MID-ATLANTIC RIDGE

submarine ridge lying along the north-south axis of the Atlantic Ocean; it occupies the central part of the basin between a series of flat abyssal plains that continue to the margins of the continental coasts. The Mid-Atlantic Ridge is in effect an immensely long mountain chain extending for about 10,000 miles (16,000 km) in a curving path from the Arctic Ocean to near the southern tip of Africa. The ridge is equidistant between the continents on either side of it. The mountains forming the ridge reach a width of 1,000 miles. These mountains sometimes reach above sea level, thus forming the islands or island groups of the Azores, Ascension, St. Helena, and Tristan da Cunha, among others. Running along the crest of the Mid-Atlantic Ridge is a long valley that is about 50 to 75 miles (80 to 120 km) wide. This rift contains the zone of seafloor spreading, in which molten magma from beneath the Earth's crust continuously wells up, cools, and is progressively pushed away from the ridge's flanks. This phenomenon is evinced by the fact that the crustal material on either side of the Mid-Atlantic Ridge is notably younger than that farther away from the ridge. Because of seafloor spreading, the movement of the ocean floor and of the continents outward from the ridge is resulting in an increasing widening of the Atlantic Basin at an estimated rate of 1 to 10 cm (0.5 to 4 inches) a year. Apart from seafloor spreading, the Mid-Atlantic Ridge is also the site of volcanic activity and earthquakes along some portions of its length.