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38. Isles Sous les Vents, Society Islands
The Leeward Islands within the French Society Islands in the Pacific Ocean provide an excellent illustration of the hypothesis first propounded by Charles Darwin to explain the origin of coral reefs in deep oceans. At the bottom right of the frame, the islands of Tahaa and Raiatea are old, eroded volcanos, fringed by a coral reef. Moving north through the chain, the central, island-forming volcanos are older and more heavily eroded. On the central island, Bora Bora, the barrier is prominently developed, while the volcanic island has significantly eroded and subsided. Visible in the top left of the frame, Tupai is just an atoll. All that remains of the volcano around which the reef originally grew lies below the shallow floor of the central lagoon. After their eruptive phase, when no longer buoyed by their magma chambers, volcanic islands may gradually sink several thousand meters into the oceanic crust, yet continuing coral growth may sustain an atoll. For example, the volcanic platform underlying Midway Island is now more than 2000 meters below the ocean surface.
STS-9, November-December 1983. Picture #9-46-1835