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'.
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.
Furthermore, the oceanic crust now came to be appreciated as a natural "tape recording" of the history of the reversals in the Earth's magnetic field.
As old oceanic crust was consumed in the trenches, new magma rose and erupted along the spreading ridges to form new crust.
Seafloor spreading proposed that rising magma was forming new oceanic crust along the oceanicridges and that old crust was destroyed at oceanic trenches.
Oceanic rocks along the North American and African coastlines are approximately 180 Myrs old whereas rocks adjacent to the ridge may be less than one million years old.
The presence of the older oceanic floor along the trenches was used to infer that the oceanic lithosphere was being consumed at the trenches.
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