What causes the subduction zone when an oceanic plate collides with a continental plate?

1) oceanic plates are more dense than continental plates so the oceanic plate dives under the continental plate
2) when the continental plates collide the force is so great that it causes the surrounding crust to collapse
3) continental plates are more dense than oceanic plates so the continental plate dives under the oceanic plate
4) continental plates are more dense than oceanic plates causing the oceanic plate to push upward