A chain of islands is called an archipelago. Archipelagos are usually on the ocean, but they can also be found on lakes and rivers. Most archipelagos comprise of oceanic islands, which were created from volcanoes on the ocean floor.
Island arcs, or chains of oceanic islands, form over intensely hot regions within the Earth. Islands are formed when hot spots rise just beneath the surface and cause the Earth's crust to shift. The arc of an island chain indicates the pattern in which the crust shifted. One of the best known archipelagos are the Hawaiian Islands, which were created by movements in the Pacific tectonic plate.