An Atoll of Algae
The Atol das Rocas, or Rocas Atoll, is the only atoll in the South Atlantic Ocean, located approximately 260 kilometers northeast of Natal, Brazil. It is an elliptical ring of reef measuring 3.7 kilometers east-to-west and 2.5 kilometers north-to-south. The structure sits atop a volcanic seamount that rises from the ocean floor, which is up to 4,000 meters deep in the surrounding area. Unlike most atolls, which are primarily built by coral polyps, Rocas is constructed mainly by crustose coralline algae, particularly red algae, with secondary contributions from vermetid gastropods and corals like Siderastrea stellata.
The atoll's reef growth began more than 4,800 years ago, with an accretion rate measured at 1.5 to 3.2 meters per millennium. This biological construction encloses a shallow lagoon with a total area of 7.1 square kilometers and a depth of up to 6 meters. At low tide, the reef crest is exposed, forming a natural wall about 1.5 meters high around the lagoon's tidal pools. Two small sandy islets, Farol Cay and Cemitério Island, are located on the western side of the lagoon. Their combined land area is just 0.36 square kilometers, with the highest point being a sand dune on Farol Cay that reaches 6 meters in elevation.