<P> A ridge under the Atlantic Ocean was first inferred by Matthew Fontaine Maury in 1850 . The existence of the ridge was discovered 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 the Cape of Good Hope into the Indian Ocean by the German Meteor expedition . </P> <P> In the 1950s, mapping of the Earth's ocean floors by Bruce Heezen, Maurice Ewing, Marie Tharp 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 40,000 - km - 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 Wegener's theory of continental drift and expansion in the modified form of plate tectonics . The ridge is central to the breakup of the hypothetical supercontinent of Pangaea that began some 180 million years ago . </P> <P> The Mid-Atlantic Ridge includes a deep rift valley that runs along the axis of the ridge along nearly its entire length . This rift marks the actual boundary between adjacent tectonic plates, where magma from the mantle reaches the seafloor, erupting as lava and producing new crustal material for the plates . </P> <P> Near the equator, the Mid-Atlantic Ridge is divided into the North Atlantic Ridge and the South Atlantic Ridge by the Romanche Trench, a narrow submarine trench with a maximum depth of 7,758 m (25,453 ft), one of the deepest locations of the Atlantic Ocean . This trench, however, is not regarded as the boundary between the North and South American Plates, nor the Eurasian and African Plates . </P>

Where do some rift valleys form along the oceanic ridge system