Mitch Battros / Earth Changes Media
A team of geoscientists says they have solved a long-standing mystery about oceanic volcanism and plate tectonics, explaining why some islands contain so much continental material despite being so far from the continental plates.
Earth’s continents are slowly being “ripped out by the base“, concludes a new report, led by researchers at the University of Southampton and published earlier this month in the journal Nature Geoscience.
According to simulations and chemical analyzes carried out during the course of the study, these intriguing mechanisms occur because the continents are “peeled” by the restless tectonic forces of our planet, through slow, undulating “non-mantle waves”.
As the study authors suggest, when continental plates fracture and move apart, the upper mantle — hot and incredibly slow-flowing — it rips out material from their roots. This scraped material is then transported far away, enriching the ocean mantle and fueling volcanism for geological eons.
“We’ve known for decades that parts of the mantle beneath the oceans appear strangely contaminated, as if pieces of ancient continents had somehow ended up there,” he explains. Thomas Gernongeoscientist at the University of Southampton, in the United Kingdom, and lead author of the study, cited by .
The scientists they had already tried to explain this phenomenon in several ways. One hypothesis was that the oceanic mantle would have been “contaminated” by sediments recycled as the crust dives into the mantle, in a process known as subduction.
Another possibility sometimes considered was that columns of hot rock, the so-called mantle feathersbrought with them enriched material as they ascended from the depths of the Earth towards the surface.
These processes can make some contribution, but they don’t tell the whole storyespecially because some enriched areas show few signs of recycling of crust or hot plumes. Furthermore, the enrichment of the oceanic mantle is very varied and appears to have origin in a mosaic of rocks with different ages.
The new “mantle waves” theory that rip out the crust helps explain the enrichment process: when a continent fragments, it triggers a chain of instabilities — waves in the mantle — that sweep the base of the continents to depths between 150 and 200 kilometers.
Gernon et al. / Nature Geoscience
Schematic showing how the enriched mantle accumulates over billions of years. Diamond symbols show where diamonds are likely to accumulate
They are varrimento movement “unsticks” the continents from the bottomin its roots, and can transport continental material for more than 1000 kilometers to the oceanic mantle, fueling volcanic eruptions that can persist for tens of millions of years.
It’s a very slow sweep, which unfolds in geological times. To say that this happens at a snail’s pace is an understatement — these continental fragments are dragged beneath the oceans at a speed a million times slower than a snail’s.
These long time scales cause continents to leave their chemical “fingerprint” long after they separated.
“We discovered that the mantle continues to feel the effects of continental fragmentation long after the continents themselves have moved away from each other,” he says. Sascha Brunegeodynamicist at the University of Potsdam and co-author of the study.
“The system does not turn off when a new ocean basin forms — the mantle continues to move, reorganize and transport enriched material far from the place where it was originally formed”, adds the researcher.
A chain of underwater volcanoes and seamounts in the Indian Ocean provides a additional line of evidence. Once located off Northeast Australia, this chain includes Christmas Island and was formed more than 150 million years ago, when the supercontinente Gondwana disintegrated.
This region does not show strong signs of mantle plumes. Instead, it reveals a pattern of enriched volcanism that occurred in the 50 million years following continental breakup. This enrichment slowly decreased over time, in line with the predictions of the model developed by the researchers.
In addition to helping to solve the mysteries of the presence of continental material in the middle of the ocean and unexpected volcanism far from the limits of tectonic plates, the study also unearthed other secrets of geoscience.
Scientists have concluded that slow, rippling waves in the mantle may also cause eruptions of diamond-rich magmas from great depths within the Earth.
And, finally, they showed that these same waves in the mantle can cause the continental upliftcausing apparently stable parts of the continents to rise more than a kilometer, giving rise to some of the “largest topographic structures on the planet”.
