Question

What is it called when a string of islands that forms where two convergent boundaries collide?

Answer 1

When two convergent boundaries collide, especially in the ocean, an Island arc is formed.

When these  two convergent plates collide, the older plate which is denser subducts beneath the younger plate.  As the subducting plate is pushed deeper into the mantle, it melts. The magma this creates rises and erupts. This forms a line of volcanoes known as an island arc , with considerable earth quakes created.

Examples of island arcs include Japan, Indonesia, the Phillipine Islands and the Aleutian Islands  of Alaska.