They are geologically active, with new magma constantly emerging onto the ocean floor through a gap called a rift in the earth's crust. In most instances, it accumulates and forms new crust. However, submarine volcanoes might also be formed.
The rocks making up the sea floor are usually younger near the centre of the ridge and the older ones located farther away from the rift. This is evidence that new magma constantly emerges through a rift.
Also, the sea floor is made up of rocks generally much younger than the Earth itself. This suggests that the sea floor is in a constant state of 'renewal'.
Before the 19th century, the depths of the open ocean were largely a matter of speculation, and most people thought that the ocean floor was relatively flat and featureless.
In 1961, scientists began to theorize that mid-ocean ridges mark structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest.
In effect, the ocean basins were perpetually being "recycled," with the creation of new crust and the destruction of old oceanic lithosphere occurring simultaneously.
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