A long, seismically active submarine ridge system situated in the middle of an ocean basin and marking the site of the upwelling of magma associated with sea-floor spreading. An example is the Mid-Atlantic Ridge.
Although Leg 2 was dogged with technical problems, it proved beyond doubt that the deepest sediments do indeed become younger as you approach the spreading centers at the mid-ocean ridges.
The active volcanic vents along the spreading mid-ocean ridges create ideal environments for the circulation of fluids rich in minerals and for ore deposition.
The mid-ocean ridge is two chains of mountains separated by a large depression that form at a spreading center (or where two plates are drifting apart).
Unlike volcanic activity at mid-ocean ridges, island arc volcanoes can remain fixed over their magma sources for thousands of years, allowing them to sometimes grow above water level and become islands.
This is simply because new ocean floor, as we have seen, continuously forms at the mid-ocean ridges and then spreads away on either side, aging and collecting a progressively thicker sediment blanket as it does so.