The eruptions of some mid-ocean volcanoes may be the echoes of supercontinent breakups that persisted for tens of millions of years after the rearrangement of Earth’s surface, a new study suggests.
The new research hints that long after continents rift apart, instabilities in the mantle created by the breakups continue to eat away at the bases of continents, peeling off crust and feeding ocean volcanoes with unusual magma.
This phenomenon could explain why these volcanoes exist and create ocean outposts like the Christmas Island Seamount, a mountain chain in the Indian Ocean. One of these mountains, Christmas Island, pokes above sea level. It’s a nature preserve famous for its lush rainforests and the annual migration of millions of crabs (Gecarcoidea natalis) — an event that coats the island in red carapaces.
The discovery is a “completely new mechanism” that also shapes the composition of the mantle, Thomas Gernon, a professor of geology at the University of Southampton in the U.K. and lead author of the new study, said in a statement.
The Christmas Island Seamount and similar undersea volcanoes have magma with odd compositions; they contain minerals that seem more like continental crust than oceanic crust. Researchers have hypothesized that perhaps these volcanoes are dredging up the remnants of oceanic crust that, long ago, was subducted into the mantle, carrying coastal sediments from the continents along with it.
Another idea is that mantle plumes — upwellings of rock from the deep mantle — are carrying ancient continental material back to the surface. But the unusual magmas are different enough that there may not be a single source that explains all of them, Gernon and his colleagues wrote in their new paper, published Nov. 11 in the journal Nature Geoscience.
Instead, Gernon and his colleagues suggest that these volcanoes may be fed by continental rocks of various ages and compositions that peel off into the mantle after cataclysmic continental breakups. They examined volcanic rocks from the Walvis Ridge, an ocean ridge that stretches away from Africa starting near northern Namibia. These rocks showed a pattern where older eruptions contained magma that was more continent-like and gradually transitioned to more ocean-rock-like compositions.
Using computer models, the researchers found that after a continental breakup, a series of roiling waves in the mantle can travel toward the interior of the shifting continent, scraping continental crust off the bottom like a peeler against potato skin. This mineral-enriched material enters the mantle within a few million years of the continental breakup and does not return to the surface for about 5 million to 15 million years, the simulations showed. The process supplies tens of millions of years’ worth of continental rock to the mantle, peaking about 50 million years after the rift of continents.
To test these ideas in the real world, the researchers next turned to the Christmas Island Seamount, again studying the ages and compositions of the volcanic rocks there. They found a pattern that matched the simulations: About 116 million years ago, 10 million years after India split from what would become Antarctica and Australia, the first volcanoes at the seamount started to erupt. The magmas were rich in continent-like minerals — a pattern that peaked within 40 million to 60 million years of the breakup. This enrichment gradually declined over time so that the magma looked more typical of oceanic rock.
The discovery points to the long-lasting impacts of a continent’s breakup, the study authors said.
“We found that the mantle is still feeling the effects of continental breakup long after the continents themselves have separated,” study co-author Sascha Brune, a geodynamicist at GFZ Potsdam in Germany, said in the statement. “The system doesn’t switch off when a new ocean basin forms — the mantle keeps moving, reorganising, and transporting enriched material far from where it originated.”
