The Sinking Mountain The famous peak of Mount Otemanu, rising 727 meters (2,385 feet) over Bora Bora, is the highest remaining point of a much larger, and now mostly submerged, shield volcano.%%CITE_1%%%%CITE_2%% Geological dating using potassium-argon methods reveals the volcano was active during the Upper Pliocene epoch, between 3.1 and 3.45 million years ago.%%CITE_3%%%%CITE_4%%%%CITE_5%% Formed by a stationary mantle hotspot, the entire volcanic edifice has been slowly sinking as the Pacific Plate moves northwestward at about 11 cm per year.%%CITE_4%%%%CITE_5%% This process of gradual sinking, or subsidence, shows the geological lifecycle that Charles Darwin first proposed to explain the formation of coral atolls.%%CITE_6%%%%CITE_7%%
As the original volcano sank, a race against time began. Coral polyps, tiny marine animals that build reefs, started to grow in the shallow, sunlit waters around the volcanic island. They formed a fringing reef that kept growing upward, tracking the rising sea level and the sinking landmass. The result is the modern barrier reef that encloses the lagoon. The main island itself is sinking at a rate of about one centimeter per century.
A Caldera's Aquatic Second Act
The lagoon is not the volcano's original crater, but rather the space created between the sinking central island and the upward-growing barrier reef. This protected body of water covers approximately 70 square kilometers. Its depth varies significantly, with vast shallow sand flats only a few meters deep, perfect for snorkeling, while its deepest points reach down approximately 40 to 48 meters (about 130 to 157 feet).
Seismic profiles and sediment cores show the Holocene-era lagoon floor is composed of up to 10 meters of accumulated sediment overlying a much older Pleistocene-era karst surface. The sediment is primarily carbonate mud, composed of aragonite needles from algae like Halimeda and the shells of mollusks and foraminifera. Despite the proximity of the volcanic island, siliciclastic content (eroded volcanic rock) is low, typically less than 10%, because the island lacks any major permanent rivers to transport it into the lagoon.
The lagoon connects to the open Pacific Ocean through a single deep channel, Teavanui Pass, which is 48 meters deep. This pass allows for water circulation, which helps the health of the coral communities within. The corals themselves, dominated by genera like Porites and historically Acropora, continue their slow upward growth, a limestone wall built on the shoulders of a sinking giant.