hey are oceanic ridges where new ocean floor is created and spreads apart is what they all have in common They are not all at the edges of tectonic plates. Neither the Atlantic Mod ocean ridge nor the Mid Indian ocean ridge are at the edge of tectonics plates. While oceanic or ridges may not be exactly in the middle of a tectonic pate. They CREATE the tectonic even some or part of an oceanic are being subjected.
"...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...."
see the map for other details
