A mid-ocean ridge is a general term for an underwater

Underwater

Underwater is a term describing the realm below the surface of water where the water exists in a natural feature such as an ocean, sea, lake, pond, or river. Three quarters of the planet Earth is covered by water...

 mountain system

Mountain range

A mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...

 that consists of various mountain ranges (chains), typically having a valley

Valley

In geology, a valley or dale is a depression with predominant extent in one direction. A very deep river valley may be called a canyon or gorge.The terms U-shaped and V-shaped are descriptive terms of geography to characterize the form of valleys...

 known as a rift

Rift

In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....

 running along its spine, formed by plate tectonics

Plate tectonics

Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...

. This type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor spreading

Seafloor spreading

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics....

. The uplifted seafloor results from convection

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 currents which rise in the mantle as magma

Magma

Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...

 at a linear weakness in the oceanic crust

Oceanic crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...

, and emerge as lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

, creating new crust upon cooling.