The corner stone of plate-tectonics is the theory of sea-floor spreading. According to this theory the floor of the ocean is moving or spreading away from ridges, by repetitive magma intrusion that split and spread the older sea-floor, causing them to be moved away from the ridge in a nearly continuous horizontal position.
This is also confirmed by the occurrence of progressively older rocks with increasing distance from the ridge. The lack of pelagic sediments at the ridge crest can also be explained by sea-floor spreading.
The new floor at the crest is too young to have received a blanket of pelagic sediments. The idea of sea-floor spreading could provide an explanation for aspects of ridge vulcanism, seismicity, and physiography, and for the apparent youthfulness of sea-floor rocks compared with the continental ones.
Plate tectonics incorporates sea-floor spreading as one of its essential parts and makes explicit the idea that the sea-floor and the litho-sphere beneath it are simultaneously being created at ridges.
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According to it, a diverging boundary on the sea-floor is marked by the crest of the mid-oceanic ridge. It is also believed that upward convection is a result of plate divergence (for details see Plate-tectonics).