Answer:
slab pull-gravity pulls older and denser lithosphere downward
Step-by-step explanation:
The subduction occurs only at the boundaries where an oceanic plate is involved, be it a boundary between two oceanic plates, or between oceanic and continental plates. The oceanic plate which is denser, heavier, because of the gravity is moving downward, thus it moves below the other plate. As it does, the plate gradually reaches the mantle layer where it starts to melt because of the high temperatures, thus it is getting recycled in a way. In this way, the oceanic plates are constantly getting destroyed, which is why the oldest oceanic crust is much younger than the oldest continental crust.