mid-ocean ridge

Long submarine mountain range that winds along the middle of the ocean floor. The mid-ocean ridge system is essentially a segmented, linear shield volcano. There are a number of major ridges, including the Mid-Atlantic Ridge, which runs down the centre of the Atlantic; the East Pacific Rise in the southeast Pacific; and the Southeast Indian Ridge. These ridges are now known to be spreading centres, or divergent margins, where two plates of oceanic lithosphere are moving away from one another (see plate tectonics). Ocean ridges can rise thousands of metres above the surrounding seabed, and extend for up to 60,000 km/37,000 mi in length.

Ocean ridges usually have a rift valley along their crests, indicating where the flanks are being pulled apart by the growth of the plates of the lithosphere beneath. The crests are generally free of sediment; increasing depths of sediment are found with increasing distance down the flanks.