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Mid-Atlantic Ridge (MAR)

Every physical geographer who had written upon the Atlantic noticed the curious parallelism between its eastern and its western borders,their salient and retiring angles corresponding very closely to each other. Thus, beginning at tho north the projection formed by the British Islands {which extends much further westwards at 100 fathoms below the surface than it does above the sea-level), answers to the wide entrance to Baffin's Fay; while, on the other hand, the projection of the American coast at Newfoundland answers to the Bay of Biscay. Further south, the great rounded prominence of Northern Africa corresponds with the vast bay that stretches from Nova Scotia to St Thomas; while the angular projection of South America towards the east corresponds with that receding portion of the mid-African coast-line which is known as the Gulf of Guinea.

The global mid-ocean ridge system is the largest single volcanic feature on the Earth, encircling it like the seams of a baseball. Here the Earths crust is spreading, creating new ocean floor and literally renewing the surface of our planet. Older crust is recycled back into the mantle elsewhere on the globe, typically where plates collide. The mid-ocean ridge consists of thousands of individual volcanoes or volcanic ridge segments which periodically erupt.

Seamounts are individual volcanoes on the ocean floor. They are distinct from the plate-boundary volcanic system of the mid-ocean ridges, because seamounts tend to be circular or conical. A circular collapse caldera is often centered at the summit, evidence of a magma chamber within the volcano. Large seamounts are often fed by "hot spots" in the deep mantle. These hot spots are associated with plumes of molten rock rising from the deep within the Earth's mantle. These hot spot plumes melt through the overlying tectonic plate and supply magma to seamounts.

Hot spot plumes are long lived. Therefore, as a moving tectonic plate passes over a mantle hot spot, a chain of volcanoes is produced with a systematic age progression - from older to younger. The Hawaiian Islands, the Galapagos Islands, the Azores, and the Cobb-Eikelberg chain that includes Axial volcano are all examples of hot spot chains. When a hot spot interacts with a mid-ocean ridge, the affected ridge segments tend to receive a greater-than-normal supply of magma from the mantle, leading to more frequent eruptions, and formation of volcanic edifices right on the ridge. Iceland and Axial Seamount are both examples of a hot spot located on the axis of a mid-ocean ridge.

The mid ocean ridge systems are the largest geological features on the planet. The Mid-Atlantic Ridge (MAR) is a mostly underwater mountain range in the Atlantic Ocean that runs from 87N -about 333km south of the North Pole- to subantarctic Bourvet island at 54S. The MAR is about 3 km in height above the ocean floor and 1000 to 1500 km wide, has numerous transform faults and an axial rift valley along its length.

The ridge was discovered in the 1950s. Its discovery led to the theory of seafloor spreading and general acceptance of Wegener's theory of continental drift. The MAR separates 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. These plates are still moving apart, so the Atlantic is growing at the ridge, at a rate of about 2.5 cm per year in an east-west direction.

Most of the ridge system is under water but forms land as a set of volcanic islands of varying size that run the length of the Atlantic Ocean.

Important information as to the changes which the seabed of the Atlantic has undergone within the later geological periods, may be gathered from the structure of the islands which lift themselves above its surface. Along its eastern border, at no considerable distance from the coast of North Africa, there are three principal groups, the Madeiras, Canaries, and Cape Verde all of which have an evidently volcanic origin, and rise up from the eastern slope of the basin, where it is progressively shallowing towards its continental shore-line.

Further out, in midocean, lies the group of the Azores, which also is volcanic, but between this area and the slope from which the Madeiras and Canaries are based is a very deep channel, ranging downwards to at least 15,000 feet; and a like depth is also found between the Azores and the coast of. Portugal. The structure of all these groups of islands gives obvious indications of their formation by separate igneous eruptions in a sea of great depth; and the earliest of these eruptions seems to have taken place in the later Miocene period.

As soon as the first solid lavas raised their heads above water, and were thus exposed to the action of the waves, fragments were dotached and rounded on the shore; and these being swept off, with the debris resulting from their attrition, formed deposits of various kinds upon the slope of the cone, in which corals, shells, stc, were embedded.




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