Some 2,000 kilometers beneath our feet, there are enormous masses of hot mantle material that have baffled scientists for the last 4 decades.
The blobs, as some scientists have taken to calling them, are the length of continents and stretch 100 times higher than Mount Everest. They sit at the bottom of Earth’s rocky mantle above the molten outer core, a place so deep that Earth’s elements are squeezed beyond recognition. The blobs are made of rock, just like the rest of the mantle, but they may be hotter and heavier and hold a key to unlocking the story of Earth’s past.
Scientists first spotted the blobs in the late 1970s. Researchers had just invented a new way to peer inside Earth: seismic tomography. When an earthquake shakes the planet, it lets out waves of energy in all directions. Scientists track those waves when they reach the surface and calculate where they came from. By looking at the travel times of waves from many earthquakes, taken from thousands of instruments around the globe, scientists can reverse engineer a picture of Earth’s interior. The process is similar to a doctor using an ultrasound device to image a fetus in the womb.
“Ultimately, a lot of people believe plate tectonics are one of the reasons why we have life on Earth,” said geophysicist Harriet Lau at Harvard University. Scientists believe these blobs play a role in many of the processes of the deep Earth, including plate tectonics and volcanism.
Once researchers began to form a picture of inner Earth, they started to see things they never imagined.
“It was very clear in those models from the get-go that at the bottom of Earth’s mantle, nearly halfway to the center, there were these huge zones where the waves traveled more slowly,” said Ed Garnero, professor of Earth and space exploration at Arizona State University.
The slow-wave velocity zones are concentrated in two locations: One lies under the Pacific Ocean, and the other sits under Africa and part of the Atlantic Ocean. They appeared like “massive mountains on the core-mantle boundary,” said seismologist Sanne Cottaar from the University of Cambridge. Other researchers describe them as conical pits of gravel sitting “all on top of each other” or like giant sand piles. The blobs are so large that if they sat on Earth’s surface, the International Space Station would need to navigate around them.
“They’re basically unmissable,” said seismologist Karin Sigloch at the University of Oxford. “They just show up on everybody’s pictures.”
There is little doubt that the blobs exist, yet scientists have no idea what they are. A recent paper said the blobs “remain enigmatic.” Scientists can’t even decide on what to call them. They go by many names, most commonly LLSVP, which stands for large low shear velocity provinces.
Part of the reason for this mystery is what Earth scientists have always struggled with: They will never be able to visit the inside of Earth. “We know less about what’s deep below our feet than the surface of the Sun or the Moon or Mars,” said University College London researcher Paula Koelemeijer. Scientists are constantly trying to come up with new ways to peek inside Earth indirectly.
Fortunately, technological advancements in sensing miniscule wobbles within Earth, as well as efforts to outfit more locations with instruments, have been propelling the field forward. Several recent studies in cutting-edge techniques are bringing new insights to the table.
Are You Dense or What?
Much of the blobs’ mystery hinges on pinpointing what they are made of. Most seismic readings cannot determine the density of the material because changes in wave velocity depend on multiple factors, such as rock composition. Not knowing the density leaves many “doors open,” said mineral physicist Dan Shim from Arizona State University.
Shim has seen the debate about the blobs’ material raging since he was a graduate student in the 1990s. “I’ve watched this whole controversy throughout my career,” he said. Researchers have argued back and forth about whether the masses are made of dense piles of chemically unique rock or bouncy lava lamp plumes that are headed for the crust above.
Researchers speculate that the blobs may feed hot spot volcanoes, which form ocean island chains like Hawaii. And other scientists wonder if the blobs could have fueled supervolcanoes in the past, potentially contributing to Earth’s biggest extinction events. But Shim said that until the density of the blobs is understood, “we cannot go to the next level of questions.”
Two recent studies, which found a way to measure density without traditional seismic methods, suggest a more complex view than before.
Earth Doing the Wave
Twice a day, Earth’s crust rises and falls with the tides. Although we’re more familiar with ocean tides, the solid Earth experiences the same forces as our oceans. As the Sun and the Moon pull on Earth, the entire planet flexes and stretches. In some places, the surface of Earth rises and falls by as much as 40 centimeters.
Scientists can track this movement using highly sensitive GPS measurements. A group of researchers led by Linguo Yuan at Academia Sinica in Taiwan analyzed measurements from GPS stations across the globe over 16 years and found that the Earth tide wasn’t what they expected: It seemed to be off-kilter just above where the blobs were located. The tides, they wrote in their 2013 paper, “provide significant information on the solid Earth’s deeper interior.”
Harriet Lau, a postdoctoral researcher at Harvard University, heard about Yuan’s work and saw an opportunity with the global data set. “It just so happens that body tides, or solid Earth tides, are very sensitive to density structure,” she said. These tides could fill in the knowledge gap that traveling waves used in seismic tomography could not.
Lau created dozens of models to explain the skewed Earth tides and compared them with Yuan’s data. She found that the models that fit the real-world data the best were those with blobs denser than the surrounding mantle. These findings, published in Nature in 2017, argued that the blobs have some sort of “compositional differences” than the rest of the mantle.
Meanwhile, another study suggested the opposite of what Lau’s study found.
Paula Koelemeijer began studying normal mode oscillations as a graduate student in 2008. “At the time, not many people were using them,” she said, despite the fact that they are a “very powerful way of thinking about the Earth.” Normal modes reveal details that more conventional seismic methods miss but are “difficult to develop an intuition for,” she said.
