Tetiaroa Island #
This astronaut photograph illustrates the typical circular appearance of a fully developed atoll. The ring of islands - covered in green vegetation and white-to-tan sandy beaches - develops on coral reefs, which originally form around a volcanic island.
As the volcanic island gradually disappears due to subsidence and erosion, the coral reefs continue to grow upwards. Over time, the central volcanic island is completely submerged, leaving a ring of coral reefs and islands that surround a lagoon.
The shallow lagoon waters appear blue-green in this image, and contrast with darker and deeper Pacific Ocean waters surrounding the atoll. One of the motus in the southern portion of the atoll, Tahuna Rahi, is a protected bird sanctuary.