An underwater mountain range developed where magma rises up through a cracking and widening ridge in the oceanic crust. Some magma cools below the crust, some forces into fractures, and much flows out to form new crust, which is then pushed away from the ridge. McCarthy (2004) J. Geophys. Res. 112, B03410 explains that the disparity in spreading rates between the Northern and Southern Hemispheres is correlated with the rate of latitudinal circumferential extension.

As new crust is created each side of the ridge, it takes up the prevailing magnetic polarity of the Earth, which reverses from time to time. As a result, symmetrical bands of crust, with alternating polarity, develop on either side of the ridge. These magnetic patterns are used to calculate the rate of the sea-floor spreading; see Michaud et al. (2006) Geology 34, 1. The term mid-oceanic ridge properly refers to the ridge at the centre of the Atlantic Ocean, which comes to the surface at points such as Tristan da Cunha and Ascension Island. Other ridges, such as the Pacific–Antarctic ridge, are not truly at the centre of the ocean.