An archipelago is a group of islands closely scattered in a body of water. Usually, this body of water is the ocean, but it can also be a lake or river.

Most archipelagoes are made of oceanic islands. This means the islands were formed by volcanoes erupting from the ocean floor. An archipelago made up of oceanic islands is called an island arc.

Many island arcs were formed over a single “hot spot.” The Earth’s crust shifted while the hot spot stayed put, creating a line of islands that show exactly the direction the crust moved.

The Hawaiian Islands continue to form this way, with a hot spot remaining relatively stable while the Pacific tectonic plate moves northwest. There are 137 Hawaiian islands, reefs and atolls, stretching from Kure and Midway in the west to the "Big Island" of Hawaii in the east. The Big Island is still being formed by the active volcanoes Mauna Loa and Kilauea. The island arc will grow as Loihi, a seamount southeast of the Big Island, eventually punctures the ocean surface as Hawaii's youngest island.

Japan is another island arc. The Japanese archipelago consists of four large islands, from Hokkaido, in the far north, through Honshu, Shikoku, and Kyushu in the far south. Japan also includes more than 3,000 smaller islands. In several places in the Japanese archipelago, volcanoes are still active.

Volcanoes do not form all archipelagoes. Many archipelagoes are continental islands formed only after the last ice age. As glaciers retreated, sea levels rose and low-lying valleys were flooded. Coastal mountain ranges became archipelagoes just off the mainland.

The largest archipelago in the world was formed by glacial retreat. The Malay Archipelago, between the Pacific and Indian Oceans, contains more than 25,000 islands in Southeast Asia. The thousands of islands of Indonesia and Malaysia are a part of the Malay Archipelago. At least some of these islands—and the straits that separate them—were part of mainland Asia during the last ice age.

Finland’s Archipelago Sea, part of the Baltic Sea, also emerged after the last ice age. There are more than 50,000 islands in the Archipelago Sea, although many of them do not measure half a hectare (one acre). Some of the islands are close enough to be connected by bridges.

Islands of the archipelago sea were never coastal mountaintops, however. They were formed by post-glacial rebound. In this process, land that was squashed by the weight of heavy glaciers during the Ice Age slowly regains its shape, like a sponge. Because post-glacial rebound is still occurring, islands continue to rise from the Archipelago Sea.