Sixty-five million years ago, in the waning days of the dinosaurs, when India was still floating alone near Madagascar, an upwelling of hot rock from deep in the Earth’s mantle called a plume broke through the continent, depositing a 2-kilometer-thick blanket of volcanic material that can still be seen today. Then India migrated northeastward, eventually slamming into Eurasia. But the plume stayed put. And as the Indian and African plates passed over it, it spawned a chain of volcanic islands that now decorate the floor of the Indian Ocean. Today, that plume sits under Réunion, a French island located east of Madagascar.
That’s the hypothesis anyway. And Karin Sigloch is determined to test whether it’s true. She’s a geophysicist at the University of Oxford, in England, where she uses vibrations from earthquakes to image the planet’s interior. By observing the waves’ arrival at recording stations called seismometers, she is able to model, using supercomputers, how the waves slow and scatter as they travel through the Earth, revealing hidden subterranean structures.
Her career may seem an unusual choice for an engineering major. But Sigloch believes she couldn’t have found a job more thrilling. After all, she is illuminating the very rock beneath our feet—a place just as dynamic and yet less studied than outer space.
Sigloch, who grew up in Germany, wasn’t always into earth science. As a child, she was fascinated with music, particularly the nature of sound. In seventh grade, her favorite book was a beautifully illustrated tome called The Science of Musical Sound, written by John R. Pierce, who helped pioneer communications satellites at Bell Telephone Laboratories. In high school, Sigloch decided to pursue engineering, imagining she might one day design acoustics for concert halls.
She entered a joint-degree program at the University of Karlsruhe, in Germany, and the Grenoble Institute of Technology, in France, earning the equivalent of both bachelor’s and master’s degrees in electrical and computer engineering in only five years. During her final year, in 2001, she had the option to finish her studies outside Europe, and so, inspired partly by Pierce’s book, she went to Bell Labs in Murray Hill, N.J. The decision turned out to be a pivotal one.
Although AT&T’s spin-off company Lucent Technologies now owned the labs, the place retained much of its freewheeling, collegial culture. “It was incredibly creative, very heady, and very, very fun,” Sigloch remembers. Researchers regularly used their lunch breaks to solve one another’s problems and debate wild ideas. “After that experience, I thought the coolest thing would be to be a researcher,” Sigloch says. But what kind?
During the 18 months she worked at Bell Labs, Sigloch experimented with new wireless-transmission schemes. Although the work satisfied her, she didn’t think she wanted to spend her career studying cellphones. “So I asked the lunch table what I should do.” Her colleagues pushed her to consider subjects beyond pure engineering. “They said, ‘You have to find a beautiful field for yourself, something that appeals to you on a gut level,’ ” she remembers.
She took the advice to heart, considering doctoral programs in space weather and neuroscience before zeroing in on a geoscience lab at Princeton. There, she could use her knowledge of waveforms to peer into the deepest recesses of the Earth—places people had theorized about but never seen. “I just thought that would be a great privilege,” she says.
At Princeton, Sigloch perfected the computational techniques she would need to turn seismic signals into 3-D pictures of the Earth’s interior. And with help from her thesis advisor, Guust Nolet, she began developing new algorithms that could produce high-resolution images. To test them, the scientists needed data.
The set they chose came from a grid of 400 movable seismometers called the USArray, which was slowly making its way across the contiguous United States. Beginning in California in 2004, seismologists buried the basketball-size instruments in 2-meter-deep holes, where they remained for two years before being dug up and transplanted further east, ending in Maine last October. When Sigloch got hold of the data, in 2006, the array had scanned only the westernmost states. But that was enough to take a snapshot of what lay beneath.
The picture she and her colleagues produced was game-changing. For decades, geologists believed that when North America and Africa began drifting apart some 200 million years ago, the vast Pacific Ocean sat atop an enormous sheet of rock called the Farallon Plate. They assumed that as North America pushed westward, the Farallon subducted under it, sinking into the hot mantle below as mountain chains rose above. But Sigloch’s study showed that whatever lay beneath North America didn’t descend as one continuous slab. Instead, it submerged in fragmented chunks, like a series of lumpy walls.
For years, she struggled to explain what she was seeing. But eventually, she pieced together a story. The ancient Pacific, she realized, wasn’t made up of one plate but at least three. And where these plates collided, molten rock welled up, forming island chains such as in Indonesia. Each plate then subducted separately, creating the mysterious walls that Sigloch had discovered under North America. Meanwhile, as the continent moved west, it plowed into the island chains, erecting the mountain ranges that now stretch from Alaska to Mexico.
After receiving her Ph.D. from Princeton in 2008, Sigloch took a job as an assistant professor at Ludwig-Maximilians-Universität München, in Germany, where she worked until moving to Oxford last October. One day, she got an e-mail from a French colleague, Guilhem Barruol. He said he was moving to Réunion and wanted to investigate the island’s volcanic source. Sigloch told him that the German government had recently bought 80 ocean-bottom seismometers for scientific research, and she suggested they distribute them near the island in hopes of imaging the hypothesized plume below.
By 2011, they had raised enough funds from the French and German governments for 57 ocean-bottom stations and 37 new land stations on Réunion and nearby islands. That year, they set up the land stations, sailing to pristine coral atolls, some of them uninhabited nature preserves where they watched sea turtles hatch and hordes of giant hermit crabs snatch liver pâté from their sandwiches.
The next year, with help from a small crew of colleagues and students, they deployed the ocean-bottom stations. The cruise lasted five weeks. Each day, the researchers assembled the instruments in a makeshift workshop inside the ship’s helicopter hangar and hauled one or two overboard using the ship’s crane. At night, they ate cheese and sipped wine and rum punch. The team returned to Réunion to retrieve the ocean-bottom sensors in 2013, but it will take years to process all of the data.
Sigloch concedes that her work can sometimes feel tedious. And she probably could have made more money had she stuck with the cellphone industry or used her training in geophysics to get a job with an oil company. “But that wouldn’t bring the same intellectual excitement,” she says. “You can only discover something once. If we find a plume under La Réunion, then for this place on Earth, we will have settled the question of its existence once and for all.”
This article originally appeared in print as “Deep-Earth Detective.”