• 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.

    The archipelago that makes up Bermuda has 181 islands.

    It's All Greek to Me
    Archipelago was what the ancient Greeks called the Aegean Sea.

    Archipelagic Apron
    An archipelagic apron is the smooth, gentle slope of the seafloor surrounding oceanic islands or seamounts (underwater volcanoes).

  • Term Part of Speech Definition Encyclopedic Entry
    archipelago Noun

    a group of closely scattered islands in a large body of water.

    Encyclopedic Entry: archipelago
    atoll Noun

    a coral reef or string of coral islands that surrounds a lagoon.

    Encyclopedic Entry: atoll
    continental island Noun

    land once connected to a continent but broken off by shifting tectonic plates.

    crust Noun

    rocky outermost layer of Earth or other planet.

    Encyclopedic Entry: crust
    glacial retreat Noun

    process by which glaciers melt faster than precipitation can replace the ice.

    glacier Noun

    mass of ice that moves slowly over land.

    Encyclopedic Entry: glacier
    hot spot Noun

    intensely hot region deep within the Earth that rises to just underneath the surface. Some hot spots produce volcanoes.

    Encyclopedic Entry: hot spot
    Ice Age Noun

    last glacial period, which peaked about 20,000 years ago.

    island Noun

    body of land surrounded by water.

    Encyclopedic Entry: island
    island arc Noun

    chain of volcanic islands.

    oceanic island Noun

    land formed from the eruption of a volcano on the ocean floor.

    post-glacial rebound Noun

    process in which land that was crushed by a glacier regains its shape.

    reef Noun

    a ridge of rocks, coral, or sand rising from the ocean floor all the way to or near the ocean's surface.

    Encyclopedic Entry: reef
    sea level Noun

    base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

    Encyclopedic Entry: sea level
    seamount Noun

    underwater mountain.

    stable Adjective

    steady and reliable.

    strait Noun

    narrow passage of water that connects two larger bodies of water.

    Encyclopedic Entry: strait
    tectonic plate Noun

    massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

    volcano Noun

    an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

    Encyclopedic Entry: volcano