The Mid-Atlantic Ridge is an underwater mountain range of the Atlantic Ocean that runs from Iceland to Antarctica, and is the longest mountain range on Earth. The ridge was discovered by Bruce Heezen in the 1950s. The discovery of this ridge led to the theory of seafloor spreading and general acceptance of Wegener's theory of continental drift. According to plate tectonics, this ridge runs along a divergent boundary.

This ridge is an oceanic rift that separates the North American PlateThe North American Plate is a continental tectonic plate covering the continent of North America and extending eastward to the Mid-Atlantic Ridge. The easterly side is a divergent boundary with the Eurasian Plate to the north and the African Plate to the from the Eurasian PlateThe Eurasian Plate is a continental tectonic plate covering the continent of Europe and central Asia and extending westward to the Mid-Atlantic Ridge. The easterly side is a boundary with the North American Plate to the north, a convergent boundary with t in the North Atlantic, and the South American PlateThe South American Plate is a continental tectonic plate covering the continent of South America and extending eastward to the Mid-Atlantic Ridge. The easterly side is a divergent boundary with the African Plate forming the southern part of the Mid-Atlant from the African Plate in the South Atlantic. The ridge actually sits on top of the mid-Atlantic rise which is a progressive bulge that also runs the length of the Atlantic Ocean with the ridge resting on the highest point of this linear bulge. This bulge is thought to be caused by upward convective forces in the asthenosphereThe asthenosphere (from the Greek asthenia "weakness") is the region of the Earth between 100-200 km below the surface, but may extend as deep as 400 km. It is the weak or "soft" zone in the upper mantle just below the lithosphere that is involved in plat pushing the oceanic crustOceanic crust is the part of Earth's lithosphere which underlies the ocean basins. It is thinner (generally less than 10 km thick) but more dense than continental crust, about 3. 3 g/cc (grams per cubic centimeter). Oceanic crust is composed of mafic basa and lithosphere.

This divergent boundary first formed in the Triassic period when a series of three-armed grabens coalesced on the supercontinent Pangaea to form the Ridge. Usually only two arms of any given three-armed graben become part of a divergent plate boundary. The failed arms are called aulacogens and the aulacogens of the Mid-Atlantic Ridge eventually became many of the large river valleys seen along the Americas, and Africa (including the Mississippi River, Amazon River and Niger River).