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In the general case, sea floor spreading starts as a rift in a continental land mass, similar to the Red Sea- East Africa Rift System today. The process starts with heating at the base of the continental crust which causes it to become more plastic and less dense. Because less dense objects rise in relation to more dense objects, the area being heated becomes a broad dome (see isostasy). As the crust bows upward, fractures occur that gradually grow into rifts. The typical rift system consists of three rift arms at approximately 120 degree angles. These areas are named 'triple junctions' and can be found in several places across the world today.
If spreading continues past the incipient stage described above, two of the rift arms will open while the third arm stops opening and becomes a 'failed rift'. As the two active rifts continue to open, eventually the continental crust is attenuated as far as it will stretch. At this point basaltic oceanic crust begins to form between the separating continental fragments. When one of the rifts opens into the existing ocean, the rift system is flooded with seawater and becomes a new sea. The Red Sea is an example of a new arm of the sea, and the East Africa rift is a "failed" arm that is opening somewhat more slowly than the other two arms.
Sea floor spreading can stop at any time in the process, but if it continues far enough that the continent is completely severed a new ocean basin is created. The Red Sea has not yet completely split Arabia from Africa, but a similar feature can be found on the other side of Africa that has broken completely free. South America once fit into the area of the Niger Delta. The Niger River has formed in the failed rift arm of the triple junction.
The new oceanic crust is quite hot relative to old oceanic crust, so the new oceanic basin is shallower than older oceanic basins. Since the world remains the same size as the continents move apart, the older ocean basins must shrink to make room for the new ocean basins. This shrinkage occurs by subduction of older oceanic crust beneath continents. Today, the Atlantic basin is actively spreading and the rim of the Pacific Ocean is actively subducting (and causing volcanic activity in what has been termed the Ring of Fire).
Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean.
At the Mid-Atlantic Ridge (and other places), material from the upper mantle rises through the faults between oceanic plates to form new crust as the plates move away from each other, a phenomenon first observed as continental drift.
It is still a matter of some debate whether seafloor spreading is driven primarily by the force of rising magma at these locations, or if it is driven by the force of sinking magma elsewhere and these upwellings are merely a side effect.