Here's why:

* Subduction: One denser plate (usually the older one) is forced beneath the less dense plate. This process is called subduction.

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

* Volcanic Activity: The magma erupts through the surface, creating volcanoes. Over time, these volcanoes build up and form a chain of islands called an island arc.

* Examples: The Japanese archipelago, the Aleutian Islands, and the Lesser Antilles are examples of island arcs formed by the collision of oceanic plates.

Let me know if you have any other questions!