There are islands of various types, depending on their origin. They can be volcanic, coral, extensions of a continent or tops of underwater mountains.
The islands are sometimes grouped together, forming a geographical unit generally called "archipelago." When the islands are very small they are often called "islets".
They are remote islands of the continents and have a different origin from these. They can appear when a mountain or underwater dorsal rises above the sea surface. For this reason, they tend to have an abrupt relief. Other times they are the result of large folds or portions of the original supercontinent (Pangea) that did not merge with the current ones.
Madagascar and New Zealand are two examples of large oceanic islands.
They are islands that are in the vicinity of a continent, separated by a shallow strait that at some geological times might have emerged. These islands are an extension of the continent, as evidenced by the fossils and types of rocks they contain.
The British Isles are an archipelago that is part of the European continental shelf.
This type of islands is the result of the volcanic activity that takes place in the ocean ridges or in other centers of this type dispersed by the oceans. They often appear in a group. These islands are still forming or disappearing today, such as the island of Surtsey, in Iceland, born in an eruption in 1963. The Pacific contains a large number of volcanic islands.
The Canary Islands, born during the last stages of the folding of the Atlantic back, are another example of a volcanic archipelago.
They are formations of biological origin, due to the intense activity of the corals that, when they die, leave their calcareous skeletons forming a structure that serves as the basis for the development of new corals. Thus, large structures can be formed, whose deepest parts sink, allowing growth through the upper area, where there is more light.
Emerged areas suffer the effects of erosion, creating soils where plants can develop. Obviously, they reach low altitude above sea level. They usually appear in circles, with a central lagoon that communicates with the sea. If the structure continues to grow, this lagoon can dry, joining the different islets on a single island.
The Great Barrier Reef, off the coast of Australia, is an example of coral islands.
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