seafloor spreading

noun
1.
a process in which new ocean floor is created as molten material from the earth's mantle rises in margins between plates or ridges and spreads out.
British Dictionary definitions for sea-floor spreading

sea-floor spreading

/ˈsiːˌflɔː/
noun
1.
a series of processes in which new oceanic lithosphere is created at oceanic ridges, spreads away from the ridges, and returns to the earth's interior along subduction zones Also called ocean floor spreading
sea-floor spreading in Science
sea-floor spreading  
In the theory of plate tectonics, the process by which new oceanic crust is formed by the convective upwelling of magma at mid-ocean ridges, resulting in the continuous lateral displacement of existing oceanic crust. See more at magnetic reversal.