When two oceanic plates meet at a convergent boundary, which plate subducts?

• A. The older one
• B. The lighter one
• C. The denser one
• D. Both subduct

Answer: (C)

At a convergent boundary between two oceanic plates, the sinking of one plate is driven by density. The cooler, older oceanic crust becomes denser than the younger, warmer crust above it, so it subducts beneath its partner. This downward-moving slab pulls into the mantle, creating a trench and fueling volcanic activity on the overriding plate. The plate on top stays buoyant and does not subduct. That’s why the denser plate is the one that subducts—the lighter, less dense plate simply cannot sink as readily. The idea that both subduct or that the lighter plate subducts doesn’t fit with how buoyancy and gravity govern subduction zones.