Mid-Atlantic Ridge, submerged volcanic mountain range that bisects the Atlantic Ocean along its north-south axis. The range covers the middle third of the ocean basin, running for 15,000 km (9,300 mi) through the North and South Atlantic, from Iceland in the north to a point 7,200 km (4,500 mi) east of the southern tip of South America. It is part of the submerged volcanic mountain range that encircles the earth, measuring some 65,000 km (40,000 mi) long. The Mid-Atlantic Ridge breaks the ocean's surface in several places, forming seven islands or groups of islands. From north to south, these islands are Iceland, the Azores, Saint Peter and Saint Paul Rocks, Ascension, Saint Helena, Tristan da Cunha, and Bouvet; all but Saint Peter and Saint Paul Rocks are volcanic in origin. Most of the ridge, however, lies 3,000 to 5,000 m (9,800 to 16,400 ft) below the ocean's surface. From the seafloor, the mountains generally rise 1,000 to 3,000 m (3,300 to 9,800 ft) high, and measure 1,500 km (930 mi) wide from east to west at their base.

The Mid-Atlantic Ridge is cleft by a deep valley along its crest, generally 10 km (6 mi) wide and with walls 3 km (2 mi) high. This valley is the boundary of two divergent plates of the ocean's floor, where the seafloor is spreading apart, according to the theory of plate tectonics. The valley in the Mid-Atlantic Ridge is widening at a rate of 3 cm (1.2 in) per year. Where the seafloor spreads, magma (molten rock) from beneath the earth's surface rises buoyantly. This magma becomes new ocean crust on and beneath the seafloor when it cools. The Mid-Atlantic Ridge is marked by fracture zones and other discontinuities at spacings of up to hundreds of kilometers, which offset the range from its general north-south course. The largest offsets, such as the east-west Romanche Fracture Zone, are as long as 1,000 km (600 mi) and occur near the equator. They account for the matching continental bends at the Gulf of Guinea in Africa and the northeastern coast of Brazil in South America.

See also Ocean and Oceanography: Ocean Basin Structure.