mid-ocean ridge: see plate tectonics.

Mid-Atlantic Ridge: see Atlantic Ocean.

Mid Glamorgan, former county, S Wales. Created in the 1974 local government reorganization of Wales from portions of Breconshire, Glamorgan, and Monmouthshire, in 1996 it was divided among the unitary authorities of Bridgend, Caerphilly, Cardiff, Merthyr Tydfil, Rhondda Cynon Taff, and Vale of Glamorgan.

Continuous, submarine mountain chain extending approximately 50,000 mi (80,000 km) through all the world's oceans, separating them into distinct basins. The main ridge extends down the middle of the Atlantic Ocean, passes between Africa and Antarctica, turns north to the Indian Ocean, then continues between Australia, New Zealand, and Antarctica and across the Pacific basin to the mouth of the Gulf of California. Lateral ridges extend from islands on the axis of the oceanic ridge to coasts of adjacent continents. The oceanic ridge system is the largest feature of the Earth's surface after the continents and the ocean basins themselves; it is explained by the theory of plate tectonics as a boundary between diverging plates where molten rock is brought up from deep beneath the Earth's crust. Seealso subduction zone.

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Submarine ridge lying along the floor of the central Atlantic Ocean. It is a long mountain chain running about 10,000 mi (16,000 km) in a general but curving north-south direction from the Arctic Ocean to the southern tip of Africa. The mountains sometimes reach above sea level, forming such islands or island groups as Ascension, the Azores, St. Helena, and Tristan da Cunha.

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