Many seismologists analyze waves coming out of Earth, but not all waves act alike. The images that map Earth’s interior use what’s called body waves. Similar to sound waves that travel through the atmosphere from someone’s mouth to another person’s ear across the room, these waves travel through Earth from one place to the next.
But there are certain kinds of waves that do not travel as much as they vibrate. This type of wave is called a standing wave, and it’s the type that shudders a violin string. “When you’re thinking of a standing wave, you’re looking at the whole resonating at the same time,” said Koelemeijer. “Like, it’s a bell that’s been hit and it’s vibrating as a whole.” Earthquakes trigger both types of waves, and seismometers detect them at the surface.
In Koelemeijer’s recent study, she picked a type of normal modes called Stoneley modes that vibrate depending on the density of the blobs. Her team analyzed records of ground movement in the days following large-magnitude earthquakes, looking for the low-frequency vibrations of standing waves. Comparing their results with models, they found that the blobs must be less dense than the surrounding mantle to explain several constraints, like the shape of the core.
When asked how the two studies reconcile, the researchers suggested that both papers could be correct.
“One way to perhaps reconcile Harriet Lau and my work is that this dense material is not distributed over a very large depth range,” Koelemeijer explained. Perhaps the blobs are densest in a sliver right next to the core, a detail that Koelemeijer could not rule out in her analysis. Lau echoed this suggestion. “I’m not actually worried at all about this seeming contradiction,” she said. The results simply help them “fine-tune” their conclusions, she said.
When seismologist Ed Garnero’s wife was pregnant with twins in 2002, he remembers going to the doctor for an ultrasound. Despite the new 3-D imaging technology, he said the low-resolution images on the screen were off-putting. “It looked like the brains were floating off to the side. It was really weird,” he said.
In seismic tomography, researchers deal with similar problems. The blobs received their nickname partly because of their soft, lump-like shape in seismic tomography maps. But what if their structure was actually more delicate? And could knowing the shape of the blobs better help researchers constrain their density?
Last December, doctoral student Maria Tsekhmistrenko from the University of Oxford presented some of the most revealing images of the structures to date. At a session at AGU’s Fall Meeting, Tsekhmistrenko showed her seismic tomography maps of about half of the blob under Africa. The images come from an extensive seismometer project that deployed sensors on the ocean floor around Madagascar.
Using a collection of different types of waves, Tsekhmistrenko revealed the jagged and angled sides of the blob and its plumes above it, showing very little of the softness suggested by earlier tomography maps. Taken together, the whole structure looks like a tree that branches up to hot spot volcanoes at the surface, said Tsekhmistrenko’s adviser, Karin Sigloch.
At first, Tsekhmistrenko said that they didn’t believe what they saw. “We worried that something was wrong with my data,” she said. Then she realized they were correct, even though “it looks different than expected.”
“Very 3-D,” she added.
Garnero, who saw the presentation, said that it was “the best Earth interior imaging presentation I’ve seen at AGU.” He added that scientists who study the movement of the inner Earth, called geodynamicists, may be excited to get their hands on Tsekhmistrenko’s images.
“The slope of that structure turns out to be hugely important in constraining its density,” he said. “That’s really important for dynamicists.” Tsekhmistrenko has already heard from one geodynamicist planning to simulate the structures in a future model.
Despite critical advances in seismology, the quest to understand the blobs is “an inherently interdisciplinary problem,” said geologist Ved Lekic of the University of Maryland.
Mineral physicists, for example, measure how waves travel through rocks under extraordinary pressures to improve seismology models. Geochemists scour Earth to collect rocks from volcanoes, looking for clues of unique chemical reservoirs that could be linked to the blobs. And modelers construct intricate webs of code to evolve the mantle over billions of years, simulating how the blobs came to form.
Whatever the answer may be, peering under Earth’s crust may give researchers a way to contemplate our earliest beginnings. “These questions are very romantic in some ways,” said Harriet Lau. “I’m so inspired by questions that go to the root of existence and the universe.”
Earth is the only planet known to contain plate tectonics, and recent research has suggested that tectonics may help sustain life by delivering a steady stream of nutrients, like nitrogen and phosphorus, to the surface. And yet researchers aren’t sure what causes plate tectonic movement, let alone the blobs.
“I think that their real fundamental and philosophical appeal is their mystery,” said Lekic. “They’re among the largest things inside the Earth, and yet we literally don’t know what they are, where they came from, how long they’ve been around, or what they do.”
Ultimately, the road to uncovering the mysteries may be long, said Garnero. “This is a slow-motion discovery, it’s a community thing,” said Garnero, who has worked on the blobs for the last 15 years.
• Evaluating the Resolution of Deep Mantle Plumes in Teleseismic Traveltime Tomography
• Fat Plumes May Reflect the Complex Rheology of the Lower Mantle
• Strongly Anisotropic Magnesiowüstite in Earth’s Lower Mantle
Lau, who plans to study the blobs as she begins her professorship at the University of California, Berkeley, later this year, said she isn’t fazed by the mystery. “I think science is incremental, and that’s why, for example, Paula Koelemeijer’s results didn’t particularly faze me,” she said. “I was actually more excited rather than anything else.”
—Jenessa Duncombe (@jrdscience), News Writing and Production Intern
Duncombe, J. (2019), The unsolved mystery of the Earth blobs, Eos, 100, https://doi.org/10.1029/2019EO117193. Published on 27 February 2019.
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