Andrews opined in a long blog in mid-September, 2021 that widespread volcanic magmas may be on the rise. Réunion, a French island in the western Indian Ocean, is like a marshmallow hovering above the business end of a blowtorch. It sits above one of Earth’s mantle plumes — a tower of superheated rock that ascends from the deep mantle and flambés the bases of tectonic plates, the jigsaw pieces that make up the ever-changing face of the world. The plume’s effects are hard to miss: One of the island’s two massive volcanoes, the aptly named Piton de la Fournaise or “peak of the furnace,” is one of the most hyperactive volcanoes on the planet.
But the plume’s modern-day punch so far is nothing compared to its past. Around 65 million years ago, when the plume was under what is now India, a series of lava floods named the Deccan Traps smothered 1.5 million square kilometers of land — enough to bury Texas, California and Montana — in a mere 700,000 years, a geologic heartbeat. A giant asteroid strike would be the coup de grâce for the dinosaurs, but the Deccan Traps have long muddled the picture of the climatic conditions the dinosaurs had to contend with. Rivers of intensely hot material flow from Earth’s core and mantle up to its surface, dictating where volcanoes appear, where land is born, and where tectonic plates collide.
In 2012, a team of geophysicists and seismologists set out to map the plume, deploying a giant network of seismometers across the vast depths of the Indian Ocean seafloor. Nearly a decade later, the team has revealed that the mantle is stranger than expected. The team reported in June in Nature Geoscience that the plume isn’t a simple column. Instead, a titanic mantle plume “tree” rises from the fringes of the planet’s molten heart, with superheated branchlike structures appearing to grow diagonally out of it. As these branches approach the crust, they seem to sprout smaller, vertically rising branches — super hot plumes that underlie known volcanic hot spots at the surface.
The discovery of this massive structure beneath Réunion nearly coincides with another recent discovery, reported in November, that found additional structures in the plumes under Africa. Taken together, the two findings represent a significant scientific advance: They suggest that plumes can be more idiosyncratic, and can have more elaborate backstories, than traditional models presumed.
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The Cataclysms to Come
If the team’s theoretical model is correct, it bolsters two long-held trains of thought. The first, said Goes, is that Earth’s plumes are “not as simple as just making an upwelling in a box of syrup in a laboratory.” Nature is complex, and in oft-surprising ways. The second is that these giant blobs have played, and will continue to play, a pivotal role in the planet’s tumultuous history.
Some geoscientists suspect that plumes from the African giant blob spent at least 120 million years tearing the ancient supercontinent of Gondwana into shards. As the plumes rose into its base, they heated it and weakened it; like moles making hills, they caused the land atop these plumes to dome upward, then slide downhill. Australia was unzipped from India and Antarctica, Madagascar from Africa, and the Seychelles microcontinent from India — an act of destruction that made the Indian Ocean.
Should the plume or plumes beneath East Africa sustain their onslaught, they will contribute to the future disintegration of the African continent: specifically, the breakup of East Africa and the creation of a new microcontinent floating beside the world’s youngest ocean. But that future tectonic divorce seems insignificant when you consider the catastrophe that may befall the continent’s southern tip. The team estimates that, in tens of millions of years, a blob of nightmarishly gargantuan proportions will pinch off from the central cusp and rise to meet what is now South Africa’s foundations. This, said Sigloch, would produce cataclysmic eruptions. The Deccan Traps were caused by what we would think of as a solitary mantle plume. This future mega-blob, though, would be capable of producing volcanism so prolific and extensive that the Deccan Traps would be a firecracker in comparison.
Envisioning future volcanic apocalypses may be disquieting. But that is precisely why painting precise pictures of plumes matters: they are arbiters of life and death. And yet, for all the chaos they cause, they are a key part of the unceasing cycle of plate tectonics, one that erratically buries and erupts carbon and water and has, miraculously, resulted in a habitable planet with a breathable atmosphere and expansive oceans — a paradise made by abyssal behemoths. “Knowing how a planet manages to do this for billions of years to basically allow human existence is important,” said Rychert.
It will be some time yet before the mantle’s monsters are thoroughly understood. Until that day arrives, geoscientists will keep sketching out the shape-shifting mantle, all the while listening to the many beasts stirring far below their feet.
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