At the Mid Oceanic Ridges the two lithospheric plates move away from each other resulting in the spreading of .
The Sea Floor Spreading was put forth by Harry Hess. The hypothesis is as follows:
The divergent boundaries are marked by Mid Oceanic Ridges and the continental rift zones. At the Mid Oceanic Ridges the two lithospheric plates move away from each other and the magma upwells. This makes the Crust at the mid oceanic ridges warmer and buoyant. Thus the crust gets a higher topography forming a gravitational head there.
The magma also adds to the Oceanic lithosphere forming new crust at the MOR. The Magma cools and forms new Oceanic crust there. This gives rise to a ridge push force which causes the Sea Floor Spreading.
This picture below explains a continental rifting and then the Sea Floor Spreading.