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

Quick explanation

  • Oceanic crust is made mostly of dense basalt, so it sinks more easily into the mantle.
  • Continental crust is thicker and less dense (more buoyant), so it tends to stay on top.
  • Where this happens, a subduction zone forms, often with: trenches, earthquakes, and volcanic arcs along the edge of the continent.

You’ll often see this phrased in textbooks and quizzes exactly as:

“When two tectonic plates collide, the oceanic crust usually subducts beneath the continental crust.”

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