Final answer:
The old oceanic crust subducts under the young crust because the thinner and denser oceanic plates can be more readily thrust down into the upper mantle compared to the thicker continental masses.
Step-by-step explanation:
When two tectonic plates come together, one plate is forced beneath the other in a process called subduction. The old oceanic crust subducts under the young crust because the thinner and denser oceanic plates can be more readily thrust down into the upper mantle compared to the thicker continental masses.
Subduction zones are often marked by ocean trenches, such as the deep Japan trench along the coast of Asia. The subducted plate is forced into regions of high pressure and temperature, causing it to melt several hundred kilometers below the surface. The material from the subducted plate is then recycled into a downward-flowing convection current.
This process helps balance the flow of material that rises along rift zones, where new crust is formed. The amount of crust destroyed through subduction is approximately equal to the amount formed at rift zones.