The mid-ocean ridges are made almost entirely of basalt, because this is where new oceanic lithosphere is being formed. In fact, oceanic plates in there entirety are made up almost entirely of basalt.(igneous rock).he oldest rocks are located on continents, usually away from the ocean. The mid-ocean ridges are, in fact, home of some of the youngest rocks. The reason for this is wh … at is called sea-floor spreading.