Mid-Atlantic Ridge

From New World Encyclopedia

Mid-atlantic ridge map.png

Full extent of the Mid-Atlantic Ridge

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.

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.

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.

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

(There is disagreement as to whether Saint Helena, the famous last island exile of Napoleon, is actually situated on or off the ridge.)

Plate Tectonics

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.

According to plate tectonics, 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.

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.


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