7) As new molten rock continues to be extruded at the mid-ocean ridge
and added to the oceanic plate (6), the older (earlier formed) part of
the plate moves away from the ridge where it was originally created.
8) As the oceanic plate moves farther and farther away from the active,
hot spreading ridge, it gradually cools down. The colder the plate
gets, the denser (“heavier”) it becomes. Eventually, the edge of the
plate that is farthest from the spreading ridges cools so much that it
becomes denser than the asthenosphere beneath it.
9) Denser materials sink, and that’s exactly what happens to the oceanic
plate—it starts to sink into the asthenosphere! Where one plate sinks
beneath another a subduction zone forms