Nature · Earth Science
Worlds of their own, ringed by sea
An island is a body of land completely surrounded by water and smaller than a continent. Isolated by the sea, islands often evolve unique plants and animals found nowhere else, making them living laboratories of evolution and biodiversity hotspots.
Islands are born in several ways: volcanoes building up from the seafloor, corals growing atop sunken volcanoes, fragments left when continents rift apart, or sandbars and sediment banks raised by rivers and waves. Rising and falling sea levels also cut hilltops off from the mainland.
A seamount grows from the ocean floor until it breaks the surface as a volcanic island; corals may later fringe its shores.
Built by fire from the seabed
Where magma erupts on the seafloor, lava piles up until it breaks the surface. Hotspots and plate boundaries string these islands into arcs and chains.
Fragments of the mainland
These islands are pieces of continent cut off when sea levels rose or land rifted away. Because of their origin, they often share plants and animals with the nearby mainland.
Rings of reef around a lagoon
Corals build reefs around a volcanic island; when the volcano sinks, the reef keeps growing upward, leaving a ring of low coral islands around a central lagoon.
Low banks of shifting sand
Waves and currents heap sand into long, low islands that parallel the coast, sheltering lagoons and wetlands behind them while migrating with every storm.