Answer
Primarily, they move apart from each other. It causes upwelling, melting, and the reaction of new seafloor.
Work Step by Step
Divergent plates move along crest of oceanic ridges, and their movement away fractures the ocean crust, causing molten rock to migrate upwards and eventually create more seafloor. This eventually becomes new oceanic lithosphere.
See pp. 46-49 for more.
