Archipelagos are sometimes defined by political boundaries. The Gulf Archipelago off the Pacific Coast forms part of a larger archipelago that geologically includes Washington State’s San Juan Islands. While the Gulf Archipelago and San Juan Islands are geologically related, they are not technically included in the same archipelago due to manmade geopolitical borders.
The word “archipelago” comes from the medieval Italian word archi, meaning chief or principal, and the Greek word pelagus, meaning gulf, pool, or pond.
Most archipelagos are formed when volcanoes erupt from the ocean floor; these are called oceanic islands. The islands of the Hawaiian archipelago, for example, were formed by a series of volcanic eruptions that began more than 80 million years ago and are still active today.
Archipelagos can also form as a result of erosion, sedimentary deposits, rising sea level, and other geographic processes. The Florida Keys are an example of a coral cay archipelago, which form when ocean currents transport sediments that gradually build up on the reef surface.
Continental fragments are archipelagos that have separated from a continental land mass due to the Earth’s tectonic movements. The Farallon Islands off the coast of California are an example of continental fragments.
Continental archipelagos, such as British Columbia’s Inside Passage, are islands that form close to the coast of a continent.