Almost entirely hidden under the sea is a worldwide chain of low mountains with lines of volcanic activity running along their crests. Their worldwide extent was recognized in the mid-20th century, and shortly afterward the mid-ocean ridges were assigned a starring role in the new theory of plate tectonics. The ridges are the divergent zones where oceanic plates are born, spreading apart from the central valley, or axial trough.
This map shows the overall configuration of the ridges and their names. Click the image for a 900-pixel version. There are more ridges whose names don't fit: The Galápagos Ridge runs from the East Pacific Rise to Cntral America, and the northern continuation of the Mid-Atlantic Ridge is called Reykjanes Ridge south of Iceland, Mohns Ridge north of Iceland, and Gakkel Ridge in the Arctic Ocean. The Gakkel and Southwest Indian ridges are the slowest-spreading ridges, while the East Pacific Rise spreads the fastest, with the sides moving apart up to nearly 20 centimeters a year.
Mid-ocean ridges are not the only place where the seafloor spreads apartbackarc spreading zones occur near many subduction zonesbut they are so productive and so important in global geochemistry that "mid-ocean ridge basalt" is commonly known by its abbreviation MORB.