A volcanic island visibly sinking while its coral reef grows upward to keep pace - Darwin's subsidence theory of atoll formation playing out in real time. Bora Bora will eventually vanish beneath its own lagoon.
NASA Johnson Space Center, Public domain, via Wikimedia Commons
A Sinking Giant
Bora Bora is a textbook example of Charles Darwin's 1842 theory on the formation of coral atolls. The island is the eroded remnant of an extinct shield volcano that first erupted from the sea floor between 3.1 and 3.45 million years ago. This volcanic activity was driven by the Society hotspot, a stationary plume of magma in the Earth's mantle that periodically punches through the moving Pacific Plate, creating a chain of islands over millions of years. Bora Bora's volcano is now extinct, and the entire island is slowly sinking under its own weight, a process called subsidence.
The rate of this sinking is estimated to be around one centimeter per century, or between 0.05 to 0.14 meters per millennium. As the central volcanic island, with its highest peak, Mount Otemanu, at 727 meters, subsides, the surrounding coral reef continues to grow upwards. This dynamic process maintains the reef's position near the sunlit sea surface, creating the vast, protected lagoon that separates the main island from the outer barrier reef. The main island today measures only 8 kilometers north to south and 5 kilometers east to west, a fraction of its original size. The lagoon itself covers an area of approximately 78 square kilometers.
The Living Barrier
The barrier reef that encircles Bora Bora is a complex living structure, constantly growing to keep pace with the island's slow descent. The reef is composed of countless coral colonies, primarily from genera like Porites, Acropora, and Montipora. These reef-building corals create hard skeletons of calcium carbonate, which accumulate over millennia. For the lagoon to exist, the vertical growth rate of the coral, known as the accretion rate, must match or exceed the island's subsidence rate. Studies of the Holocene reef show average vertical accretion rates of 5.65 to 6.15 meters per millennium—easily outpacing the sinking of the volcano.
This process creates a distinct geography. The main volcanic island is encircled by the shallow lagoon, which has a maximum depth of around 60 meters, though large areas are much shallower. The lagoon is then enclosed by the barrier reef. On top of this reef are the "motus," small, sandy islets formed from eroded coral and sand. This entire sequence, from a high volcanic island with a fringing reef to a barrier reef island like Bora Bora, and finally to a true atoll (a ring of coral with no central island), illustrates Darwin's subsidence theory in action.
💡Fun Facts
The main volcanic crater of Bora Bora collapsed long ago; its former southwestern edge is now marked by the small islets of Toopua and Toopua-iti.
The original name for the island was Pora Pora, meaning "first born" in Tahitian; the current name arose from a later misspelling.
The Society Islands chain shows a clear age progression, with older, more eroded islands to the northwest and younger islands to the southeast, closer to the active hotspot.
Besides subsidence, the reef's development has been heavily influenced by glacio-eustatic sea-level changes—the rise and fall of oceans due to ice ages.
