak	909 m	between Argentina and South Africa
Bouvet Island	Norwegian	Olavtoppen	780 m	between Antarctica and South Africa

References

ISBN links support NWE through referral fees

• Hellemans, Alexander, and Brian Bunch. The Timetables of Science: A chronology of the most important people and events in the history of science. Simon and Schuster, 1988. ISBN 0671621300
• Redfern, Ron. Origins, the Evolution of Continents, Oceans and Life. University of Oklahoma Press, 2001. ISBN 978-0806133591
• Searle, Roger. Mid-Ocean Ridges. Cambridge University Press, 2013. ISBN 978-1107017528
• Spencer, Edgar W. Introduction to the Structure of the Earth. McGraw-Hill College, 1988. ISBN 978-0070601987

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

• Mid-Atlantic Ridge  ^history

The history of this article since it was imported to New World Encyclopedia:

• History of "Mid-Atlantic Ridge"

Note: Some restrictions may apply to use of individual images which are separately licensed.