Why do composite volcanoes occur at the boundary between oceanic crust and continental crust?

Answer 1

The simple answer:

Oceanic crust is more dense than the continental crust, so it goes under as the two plates ram into each other. (This is called subduction)

This submerged crust melts into magma, and collects into a large pool under the continental plate where it went under. The pool of magma forces its way up to form a volcano, which sits in front of us and blows when least expected.

The whole process takes a very long time.

Also, it only happens in convergent boundaries.

Answer 2

Composite volcanoes, also known as stratovolcanoes, occur at convergent plate boundaries where oceanic crust subducts beneath continental crust. The subduction of denser oceanic crust triggers the melting of rocks in the mantle, leading to the formation of viscous magma that erupts explosively through the continental crust, creating composite volcanoes with alternating layers of lava and ash.