What is the main reason that cause the subduction process?

Subduction occurs when two plates collide at a convergent boundary, and one plate is driven beneath the other, back into the Earth’s interior. Not all convergence leads to subduction. Continental rocks are too buoyant to be forced downward, so when continents collide, they crumple but stay at the surface. Only oceanic plates, which are topped with basalt, are dense enough to sink into the mantle. As a result, only oceanic plates are subducted.

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