A section of the Rio Grande Rise, a vast volcanic plateau hidden beneath the South Atlantic, was once a tropical island millions of years ago, researchers have found. The discovery came from an unexpected source and is offering new clues about the area’s past.
The Rio Grande Rise sits about 750 miles off the coast of Brazil. Scientists have known about the underwater formation for decades, but recent research has revealed that part of it spent a period of its history above sea level before slowly sinking beneath the ocean.
That sunken island is helping researchers piece together the geological history of the South Atlantic. It is also drawing attention to a region known for deposits of rare earth elements and other metals widely used in modern technology.
The Island That Shouldn’t Have Been There
The story began in 2018 when scientists from Brazil and the United Kingdom examined rocks collected from the western part of the Rio Grande Rise. Among the layers of volcanic lava, they found something unexpected: red clay.
The discovery immediately stood out. As reported by Eos, marine geologist Bramley Murton of the National Oceanographic Centre in Southampton said the deposits looked more like tropical soils than material normally found on the seafloor.
“You just don’t find red clay on the seabed,” he noted.
The unusual samples prompted years of additional research. Scientists eventually concluded that the clay formed when the area wasexposed to the atmosphere rather than submerged beneath the ocean.
A study published in Scientific Reports in 2023 found that this portion of the Rio Grande Rise likely stood above sea level between 44 and 47 million years ago during the Eocene epoch. At the time, it would have resembled a volcanic island in the South Atlantic.
Evidence Of A Long-lost Tropical Landscape
The red clay turned out to be one of the strongest pieces of evidence supporting that conclusion. Researchers identified minerals including kaolinite, hematite, and goethite, which are typically associated with intense weathering in warm and humid climates.
Researchers told Eos that the deposits closely resemble Brazil’s red tropical soils. Study co-author Luigi Jovane, a marine geologist at the University of São Paulo, explained that:
“these red clays are exactly the same, chemically and mineralogically, as the red earth orterra roxa we find all over Brazil.” She added, “We are confident that they represent the in situ, weathered upper surfaces of the lavas.”
Another key clue came from the clay’s Chemical Index of Alteration (CIA)value of 93. Geologists use this scale to measure how extensively rocks have been altered by weathering processes. Such a high value indicates prolonged exposure to tropical conditions after volcanic activity had ceased.
In comments cited by Scientific Reports, researcher Priyeshu Srivastava said the red clays could only have formed during a period when the Rio Grande Rise was exposed above sea level. The deposits therefore point to a phase when the island existed before gradually sinking beneath the ocean.
“The Rio Grande Rise and the continent have the same soil and climate,” pointed out Jovane. “In that sense there is a direct relationship between the two.”
A Geological Find Worth Watching
The Rio Grande Rise is attracting attention for more than its geological history. The plateau contains ferromanganese crusts enriched with metals such as cobalt, nickel, and lithium, materials commonly used in energy storage technologies and renewable energy infrastructure. Scientists have also identified significant concentrations of rare earth elements, including yttrium.
As detailed in the source material, yttrium is used in aluminum and magnesium alloys as well as superconductors, lasers, LED lighting, and camera-lens glass. These applications have increased interest in regions containing substantial rare earth resources.
For researchers, the most remarkable aspect of the discovery is what it reveals about the Rio Grande Rise itself. Millions of years after disappearing beneath the Atlantic, the former island has left behind a geological record.
