Which term describes a boundary where two plates move towards each other and the denser plate is subducted?

• A. Destructive plate boundary
• B. Transform plate boundary
• C. Conservative plate boundary
• D. Constructive plate boundary

Answer: (A)

Subduction happens when two tectonic plates collide and the denser plate sinks into the mantle, marking a destructive boundary. This boundary type is described as destructive because crust is recycled as one plate is forced down, driving deep earthquakes and volcanic activity as magma rises to form volcanic arcs. The oceanic plate is typically the denser one and subducts beneath the other plate, whether continental or younger oceanic crust, creating features like trenches and volcanic chains.

If plates merely move apart, new crust forms at mid-ocean ridges, which is a constructive (divergent) boundary. If plates slide past each other horizontally, that’s a transform boundary, involving lateral movement with little vertical crust production or destruction. The term conservative is sometimes used to describe boundaries with lateral movement and minimal crust creation or destruction, which doesn’t fit the subduction process described here.