Hi. I am confused about plate boundaries. In school we always drew diagrams where the continental crust sat on top of the oceanic crust. With that in mind, what is a collision boundary? Is this where continental crust sits on top of the mantle? How did India collide with Asia? Is the Indian sub-continent sat on top of oceanic crust and the approach to the Luarentia is a normal destructive plate boundary. If so, why are there no volcanoes in the Himalayas?