There are islands....then there are islands. Great Britain, for example, is a continental island, found in shallow seas off of large landmass. Those islands are formed when rising seas cause the separation of a piece of land from the larger continent.
Then there are volcanic islands. Those are formed by volcanic activity on the ocean's floor, usually near where two tectonic plates pull apart and lava flows up. Sometimes, it is an undersea volcanic eruption that breaks the surface to form land.
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