Archipelago: Definition & Examples

Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

Are islands archipelagos, or are archipelagos islands? In this lesson, we'll figure out what's what, and look at some of the different ways that archipelagos can form.

Archipelagos

The word ''archipelago'' was once the name of the Aegean Sea, off the coast of Greece. Its definitive feature: It's absolutely covered with islands. While the name of this body of water has changed, that connotation remains in modern geography. Today, an archipelago is a string or cluster of islands that are very close to each other within the same body of water. Islands don't have to be very big to count officially as part of an archipelago; generally anything over an acre qualifies. So, we can get literally thousands of tiny islands (and a few big ones) within a single archipelago. For anyone who really likes islands, archipelagos are a geographic jackpot.

Forming Archipelagos

Archipelagos contain dozens to thousands of islands within close proximity. So, how does this happen? Archipelagos aren't collections of islands that just happen to be near each other; generally they are formed as part of the same geological events. So, they're related and connected in a way.

There are a few ways that archipelagos can form, but the most common is through undersea volcanic activity. Here's the gist: Magma boils from the earth's mantle to the surface at locations called hot spots. Many of these can be found on the floor of the ocean. As magma pours into the ocean, it builds up over time, creating a seamount. Once that seamount becomes so tall that it reaches the surface of the water, it becomes a volcanic island.

That's how many islands are formed. However, hot spots are significant because their source originates below the tectonic plates. They also occur in the middle of plates, and not at plate boundaries. So, what's that mean? Basically, it means that the surface of the earth keeps moving as tectonic plates move, but the hot spot stays in the same location. As one island is formed, the plates continue to move and eventually the island moves off the hot spot. The volcano becomes inactive, and at the bottom of the seafloor a new seamount starts to form.

If you remove the ocean, you can see how the Hawaiian archipelago is created from one hot spot, with the plate moving in the direction of the arrow.
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This is how most archipelagos form. A hot spot keeps creating islands as the tectonic plates keep shifting. If you look at archipelagos created this way, you can basically draw the path that the plates are moving as a line from island to island. The most famous archipelago to be created this way might be Hawaii. While we think of the big islands, Hawaii's archipelago technically contains more than 100 smaller islands stretched out more than 1,500 miles, all created as the Pacific Plate moves over the same hot spot. In fact, southeast of the island of Hawaii is the next island in the chain, still forming. This is Lo'ihi seamount, which is currently about 10,000 feet tall but still 3,000 feet below sea level. When it reaches the surface, it will be the newest island of the Hawaii archipelago.

Island Chains

Many archipelagos form from hot spots, like the Hawaiian Islands. However, volcanic activity can also form archipelagos in a second way. Hot spots are places were magma reaches the surface somewhere inside the borders of a tectonic plate. It's unusual because we don't expect this to be a place for magma to rise. Where we do expect this to happen, however, is at the boundaries between plates.

As two tectonic plates intersect, one dives under the other. Areas where this happens have lots of earthquakes as well as lots of volcanic activity. Because magma is often coming to the surface, the areas also tend to have archipelagos. The islands of these archipelagos appear along the border between the two plates. If you draw a line between them, it often takes an arc-shaped appearance, and as a result, we call this kind of archipelago an island arc. Japan, the Philippines, and the Aleutian Islands are all parts of island arcs, as are most islands of the Aegean.

The Aleutian Islands form a clear island arc.
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The Ice Age and Archipelagos

Most archipelagos were created through volcanic activity under the sea, but not all. In fact, some were created not by fire, but by ice. Sort of.

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