Here's why:

* Subduction: When two oceanic plates collide, the denser plate (usually the older one) is forced beneath the less dense plate. This process is called subduction.

* Magma Formation: As the subducting plate descends, it melts due to the intense heat and pressure. This molten rock, called magma, rises towards the surface.

* Volcanic Arc: The magma eventually erupts through the overriding plate, creating a chain of volcanoes known as a volcanic arc. These arcs are often found parallel to the trench where the plates meet.

So, the collision of oceanic plates results in subduction, which leads to magma generation and the formation of volcanic arcs.