- any of several seismically active submarine mountain ranges that extend through the Atlantic, Indian, and South Pacific oceans: each is hypothesized to be the locus of seafloor spreading.
- A long mountain range on the ocean floor, extending almost continuously through the North and South Atlantic Oceans, the Indian Ocean, and the South Pacific Ocean. A deep rift valley is located at its center, from which magma flows and forms new oceanic crust. As the magma cools and hardens it becomes part of the mountain range. The mid-ocean ridge is approximately 1,500 km (930 mi) wide, 1 to 3 km (0.62 to 1.86 mi) high, and over 84,000 km (52, 080 mi) long. See more at sea-floor spreading.