The oceanic crust usually subducts beneath the continental crust because it is denser than the continental crust.

Quick explanation

  • Oceanic crust is made mostly of basalt and other dense, mafic rocks, which makes it heavier per unit volume.
  • Continental crust is made largely of lighter, more felsic (granitic) rocks, so it is more buoyant and tends to “float” higher on the mantle.
  • When the two collide at a convergent boundary, the denser oceanic plate sinks (subducts) beneath the less dense continental plate, forming a subduction zone, trenches, and often volcanoes.

So, in multiple‑choice form, the correct completion is:

When two tectonic plates collide, the oceanic crust usually subducts beneath the continental crust because it is denser than continental crust.

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