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There are many definitions of an archipelago but what they all have in common is that it’s a chain or cluster of islands. The word comes from the Greek ἄρχι- arkhi, which means “chief,” and πέλαγος pélagos, meaning “sea.” There are various ways to pronounce this compound word depending on your native language: ARKHIPELAGOS (English), ˈɑrkhaipelagos/ (British English) or IPA:, äˌʃi̯paltaˈgosi͜o̞/
The Arcipelago was once used as the name for the Aegean Sea, and later came to be used for the islands within it. The word is cognate with archipelago (Italian archipiélago) or archipel des Indes (French).
Today this suite of words is sometimes extended to encompass “Bigger archipelagos,” which may also be composite lands like Great Britain and Ireland, Madagascar, Java, Sumatra, Sakhalin, etc. or large archipelagic states: Indonesia (the second-largest archipelagic state in the world), Argentina (which includes Tierra del Fuego archipelago), Chile, Philippines; but generally excluding small outlying islands like Norr å land In Canada, Australia, and New Zealand, organized island groups are called archipelagoes. In the Philippines, the archipelago is called Luzon, Visayas, and Mindanao (Luzon is named after a Spanish archbishop). In Indonesia, archipelagic states are also known as the Lesser Sunda Islands.
An archipelago may have anywhere from totally isolated islands to groups of hundreds of closely spaced islands. There are four main types of archipelagos: volcanic, orogenic, coralline, and non-volcanic or coastal. A volcanic archipelago is one arising from volcanism. An orogenic archipelago results from progeny, with islands piled up on top of each other in a succession of island arcs by seismic activity, subsidence of nearby land areas, and most commonly a mixture of both. A coralline archipelago is created by physical or biological processes (such as erosion) working on rock formations in the sea. One can consider such archipelagos like a city with many buildings, where each building represents an island.
Assemblage archipelagoes are archipelagos that originated from various types of transported material forming an accretionary prism; they may have formed at their present location or somewhere else entirely within the ocean basin before being moved there by plate tectonics. Some scientists use the term archipelagic state to refer to all islands within an entire oceanic basin, including smaller surrounding islands, for example, Hawaii and New Zealand, which sit astride the boundary of the Pacific and Tasman basins.
There are five main types of archipelagos that exist in the world today: volcanic, orogenic, coralline, non-volcanic, or coastal, and assemblage. Each type is typically formed by a different geologic process or combination of processes.

Coastal archipelagos are archipelagos that began their formation process as island chains along continental coastlines in areas where sea levels were rising, such as oceanic crust extending underwater, says Brian Atwater, a geologist at the U.S. Geological Survey in Seattle. Coastal archipelagos include archipelagos such as Galapagos Islands archipelago; Hawaiian archipelago (consists of eight main islands, with dozens of smaller ones extending from the main island chain) archipelago; and Banks Island archipelago. Non-volcanic or coastal archipelagoes are altered by isostatic uplift or subsidence during tectonic processes like orogeny. They include archipelagoes like the Iceland archipelago, the West Siberian Plain archipelago, and the St. Lawrence Island archipelago.

Coralline archipelagoes are made of coral which is primarily composed of carbonate minerals and CaCO3 (calcium carbonate). They are created when the ocean tectonics forces a salt-water current to flow over a volcanic archipelago. When these two forces meet, limestone is precipitated on top of the reef structure. Through time, this single island grows above sea level into a coral archipelago, with a fringing reef.

Volcanic archipelagoes are formed when volcanoes erupt underwater, which then cools to form islands made primarily of rock surrounded by lava. The resulting land masses range in size from small rocky outcroppings less than one hectare to large land masses many hundreds of kilometers across. The mere act of formation can create island landforms such as atolls and archipelagos. Over millions of years, tectonic movement causes the atoll to build vertically while seawater erodes away the shallow margins. Atolls are archipelagoes created from coral reef formations in a ring surrounding an underwater volcanic island or seamount. The volcano forms a central island which is often only above sea level with a series of small surrounding islands; these surrounding islands form the atoll. [CUNY Journal Article: Collision archipelagoes]

Some archipelagos are formed by glacial and river erosion, such as the Columbia River archipelago and Mackenzie River archipelagoes; others were formed by glaciers around eroded mountains, like the Saint Elias archipelago and the archipelago of Franz Josef Land. An archipelago formed by glaciers is called a glacial archipelago. The largest archipelagoes in the world, created by plate tectonics, are the Lesser Antilles archipelago and Solomon Islands archipelago.

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