Spreading Rate

Spreading rate is the velocity at which new oceanic crust forms and moves away from a mid-ocean ridge axis, usually given as a full rate in cm/yr (the half rate is one limb). Rates span about 2 to 5 cm/yr at slow ridges such as the Mid-Atlantic Ridge up to 15 to 20 cm/yr at the fast East Pacific Rise. Rate sets ridge shape: slow ridges build a deep axial rift valley, fast ridges form a smooth rise. It is measured from the spacing of dated magnetic-anomaly stripes, which record geomagnetic reversals symmetrically about the axis, and from the deepening of the seafloor with crustal age.