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1. Any of various underwater mountain ranges forming a chain that extends almost continuously for about 66,000 kilometers (41,000 miles) through the North and South Atlantic Oceans, the Indian Ocean, and the South Pacific Ocean at the boundaries between divergent tectonic plates. Magma escapes from rifts along the tops of these ranges, adding new material to the earth's crust.
2. The system of these mountain ranges considered as a single geologic feature.
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. See more at tectonic boundary.