The Earth is full of natural phenomena that change its surface—rivers transport sediment, glaciers carve valleys and colliding tectonic plates build mountains. But one of the most impressive of its geologic skills is the formation of islands. Island formations come in many shapes, sizes and types, with different underlying geologic origins. If several islands are found together in the same area of a body of water, they are called archipelagos.

Islands have always been a special feature of the world’s geography and are commonly described as a “slice of land completely surrounded by water.” The isolation of islands also affects the plants and animals that live on them, which may seem to be an odd assortment compared with the biota of continents. The skewed distribution is due in part to the small size of most islands, which cannot support a representative zoo or allow migration from outside. This can also be caused by the fact that islands tend to develop independently of continental life, with a unique and often very peculiar array of endemic species.

The most common type of island is formed by volcanic action related to the movement of lithospheric (continent-bearing) plates, such as the Hawaiian islands. Other types of islands include oceanic islands that grow from the continental lithosphere and those that form as sea-floor spreading or along the mid-ocean ridges. Then there are coral islands, which grow from corals that colonize and spread around a volcano as it sinks into the sea. Finally, there are river islands, which are created when sediment is deposited in place by erosion.