Collission or convergent boundaries. Where plates collide. Plate
edges may be either oceanic crust or continental crust. So when plates collide,
we have only three possibilities: oceanic-oceanic, oceanic-continental, or
continental-continental collisions. If oceanic crust collide with continental
crust, the denser oceanic crust is subducted under the less dense continental
crust (as at the Ring of Fire). If continental crust collide with continental
crust they push each other up in a mountain range (like the Himalayas).