mid-ocean ridge

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun The system of these mountain ranges considered as a single geologic feature.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a mid-oceanic ridge