A Huge Subterranean ‘Tree’ Is Moving Magma to Earth’s Surface

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.

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Maria Tsekhmistrenko (right) and other technicians during the month-long cruise to deploy 57 seismometers on the Indian Ocean seafloor in October 2012. The seismometers were retrieved a year later.Courtesy Guilhem Barroul

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.”

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