Why is the seafloor crust younger than the continental crust?
Subduction is the process where the ocean floor plunging back into the interior of the earth.
Older ocean floor is pushed away from the midocean ridges as new ocean floor is formed.
When the rocks are pushed deep enough, they are melted by the heat of the earth. Some of the molten rock will rise up and produce volcanoes. But most of the molten part will become part of the mantle. So as new rocks are formed along the midocean ridges, older rocks are subducted intothe trenches.
(Trenches are deep, v-shaped valleys that lie along the bottom of the oceans)