A long, linear, elevated, volcanic structure often lying along the middle of the ocean floor. Such ridges tend to occupy central positions because the oceans have formed by the symmetrical spreading of two lithospheric plates from the ridge sites. Ocean ridges occur in all the Earth's oceans, but may be offset from a central position, e.g. the E. Pacific ridge, where one side of the oceanic crust is being consumed along a subduction zone. At mid-ocean ridges, ocean floor is being formed. At the centre there is a rift valley, formed as discrete segments, bordered by high mountains on both sides. At a fast-spreading ridge (opening at up to 15 cm a year) the crust is smoother, with flat lavas flowing from fissures, than at a slow-spreading ridge (about 2 cm a year), where the median valley contains a chain of small volcanoes linked by fissure eruptions.
Subjects: Earth Sciences and Geography.