Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to form new sea floor. Older rocks will be found further away from the spreading zone while younger rocks will be found nearer to the spreading zone.
Earlier theories (e.g. by Alfred Wegener and Alexander du Toit) of continental drift were that continents "plowed" through the sea. The idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. The theory is well-accepted now, and the phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere.
Read more about Seafloor Spreading: Incipient Spreading, Continued Spreading and Subduction, Debate and Search For Mechanism, Sea Floor Global Topography: Half-space Model
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