The ocean ridges
The mid-ocean ridges are underwater ridges located in the central part of the oceans. The ridges are the manifestation of divergent or constructive edges. They have been produced as a consequence of the updrafts of the mantle, which have risen to the surface at this point, reaching an altitude of 2000 - 2500 meters above the ocean floor.
In the axis of the ridge there is a central groove, the rift, with great volcanic activity, where the magma from the mantle comes out, forming new volcanoes and a new oceanic lithosphere. The rocks in the center of the ridge are more modern than those in the periphery.
It is also possible to observe, on both sides of the ridge, a symmetry of magnetic bands parallel to the axis of the ridge, which alternately have normal (current) and inverse polarity. This shows that there have been numerous reversals of the magnetic field, and that as ferromagnetic minerals are oriented according to the magnetic field in their crystallization, the oceanic lithosphere has been growing since the rift due to the departure of new materials.
By United States Geological Survey [Public domain or Public domain], via Wikimedia Commons
In this way, the ocean grows over millions of years, and the continents on both sides will move away. For example, in the Atlantic Ocean, the growth of the oceanic lithosphere, for 180 million years, causes the separation of Europe and Africa from America.
Since the Earth is spherical, the ridges are divided into segments displaced by the transform faults .