Mid-ocean ridge

The rate of seafloor spreading determines the morphology of the crest of the mid-ocean ridge and its width in an ocean basin.

The melt rises as magma at the linear weakness between the separating plates, and emerges as lava, creating new oceanic crust and lithosphere upon cooling.

[2][3][9][10] By contrast, fast-spreading ridges (greater than 90 mm/yr) such as the East Pacific Rise lack rift valleys.

The flanks of mid-ocean ridges are in many places marked by the inactive scars of transform faults called fracture zones.

[3] The oceanic crust is in a constant state of 'renewal' at the mid-ocean ridges by the processes of seafloor spreading and plate tectonics.

[17][18] Hydrothermal vents fueled by magmatic and volcanic heat are a common feature at oceanic spreading centers.

[24][26] A process previously proposed to contribute to plate motion and the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" due to deep convection (see image).

[27][28] However, some studies have shown that the upper mantle (asthenosphere) is too plastic (flexible) to generate enough friction to pull the tectonic plate along.

[29][30] Moreover, mantle upwelling that causes magma to form beneath the ocean ridges appears to involve only its upper 400 km (250 mi), as deduced from seismic tomography and observations of the seismic discontinuity in the upper mantle at about 400 km (250 mi).

[34] Sealevel change can be attributed to other factors (thermal expansion, ice melting, and mantle convection creating dynamic topography[35]).

Over very long timescales, however, it is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of seafloor spreading along the mid-ocean ridges.

[38] Fast spreading rates will expand the mid-ocean ridge causing basalt reactions with seawater to happen more rapidly.

[39][42] The first indications that a ridge bisects the Atlantic Ocean basin came from the results of the British Challenger expedition in the nineteenth century.

[44] It was not until after World War II, when the ocean floor was surveyed in more detail, that the full extent of mid-ocean ridges became known.

A team led by Marie Tharp and Bruce Heezen concluded that there was an enormous mountain chain with a rift valley at its crest, running up the middle of the Atlantic Ocean.

Other research showed that the ridge crest was seismically active[45] and fresh lavas were found in the rift valley.

The German Meteor expedition traced the mid-ocean ridge from the South Atlantic into the Indian Ocean early in the twentieth century.

He stated: "the Mid-Atlantic Ridge ... zone in which the floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot sima [rising] from depth".

Following the discovery of the worldwide extent of the mid-ocean ridge in the 1950s, geologists faced a new task: explaining how such an enormous geological structure could have formed.

The discovery of mid-ocean ridges and the process of seafloor spreading allowed for Wegener's theory to be expanded so that it included the movement of oceanic crust as well as the continents.