<P> Two processes, ridge - push and slab pull, are thought to be responsible for the spreading seen at mid-ocean ridges, and there is some uncertainty as to which is dominant . Ridge - push occurs when the growing bulk of the ridge pushes the rest of the tectonic plate away from the ridge, often towards a subduction zone . At the subduction zone, "slab - pull" comes into effect . This is simply the weight of the tectonic plate being subducted (pulled) below the overlying plate dragging the rest of the plate along behind it . The slab pull mechanism is considered to be contributing more than the ridge push . </P> <P> The other process proposed to contribute to the formation of new oceanic crust at mid-ocean ridges is the "mantle conveyor" (see image). However, there have been some studies which have shown that the upper mantle (asthenosphere) is too plastic (flexible) to generate enough friction to pull the tectonic plate along . Moreover, unlike in the image above, 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 from studies of the seismic discontinuity at about 400 km (250 mi). The relatively shallow depths from which the upwelling mantle rises below ridges are more consistent with the "slab - pull" process . On the other hand, some of the world's largest tectonic plates such as the North American Plate are in motion, yet are nowhere being subducted . </P> <P> The rate at which the mid-ocean ridge creates new material is known as the spreading rate, and is typically measured in mm / yr . As a general rule, fast ridges have spreading (opening) rates of more than 90 mm / year . Intermediate ridges have a spreading rate of 50--90 mm / year while slow spreading ridges have a rate less than 50 mm / year . The spreading rate of the North Atlantic Ocean is ~ 25 mm / yr, while in the Pacific region, it is 80--120 mm / yr . Ridges that spread at rates <20 mm / yr are referred to as ultraslow spreading ridges (e.g., the Gakkel Ridge in the Arctic Ocean and the Southwest Indian Ridge) and they provide a much different perspective on crustal formation than their faster spreading brethren . </P> <P> The mid-ocean ridge systems form new oceanic crust . As crystallized basalt extruded at a ridge axis cools below Curie points of appropriate iron - titanium oxides, magnetic field directions parallel to the Earth's magnetic field are recorded in those oxides . The orientations of the field in the oceanic crust preserve a record of directions of the Earth's magnetic field with time . Because the field has reversed directions at irregular intervals throughout its history, the pattern of geomagnetic reversals in the ocean crust can be used as an indicator of age . Likewise, the pattern of reversals together with age measurements of the crust is used to help establish the history of the Earth's magnetic field . </P>

On this map of the south atlantic why is the mid-ocean ridge in the center of the ocean