But seismology is not omniscient. Seismic waves can detect structures within the mantle, but they cannot reveal every characteristic of those structures. “You can slow down a seismic wave by heating a material up,” said Harriet Lau, a geophysicist at the University of California, Berkeley. But a change in the rock’s mineral makeup can achieve the same effect. Scientists are forced to choose which option is more likely in each measurement they do. Seismology may be a hard science, but there is an art to it.
Subcrustal structures are also equipped with camouflage. Seismic waves like to take the fast lane: They preferentially channel into colder, rigid rock. Plumes, being hot, are repulsive to seismic waves. Plumes are also thin, allowing most incoming seismic waves to dodge them with ease.
The more seismic waves you have crossing through the same point in the plume, the more confident you can be that it exists. But “earthquakes don’t happen everywhere,” said Catherine Rychert, a geophysicist at the University of Southampton. And seismic stations are mostly on land, not on the seafloor, which means oceans have poor seismic coverage.
“Theoretically, we know [plumes] have to exist,” said Lau. “But they’re just so hard to see seismically.” Consequently, seismic waves capture only slices of plumes, and their properties are often the subject of unresolvable debate.
Ideally, scientists want to produce a plume image that stretches from its base to the planet’s surface. That would require a cornucopia of seismometers spread over a vast area, forming a huge aperture that could eat up as many seismic waves as possible and thereby see a sizable segment of the mantle — a seismic equivalent of a giant telescope.
So, in 2012, scientists built one.
The Tree and the Truth
That year, two vessels zigzagged across the western Indian Ocean, occasionally stopping to make a submarine seismometer walk the plank and sink to the seafloor. In total, 57 were thrown overboard, ultimately creating a 2,000-by-2,000-kilometer aperture. This vast array was augmented by 37 seismic stations positioned on Madagascar and various smaller islands.
For 13 months between 2012 and 2013, that aperture was open. Its objective: to hunt down the Réunion plume, one of the most consequential fountains of fire to grace the planet in the past 100 million years.
A team of scientists peered through their mantle telescope. They combined their data with two other seismic data sets, and they were shocked when they saw the thin, vertical plume beneath Réunion simply vanish in the lower mantle. At that moment, Maria Tsekhmistrenko, then a student of Sigloch’s at the University of Oxford, recalls thinking, “Oh, I must have done something terribly wrong. Everything is wrong. My Ph.D. is over.”
But as the team looked at the entire region, the data began to reveal a spectacular sight. The African giant blob, 2,900 kilometers below the surface, grows up from its middle to form a “trunk,” reaching a depth of 1,500 kilometers. The top of the trunk, dubbed the cusp, appears to grow thick branches of hot matter from its western and eastern extremities. These grow diagonally upward until they reach a depth of 1,000 to 800 kilometers; at this point, the tops of these branches sprout vertically rising thin branches.
One of these thin branches reaches the underside of hyper-volcanic Réunion. Around 3,000 kilometers northwest, another diagonal branch stretches to East Africa, a region awash with volcanism and which prior seismic work has found to be home to one or perhaps two mantle plumes.
But there was a problem: this structure was difficult to reconcile with the laws of thermodynamics.
Plumes, being so hot and buoyant, rise quickly — at 10 times the speed of other mantle migrations, including the movement of plates. “The plumes are so quick. You don’t have time to tilt them” as they ascend, said Goes.
Tsekhmistrenko, Sigloch and company agree: Plumes rise straight up. The tree structure, then, is evidence for a more complex process going on in the mantle.
Here’s how they think it works: The African blob — including the trunk and cusp — gets heated by the core. The eastern and western peripheries of the hot cusp, surrounded by a large proportion of relatively cooler ambient mantle material, are considerably buoyant. Eventually, an 800-kilometer blob pinches off from each end; both rise vertically for tens of millions of years. Eventually, they reach the shallow boundary between the dense lower mantle and the less dense upper mantle. There, they spread out laterally. Several tails sprout off the top of them and rise vertically, forming those narrow towers classically referred to as plumes.
Meanwhile, as one of these two sub-blobs rises toward East Africa and one rises toward Réunion, the eastern and western extremities of the cusp — now closer to its middle — produce two new blobs, which also rise straight up. Since they leave later and are positioned to the lower right and lower left of the East African and Réunion blobs respectively, they resemble diagonal, interconnected branches. In reality, they are separate blobs, all rising vertically.
Independent scientists have largely applauded the research. Classically, the problem with imaging plume structures in high resolution is a lack of seismic data. Not so this time, said Rychert, “because they had this amazing experiment in the Indian Ocean,” one that gorged itself on a smorgasbord of seismic waves.
Combining the data from the giant array with additional seismic data sets proved instrumental, as it allowed the team to precisely resolve an entire swath of the mantle, from its greatest depths to its highest reaches. “In terms of the seismology, it is a step forward,” said Carolina Lithgow-Bertelloni, a geophysicist at the University of California, Los Angeles. “In that sense, I think it’s great.”
The tree structure is “an intriguing observation,” said Fitton, and the team’s model of how it branches up from the core is “quite a clever idea.” But he cautions that their precise model for what’s going on in the mantle is just one of several possible interpretations of what is happening. “I think that’s a really cool idea,” said Rychert. “I don’t know if it’s the right idea, but it’s cool.”
“Seismic tomography is a snapshot of today,” said Lithgow-Bertelloni. Taking snapshots of present-day structures and speculating on how they formed over millions of years, and how they will continue to evolve, is rife with uncertainty, she cautions.
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 scientists 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, scientists will keep sketching out the shape-shifting mantle, all the while listening to the many beasts stirring far below their feet.
