The long-lost submerged tropical island of Rio Grande Rise is located off the coast of Brazil

The discovery of sunken islands reveals hidden chapters in our planet's geological history, providing unique insights into the land-ocean dynamics that shaped the world as we know it today. Among these discoveries, the Rio Grande Rise, an underwater volcanic plateau located approximately 1,200 km off the coast of Brazil, stands out for its history and exceptional economic potential. Researchers associated with the University of São Paulo in Brazil and the University of Southampton in the United Kingdom conducted an extensive study, published in the journal Scientific Reports, revealing that the structure was once a tropical island rich in biodiversity and minerals, but is now submerged. This discovery not only enriches our understanding of geological processes, but also highlights the importance of resources hidden beneath the seabed, opening new avenues for scientific exploration and sustainable exploitation of mineral resources.

A lost island appears

In 2018, an international collaboration between scientists from the University of São Paulo and the University of Southampton led to an extraordinary expedition along the coast of Brazil. Specifically, this mission aimed to explore the Rio Grande uplands. This sunken island is located approximately 1,200 kilometers from the mainland. At the beginning of their research, the researchers discovered layers of red clay 650 meters below the surface of the ocean. Bramley Murton, of the National Oceanographic Center in Southampton, who is participating in the expedition, explains in a press release: “Red clay is simply not found on the seabed. The deposits resembled tropical soil.”

The existence of these red clays is therefore significant. It indicates a process of change that can only happen outdoors, under the direct influence of tropical climatic conditions. This includes high heat and humidity. This outdoor weathering leads to the formation of kaolinite, the clay mineral that dominates these red layers.

Bathymetric map of the seafloor along the southern Brazilian continental margin, including the rise of the Rio Grande. © Ana Alberoni

The presence of this clay on the ocean floor therefore strongly suggests that the Rio Grande Rise was once a tropical island. It surfaced and was exposed to the atmosphere, thus enabling the development of tropical soil before its submergence. This discovery calls into question the understanding of the geological history of the region. It provides a new perspective on the tectonic and climatic processes that influence the formation and development of underwater landscapes.

Geological history of the Rio Grande upland revealed by a sunken island

About 80 million years ago, the Rio Grande Rise Plateau emerged from the depths of the South Atlantic Ocean. Indeed, intense volcanic activity shaped it. The mantle cloud fueled this activity. It is an ascending column of molten rock that originates deep in the Earth's mantle. It broke through the oceanic crust to create new geological formations.

During this period, a large island was created, which in terms of dimensions and geological features can be compared to today's Iceland. Murton adds: “It started as a Cretaceous version of Iceland.” This island is a direct result of the accumulation of lava and volcanic materials ejected during eruptions. Over time, under the influence of tectonic processes and erosion, this landmass slowly moves. It began its inexorable descent below the surface of the ocean, carrying with it evidence of its fleeting existence.

High-resolution images captured by the HyBIS ROV. Different red and brown clays are visible between the lava flows. © P. Srivastava et al., 2024

About 40 million years after its initial formation, the Rio Grande Rise experienced a new phase of volcanic activity. This late phase was marked by less intense eruptions, but powerful enough to create additional rock formations. They were enriched with precious minerals.

Exploration and economic implications of sunken island

The first expedition, in 2018 on the Brazilian research vessel Alpha Crucis, used sonar to map the underwater terrain. Ferromanganese-rich crusts on the sea floor needed to be characterized. Mapping revealed a 30-kilometer canyon of steep cliffs, the Cruzeiro do Sul Rift, as well as fossilized beach terraces, wave-eroded platforms and submerged waterfalls. Eight months later, the team set sail on the ROV-equipped RRS Discovery. He photographed the rocks along the canyon walls and collected samples using a robotic arm.

By studying the samples, the researchers identified certain minerals formed by the crystallization of magma. They include elements such as cobalt, nickel and lithium, which are in high demand for their industrial applications. The discovery of these mineral resources on the Rio Grande Rise opens promising economic prospects. But there are challenges involved in terms of sustainability and preservation of marine ecosystems.

In December 2018, Brazil asked the UN to expand its maritime borders to include this region. It is located far outside Brazil's exclusive economic zone. Brazil must show geological similarities to the continent to justify this expansion. To succeed, Brazil must also demonstrate its ability to sustainably exploit this area. A challenge in a context where legislation on underwater mining in international waters has yet to be defined.

Source: Srivastava, P., J. Murton, B., Sant'Anna, LG et al., “Red clays indicate sub-aerial exposure of the Rio Grande Rise during the Eocene Volcanic Episode”. Sci Rep 13, 19092 (2023)

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