By Mindy Weisberger, CNN
A view over Antarctica in an illustration of Earth from space. Photo: AFP / Science Photo Library
Like most of us, Earth has a lot going on under the surface - even in what may have once seemed to be its most unassuming layer.
The mantle, a zone between our planet's thin crust and the molten core, features 2900 kilometres of mostly solid rock, with a consistency like thickened caramel that scientists long hypothesized was uniformly blended. But massive unmixed regions have been found lingering in the mantle, like lumps of chocolate in a cookie, and new findings are just beginning to reveal their secrets.
Among the enigmatic mantle lumps are two enormous "supercontinents" buried thousands of kilometres below the crust amid the remains of ancient tectonic plates. One supercontinent lies under Africa, and the other resides deep under the Pacific Ocean. Using a new method to analyze data from earthquakes, researchers recently uncovered previously unknown details about these vast island regions, revealing that they may serve as anchors in our planet's mantle and that they could be much older than previously thought.
The discovery adds to a growing body of evidence suggesting that the rocky mantle isn't as well-stirred by Earth's internal churning as once believed. And hidden structures or pockets of unblended material, such as these supercontinents, may shape mantle activity, including plate movement, in ways that are yet to be understood, scientists reported 22 January in the journal Nature.
"These findings will contribute to a better understanding of mantle convection and plate tectonics, and, therefore, phenomena we experience at the surface like earthquakes and volcanism," said Claire Richardson, a doctoral candidate in the School of Earth and Space Exploration at Arizona State University, who was not involved in the new research.
"Resolving the physical, thermal, and chemical properties of rocks ~3000km below our feet, at extreme temperatures and pressures, is a challenging problem to say the least," Richardson told CNN in an email. "Open questions abound, and each new study gets us closer to understanding what's really going on down there."
Clues revealed by waves
Researchers first spotted the subterranean supercontinents about 50 years ago when they popped up as anomalies in seismic data generated by earthquakes powerful enough to send reverberations through the planet. When seismic waves encounter unusual structures in the mantle, changes in wave speed provide seismologists with clues about Earth's deep interior.
Over the decades, seismic data revealed that these supercontinents make up about 20 percent of the mantle-core boundary. Each of the buried islands covers hundreds of thousands of miles, and in some spots they tower nearly 600 miles (965 kilometres) tall. However, little was known about what they were made of, when they sank and what role they might play in mantle flow, known as convection, said D. Sujania Talavera-Soza, lead author of the new study and a geosciences and seismology researcher at Utrecht University in the Netherlands.
"Their origin and whether they are long-lived structures - it's widely debated," Talavera-Soza said.
Earth maps in the top row show the positions of two buried supercontinents — also called large low shear velocity provinces or LLSVPs — and how they affect speed and attenuation, or damping, of seismic waves. The bottom row shows the same LLSVPs in a cross-section view of Earth. Photo: Utrecht University
Earlier research focused on the velocity of seismic waves, showing that wave speed slowed by about 2 percent upon arriving at the supercontinents. This slowing of seismic waves led geologists to name the regions large low shear velocity provinces, or LLSVPs.
Velocity loss in seismic waves suggested that these mantle zones were hotter than the rocks around them, Talavera-Soza said. But it was unknown whether the LLSVPs differed structurally from nearby regions. Scientists were also uncertain whether the supercontinents were actively involved in convection, or if they were "kind of dense piles that would just be sitting there," said study coauthor Dr Arwen Deuss, a professor of structure and composition of Earth's deep interior at Utrecht University.
"There was no information about that," Deuss said. "We only knew that seismic waves slowed down."
In the new study, the authors used a different approach for studying the LLSVPs to see if they could tease out details about the zones' composition and activity. They looked at the attenuation, or intensity, of seismic signals as they traveled through the mantle to see how much energy the vibrations from quakes lost.
In music, attenuation is comparable to damping a tone, which produces a lower volume, Deuss said. Examining the attenuation of the waves - along with changes in wave speed - could provide previously unseen clues about LLSVPs' makeup. The best data for this is from waves produced by earthquakes that are magnitude 7.8 or higher, Talavera-Soza added.
Wave speed and energy loss were known to be affected by mineral grain size as well as temperature, so the authors used a physics model that linked seismology and mineral physics. Waves are damped more when they encounter material made of smaller grains; if lots of grains are packed together, there are more boundaries between the grains that can sap a wave's energy.
Older than 'slab graveyards'
Other studies revealed that the supercontinents had company in the deep mantle. Around them were "slab graveyards" of sunken tectonic plates, Deuss said. They were cooler than the LLSVPs, so seismic waves moved through them more quickly.
However, the new model showed that while seismic waves' velocity dipped when they reached LLSVPs, the waves didn't lose much energy. By comparison, there was significant damping among the cooler graveyards around the LLSVPs.
Researchers believe those differences come down to the comparative ages of the structures. Over millions of years, as rocky material descends through the boundary between the upper and lower mantle, mineral crystals are compressed and reformed into tinier grains that then regrow over time. Younger regions therefore have smaller crystals, which suck more energy from seismic waves, so the amount of damping in a region hints at how old it is.
"The fact that the LLSVPs show very little damping, means that they must consist of much larger grains than their surroundings," Talavera-Soza said. Larger mineral grains suggested that the supercontinents were significantly older than the tectonic graveyards around them, as their grains must have had more time to grow, according to the study. Larger building blocks would also make the supercontinents more rigid, keeping them separate from mantle convection, or movement of materials in that layer due to heat transfer.
"Our study points to the LLSVPs being long-lived features, at least half a billion years old, perhaps even older," Talavera-Soza said. "This implies they act as anchors at the base of the core-mantle boundary and have survived mantle convection, meaning that the mantle is not well-mixed."
This discovery follows another recent revelation about even more "sunken worlds" that contradict the notion of a blended mantle. Buried plates in tectonic graveyards tend to accumulate in alignment with Earth's subduction zones - regions where the edges of two plates meet, and where one slides beneath the other. But earlier this year, 2 another team of scientists identified sunken tectonic slabs far from these boundaries in locations under continents' interiors and beneath oceans, where sunken plates had never been found before.
"Apparently, such zones in the Earth's mantle are much more widespread than previously thought," said Thomas Schouten, lead author of that investigation and a researcher at the Geological Institute of ETH Zurich, the Swiss Federal Institute of Technology, in a statement.
The model in the new study - the first 3D attenuation model for the entire mantle - will help seismologists to better understand what lies thousands of kilometres below Earth's surface, said Richardson, the doctoral candidate.
"It maps regions of the Earth that weaken seismic energy, ultimately affecting the measurements many seismologists use to understand other physical and chemical properties of Earth's interior," she said.
The findings could transform researchers' understanding of plate tectonics and how plate movement might be shaped by these ancient, fixed anchors near Earth's core, Deuss said. Further analysis of the supercontinents could also reveal if they are the source of geochemical elements nearly as old as Earth itself that are found in lava from certain types of volcanoes, she added.
"These LLSVPs have been there for a long time - if they've been there for a billion years, they might have also been there for 4 billion years. They might well be that hidden reservoir where these chemical primordial elements might be located. We can't prove that now, but geochemists can investigate this," Deuss said.
"From this study, I think there will be a lot of extra research that might answer a lot of outstanding questions that have been confusing scientists for ages."
- CNN
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