Practically every different type of oceanic island can be found in the Cooks.
Rarotonga is the only high volcanic island of the Tahiti type. Aitutaki, like Bora Bora, consists of a middle-aged volcanic island surrounded by an atoll-like barrier reef, with many tiny islets defining its lagoon.
Atiu, Mangaia, Mauke, and Mitiaro are raised atolls with a high cave-studded outer coral ring (makatea) enclosing volcanic soil at the center. The outer crust of earth is elastic, and when this envelope is stretched taut, the tremendous weight of a volcano is spread over a great area, deforming the seabed. Thus Atiu, Mangaia, Mauke, and Mitiaro were uplifted during the past two million years due to the weight of Rarotonga on the earth's crust.
There are low rolling hills in the interiors of both Atiu and Mangaia, while Mauke and Mitiaro are flat. The swimming and snorkeling possibilities at Atiu, Mangaia, Mauke, and Mitiaro are limited, as there's only a fringing reef with small tidal pools. Aitutaki and Rarotonga have protected lagoons where snorkeling is relatively safe. The rich, fertile southern islands account for 89 percent of the Cooks' land area and population.
Manihiki, Manuae, Palmerston, Penrhyn, Pukapuka, Rakahanga, and Suwarrow are typical lagoon atolls, while tiny Takutea and Nassau are sand cays without lagoons. All of the northern atolls are so low that waves roll right across them during hurricanes, and you have to be within 20 km to see them. This great variety makes the Cook Islands a geologist's paradise.
As the volcanic portion of an island subsides, the fringing reef is converted into a barrier reef. After the volcanic core has disappeared completely into the lagoon, the remaining reef island is called an atoll.