Christmas Island lies around 217 miles (about 350 kilometers) from Indonesia, while mainland Australia is nearly five times as distant. But if you dig into the rocky base of the island, you’ll find materials traceable to large continents.
How these substances ended up in the middle of the ocean has perplexed geoscientists for decades. New research, led by a team at the University of Southampton and published in Nature Geoscience, may now provide an answer.
The work suggests seismic activity scrapes away rock from beneath large continents, like shallow reefs scratching the paint off the underside of a boat. This rock then flows into the oceanic mantle, a layer of hot, semi-solid rock beneath the ocean floor. Subsequent volcanic activity over millions of years, say the researchers, explains how this continental rock ends up in places like Christmas Island, which is the above-water peak of a large and dormant undersea volcano.
This proposed mechanism, say the researchers, could build on the long-standing puzzle of how our Earth is carved into shape by flowing volcanic activity.
Read More: Global Thaw 10,000 Years Ago May Have Fueled Volcanoes and Sped Up Continental Drift
How Did Continental Rock End Up In Ocean Volcanoes?
“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 study lead author Thomas Gernon, an earth scientist at the University of Southampton, in a press release. “But we haven’t been able to adequately explain how all that continental material got there.”
In earlier research, the team had shown that separating continents releases powerful ripples of energy at their bases. This “mantle wave” travels along the underside of the continents, up to about 124 miles (200 kilometers) below the surface, stripping away rock as it moves.
To understand the process in more detail, the team created computer simulations of continent and mantle movement in response to shifting tectonic energies. The simulations showed that the stripped rock could travel more than about 621 miles (1,000 kilometers) into the oceanic mantle.
“We found that the mantle is still feeling the effects of continental breakup long after the continents themselves have separated,” said co-author Sascha Brune, researcher at the GFZ Helmholtz Centre for Geosciences, in a press release.
An Ancient Supercontinent Gave Rise to Christmas Island
The team also analyzed geochemical data from around the Earth, including from Christmas Island, which formed over 100 million years ago after the shattering of the supercontinent Gondwana.
The combination of simulation and chemical data showed that after Gondwana broke apart, magma enriched with continental elements bubbled to the surface.
These enriched deposits had previously been thought to be leftovers from rock recycled after continents plunged into the mantle or to have spouted from hot rock columns fired up from deep within Earth called mantle plumes.
But there was little sign of recycling or mantle plumes in the formation of Christmas Island, suggesting that the phenomenon of mantle “waves” was instead responsible.
“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,” said Gernon. “Mantle waves can carry blobs of continental material far into the oceanic mantle, leaving behind a chemical signature that endures long after the continents have broken apart.”
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