Where does continental material on islands come from?

(Press-News.org) Many oceanic islands far from active plate tectonic boundaries contain materials that clearly originate from continents, even though they are located in the middle of an oceanic plate. Where do the continental remnants come from? Are they sediments that are recycled when oceanic plates subduct into the mantle? Or do they originate from the depths of the Earth's mantle and are carried upward by hot currents, known as mantle plumes? Both explanations are being discussed, but they fall short. This is because some volcanic regions show little evidence of crustal recycling, while others are too cool to be driven by mantle plumes.

Researchers at the University of Southampton and the GFZ Helmholtz Centre for Geosciences have now proposed a new explanation. Geochemical analyses and modelling have revealed the following picture: When continents break apart, a wave of instability is created at a depth of more than a hundred kilometres. This “mantle wave” scrapes material from the underside of the continents along the base, which is then transported sideways into the Earth's mantle beneath the oceans.

There, these remnants of continental roots feed volcanic eruptions in the ocean crust over millions of years. Sometimes the material travels more than a thousand kilometres from the continental interiors before it forms oceanic islands.

Professor Sascha Brune from GFZ in Potsdam and co-author of the study, says: “We have found that the mantle continues to ‘feel’ the effects of continental rifting long after the continents have separated. The system does not shut down when a new ocean basin forms—the mantle continues to move, reorganize, and transport enriched material far away from its place of origin.”

The team analyzed geochemical data from various regions of the Earth, including the Seamount Province in the Indian Ocean, a chain of volcanic formations that formed after the breakup of the supercontinent Gondwana over 100 million years ago.

Through simulations and chemical analyses, the team discovered that shortly after Gondwana broke apart, unusually enriched material with a continental fingerprint appeared beneath the new-born ocean and generated melts that can now be found at oceanic islands and submerged seamounts. Over millions of years, this chemical signal faded as the flow of material from the interior of the continent slowed.

This happened without there having been a mantle plume. Thomas Gernon, Professor of Earth Science at the University of Southampton and lead author of the study, says: "We are not ruling out mantle plumes, but our discovery points to a completely new mechanism that also influences the composition of the Earth's mantle. Mantle waves can transport continental material far into the oceanic mantle, leaving behind a chemical signature that persists long after the continents have broken apart."

The study builds on the team's recent discovery that “mantle waves” can also cause dramatic changes deep within continents. Their previous work showed that these slow, rolling motions in the Earth's mantle can help trigger diamond eruptions and even reshape landscapes thousands of kilometres away from the edges of tectonic plates.

The study, published in Nature Geoscience, was led by the University of Southampton and conducted in collaboration with the GFZ Helmholtz Centre for Geosciences in Potsdam, the University of Potsdam, Queen's University Canada, and Swansea University.

Original study: Thomas Gernon et al.: Enriched mantle generated through persistent convective erosion of continental roots; in: Nature Geoscience https://doi.org/10.1038/s41561-025-01843-9

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