<P> The Mid-Atlantic Ridge (MAR) is a mid-ocean ridge, a divergent tectonic plate or constructive plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world . In the North Atlantic it separates the Eurasian and North American Plates, and in the South Atlantic it separates the African and South American Plates . The ridge extends from a junction with the Gakkel Ridge (Mid-Arctic Ridge) northeast of Greenland southward to the Bouvet Triple Junction in the South Atlantic . Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level . The section of the ridge that includes the island of Iceland is also known as the Reykjanes Ridge . The ridge has an average spreading rate of about 2.5 centimetres (0.98 in) per year . </P> <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>

What four major plates make up the floor of the atlantic ocean