Southampton, UK—Earth scientists have uncovered a hidden and slow-moving geological process where the lower layers of continents are gradually stripped away and swept deep into the oceanic mantle. This newly identified mechanism helps explain a long-standing puzzle: the presence of continental-like chemical signatures in volcanic islands located far from tectonic plate boundaries.
The study, led by the University of Southampton and published in Nature Geoscience, proposes that continents are not only pulled apart at the surface during rifting events but also “peeled” from below over much greater distances than previously imagined.
The Mystery of the “Contaminated” Mantle
For decades, scientists have been puzzled by volcanic islands, such as Christmas Island in the northeast Indian Ocean, which exhibit unusually high levels of “enriched” chemical elements typically associated with continental crust. These elements suggest that ancient, recycled material is being blended into the mantle, but the exact source was unclear.
Until now, the leading explanations involved sediments sinking with oceanic plates, or mantle plumes upwelling columns of hot rock rising from deep within the Earth. However, these explanations fell short in regions that were too cool or lacked evidence of crustal recycling.
“We’ve known for decades that parts of the mantle beneath the oceans look strangely contaminated, as if pieces of ancient continents somehow ended up in there,” said Professor Thomas Gernon, lead author of the study and Professor of Earth Science at the University of Southampton. “But we haven’t been able to adequately explain how all that continental material got there.”
The Mechanism: Mantle Waves and Convective Erosion
The new study proposes a novel mechanism involving the movement of “mantle waves” slow-moving instabilities that sweep along the base of continents at depths of 150 to 200 kilometers.
The research team used simulations to model the behavior of continents and the underlying mantle as they are stretched by tectonic forces, such as during continental breakup. They found that:
• Mantle Instability: The stretching triggers a wave of instability that sweeps along the deep roots of the continent.
• Peeling Action: This sweeping motion, moving at an incredibly slow pace a millionth the speed of a snail gradually strips material from the lowermost continental crust and mantle.
• Sideways Transport: These “peeled” fragments are then swept sideways into the oceanic mantle, sometimes travelling over 1,000 kilometers away from their continent of origin.
• Fueling Volcanism: This continental material then feeds volcanic eruptions in the ocean, driving activity for tens of millions of years after the continents have physically separated.
“We found that the mantle is still feeling the effects of continental breakup long after the continents themselves have separated,” explained study co-author Professor Sascha Brune of GFZ in Potsdam.
Evidence from the Indian Ocean
The team used their simulations to analyze geochemical data from the Indian Ocean Seamount Province, a chain of volcanic features formed after the supercontinent Gondwana broke apart more than 100 million years ago.
Their analysis showed that soon after the breakup, a burst of enriched magma rose to the surface. This chemical signal gradually faded over tens of millions of years, consistent with the slow flow of stripped material from beneath the continent waning a pattern that occurred without the need for a deep mantle plume.
“We’re not ruling out mantle plumes, but this discovery points to a completely new mechanism that also shapes the composition of the Earth’s mantle,” Professor Gernon concluded.
The findings build on the team’s earlier work, which showed that these slow, rolling movements in the Earth’s mantle can also trigger dramatic changes deep within continents, including driving diamond eruptions and reshaping landscapes thousands of kilometers from plate edges.