Rio Grande Rise

The Rio Grande Rise is an aseismic ocean ridge in the southern Atlantic Ocean off the coast of Brazil. Together with the Walvis Ridge off Africa, the Rio Grande Rise forms a V-shaped structure of mirrored hotspot tracks or seamount chains across the northern South Atlantic. In 2013, Brazilian scientists announced that they found granite boulders on the Rio Grande Rise and speculated that it could be the remains of a submerged continent, which they called the "Brazilian Atlantis". Other researchers, however, noted that such boulders can end-up on the ocean floor by less speculative means.

The Rio Grande Rise separates the Santos and Pelotas Basins and is composed of western and eastern areas, which have different geological backgrounds. The western area has numerous guyots and seamounts and a basement dated to 80 to 87 million years ago. The eastern area is covered by fracture zones and may represent an abandoned spreading centre. In the western area, volcanic breccia and layers of ash indicate widespread volcanism during the Eocene, which coincides with the formation of volcanic rocks onshore. During this period, parts of the western plateau were uplifted over sea level and short-lived volcanic islands formed.

When West Gondwana (i.e. South America) broke away from Africa during the Early Cretaceous (146 to 100 Ma), the South Atlantic opened up from its southern to its northern end. In this process, the voluminous Paraná and Etendeka continental flood basalts formed in what is now Brazil and Namibia. This event is linked to the Tristan-Gough hotspot, now located near the Mid-Atlantic Ridge, close to Tristan da Cunha and the Gough Islands. During the Maastrichtian (60 Ma), the orientation of spreading changed, which is still visible on the African side, and volcanism ended on the American side. This process resulted in the Tristan-Gough seamount chains on either side of the Tristan-Gough hotspot.

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