Islands form in three main ways. Continental islands are created when land is cut off from a continent as melting ice caps raise the sea level and one-time surface connections are flooded. Volcanic islands form when mountains rise from the sea, built by magma forced to the surface through vents called "hot spots" or as a result of plates overriding one another at plate boundaries. Coral islands, a third island type, begin with colonies of marine coral building reefs that can cover hundreds of square miles. When enough water-borne sand and wind-borne dust accumulate on the reef, an island forms.

Atolls are a distinctive kind of coral island whose low-lying reef surrounds a warm lagoon. These formations exist over sunken volcanic islands, primarily in the South Pacific. Charles Darwin is credited with proposing the foundational theory of how atolls, which are really a hybrid of two island types, develop.

Darwin's theory, which geologists today widely accept, states that as a volcano's magma source is depleted, it begins to sink under its own considerable weight. With it goes the fringing reef that formed in the shallow waters around the island. Because reef animals can only survive near the surface of the water, new animals build vertically on top of the limestone skeletons of past generations. Provided their building keeps the top of the reef near sea level, a barrier reef forms offshore. The sinking island eventually disappears completely under water, leaving an atoll composed of low coral islets that ring a shallow lagoon. The living corals' sandy waste fills the lagoon inside the reef, and that which falls seaward buttresses the entire structure.

Atolls can indicate the age of islands within a chain. Islands with prominent, active volcanoes are younger, whereas smaller or submerged formations like atolls are older, having undergone a transformation that may have taken many millions of years. In fact, if scientists were to drill down through the coral reef, they might have to drill several thousand feet before reaching the basalt that composes the volcano. This indicates that the reef has been growing for a very long time indeed.

Atolls are just one example of how biological processes — reef building, in this case — can modify Earth's surface. Coal and oil deposits (organic matter subjected to intense heat and pressure over time) and stromatolites (fossilized reef-like structures built long ago by photosynthesizing microbes) are some other examples.

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