1. Margin and explanation
An Island arc is often composed of a chain of volcanoes , with arc shaped alignment situated parallel and close to a boundary between two converging tectonic plates.
Most of these island arcs are formed as one oceanic tectonic plate subducts another one and, in most cases, produces magma at depth below the over-riding plate. However, this is only true for those island arcs that are part of the group of mountain belts which are called volcanic arcs, a term which is used when all the elements of the arc-shaped mountain belt are composed of volcanoes

2. The Ryukyu Islands form an island arc as well as the Tonga islands.
Diagram and named egs
The Ryukyu Islands form an island arc as well as the Tonga islands.

3. Formation
The oceanic crust which is denser, gets subducted underneath the continental crust. This forms an ocean trench and a succession of volcanoes which then forms grouped islands in the shape of an arc.