This animation illustrates in a very simplified manner, the rising of magma at the mid-oceanic ridge level and the creation of oceanic crust. A ridge is a zone of divergence where two plates separate from each other.
Geologists distinguish two types of divergence:
The topography of the ridge strongly depends on this speed.
Note: The "rolling carpet" model presented in this animation can lead you to believe that the rising of magma in the centre is the cause of the separation of plates. This is not the case. Geological models actually show that the deep convection currents, situated in the mantle would be the cause of a drifting movement of plates on a large scale. The ridge, like subduction, would therefore be a consequence of continental drift.
Note that the "plate techtonic theory" is a young theory, announced in 1967, and is always evolving.