As a plate slowly moves over a fixed hot spot or magma plume, a volcanic island can often form in the ocean.

arc

Explanation:

The hot spots are products of the mantle plumes. They produce volcanoes over them, and the location of the volcanoes doesn't often correspond with the plate boundaries.

The hot spots often produce volcanoes in the middle of the plates, with the Hawaii Islands being a prime example of this. Because the hot spot is staying at one place, but the plates move over it, over time a volcanic island arc is formed.

This comes to be because as the plate moves above the hot spot it stretches the connection between the hot spot and the volcano, eventually resulting in cutting it off. The volcanic island then stops its volcanic activity, but the hot spot continues to push magma upward, resulting in the formation of new volcanic island. The same process continues over and over again, so an island arc forms over the course of millions of years.