Rocks on the ocean floor are expelled from the molten mantle through Mid-Ocean Ridges, areas where the tectonic plates of the Earth are moving apart. This allows the liquid rock from underneath the plates to move upwards and cool as it hits the water. These rocks are younger than the rocks at a Subduction Zone, because there are closer to point of origination. At the subduction zone, one tectonic plate is pushing underneath another, recycling its material into the molten mantle; by definition, this plate has been moving away from the mid-ocean ridge since it was first formed. While the ocean floor moves and so rocks from far away may be pushed to new locations, generally speaking the rocks near a subduction zone will be older than those near a mid-ocean ridge.