Because a series of magnetic reversals that are semmetrical either side of EACH mid oceanic ridge have been mapped out and clearly demonstrate the way the the oceans have spread through time. In the late 1950s, a series of oceanographic expeditions was commissioned to map the magnetic character of the ocean floor, with the expectation that the ocean floors would display largely uniform magnetic properties. Surprisingly, results showed that the basaltic sea floor has a striped magnetic pattern, and that the stripes run essentially parallel to the mid-ocean ridges. Moreover, the stripes on one side of a mid-ocean ridge are symmetrically matched to others of similar width and polarity on the opposite side. In 1963, two British geoscientists, Vine and Matthews (Box 1), proposed a hypothesis that elegantly explained how these magnetic reversal stripes formed by linking them to the new idea of sea-floor spreading. They suggested that as new oceanic crust forms by the solidification of basalt magma, it acquires a magnetisation in the same orientation as the prevailing global magnetic field. As sea-floor spreading continues, new oceanic crust is generated along the ridge axis. If the polarity of the magnetic field then reverses, any newly erupted basalt becomes magnetised in the opposite direction to that of the earlier crust and so records the opposite polarity. Since sea-floor spreading is a continuous process on a geological timescale, the process preserves rocks of alternating polarity across the ocean floor (Figure 7a). Reading outwards in one direction from the mid-ocean ridge gives a record of reversals over time, and this can be matched with the record read in the opposite direction.
There is a ridge down the center of the pacific. You can trace it up far to the north and it makes a hard right which is something to really think about. What caused this ridge is that the pacific floor is spreading apart about four inches a year. That space becomes filled with magma from below and as it surges up from below makes this underwater mountain range. That is part of plate tectonics. The process starts when a plate is being pulled apart and an example of that would be the Rift in Africa. The next step is that area fills up with water and and example of that would be the Red Sea. The ends of the rift usually have a V formation. Put that V upside down and it is visible with the Red Sea. As long as the tectonic forces pull the plate apart, this process continues and you have the Pacific Ocean. Should the forces change, it will stop and an example of that is the lower part of the Mississippi River. . .down where it is called Ol' Man River. The upper part of the Mississippi is from water drainage. At some point the pressure on the our plate stopped and the it just turned into our drainage system in the US. Makes you go back to that underwater mountain range and make you wonder what happen that those mountains at the end of the chain are going in a different direction and how does that relate to the Mississippi. Plate tectonic are really interesting to me. Having lived in Missouri for some time. There is a line across the southern part of the state and huge sink holes open up. I asked that question in geology and everyone shrugged their shoulders. But couldn't that be the V I talked about. I had a friend with a farm down there and it's on this invisible line. They took me over to their neighbors to see this large hole. Their barn and a great deal of acreage had just disappeared down this hole. Interesting stuff. . .hope this helps.