Geodynamics from the Top Down

Photo by Jessica ScrantonPhoto by Jessica Scranton
By Jonathan Shaw

Plate tectonics is a scientific theory that relates the movement of thin, rigid plates on Earth’s surface to convection currents in its hot interior. Chains of volcanoes and earthquakes delineate the plate boundaries. Beneath the plates and their boundary zones, viscous rock flows at depths of hundreds of kilometers in the mantle. Sometimes, at mid-ocean ridges and mid-plate volcanoes, this hot viscous mantle reaches the surface as magma and it solidifies, creating new crust. At other times, it flows horizontally beneath the base of the plates before eventually cooling and being incorporated into them, finally descending to create a great circle—
a conveyor belt in Earth’s mantle that can build mountains and move continents. At least, that is how geophysicists have traditionally thought about plate tectonics.

But Leigh Royden ’76, RI ’10, the William and Flora Hewlett Foundation Fellow at the Radcliffe Institute and a professor of geology and geophysics at the Massachusetts Institute of Technology, takes a different perspective. She is trying to understand the large-scale motions of Earth’s lithosphere—the crust and uppermost mantle—not from the inside out, as most geodynamicists do, but from the surface down. An increasing body of knowledge—to which she is an important contributor—suggests that stresses at the surface and characteristics of the plates themselves, rather than viscous mantle flow at depth, play a primary role in organizing the global tectonic system.

Royden got into geophysics almost by accident. In 1975, while still an undergraduate at Harvard and Radcliffe concentrating in physics, she was the United States women’s national rowing champion in single sculls and a silver medalist as part of the women’s eight at the World Championships. Plate tectonics was not much on her mind. She had taken “a couple of classes in geology,” she recalls, but it wasn’t until the following year, while training for the 1976 Olympic Games in Montreal, that she learned about the field of geophysics from a fellow rower who was dating a graduate student
 in the field at MIT. “Wow, physics and geology together,” she thought. “I never knew that existed.” An introduction to the young man’s advisor—MIT professor of geophysics John Sclater—led to the offer of a summer job at Woods Hole. But Royden turned the opportunity down in favor of the chance to row in Montreal.

Then, one month before the team selection, she tore multiple ligaments in her leg in a running accident, effectively ending her Olympic aspirations. “Wiki,” as she was known, suddenly found herself on an entirely different path. She took the summer job, discovered the joys of field geology and mapping, and, with encouragement from Sclater, went on to earn her PhD from MIT in 1982. When he left the following year for a position at the University of Texas at Austin, MIT offered her a job.

Unlike many of her colleagues, who study geodynamics on a global scale, Royden has focused her career on regional-scale understanding of how topography is built, asking (because these are very messy, complicated systems), “How can I describe 90 percent of what I see by one or two processes going on in the crust or the mantle—and how can I quantify that?” For example, she has studied small, local subduction zones that move around very quickly on a geological time scale. These zones, where the lithosphere is being subsumed into the underlying, viscous asthenosphere, move by trench “rollback,” she explains. Imagine a sheet of paper suspended horizontally by its four corners. Grab a finger’s width starting at one of the edges and pull down. A strip tears across the middle of the sheet. This is what happens during trench rollback, with the difference being that the force ripping the strip at its edges is provided not by fingers but by the slab’s own weight. The dense, descending lithospheric material pulls the rest of the slab in after it.

Royden studies the forces at play in subduction zones like these. As the slab tears from the adjacent lithosphere, sinks, and rolls back, its weight forces material from in front of the slab out and around 
to the back, in a circular motion. By estimating the slab’s weight and the viscosity of the surrounding mantle, one can, in theory, quantify the forces that determine the speed at which the trench will move. Modeling this has so far proved very difficult, Royden says, “because it is a fluid dynamics problem in the upper mantle, where the fluid flows vertically and horizontally, including ‘toroidal flow’” (in a circle around the slab). Her work has nevertheless played a critical role in identifying some of the key forces at play in such local subduction zones.

Likewise, “we now know that subduction provides most of the force for driving the motion of the plates” among the major tectonic plates covering most of Earth’s surface, Royden explains. But there are many important unanswered questions: “We don’t know what organizes plate tectonics. Why do we have just a few really big plates rather than lots of small ones?” Was it always this way during the past 4 billion years of Earth’s history?

During her fellowship year, Royden has been working on a model tectonic system that might shed light on such questions. Her “toy Earth” is simple: a subduction zone, with rules for its behavior, and a spreading ridge that follows rules she has figured out over years of fieldwork on regional-scale systems. She starts with one plate and goes to two and then three, to see how they interact. She is not trying to describe Earth, but by creating a tectonic system in which she knows all the variables, she hopes to identify key regulators of behavior.

A critical question involves the early planet, which produced much more internal heat than Earth does today. “It had to be losing heat a lot faster,” says Royden, “but how? Did it have lots of little plates with many mid-ocean spreading ridges, because that is where a huge amount of Earth’s heat is lost?” Or did it have a few major plates that simply moved faster, bringing greater quantities of hot material to the surface? Her model does
 not incorporate temperature directly, but it does include viscosity of the asthenosphere—a proxy for temperature—and, therefore, may suggest whether the organization or motion of plates changes in response to higher temperature. “I do a lot of quantitative work in order to get a qualitative understanding,” she explains. “I’m interested in what a typical time sequence of evolution from one state to another might be.”

“I won’t know until I get there how much new insight is going to come out of this,” she adds,
 “but if I can come out with a qualitative understanding of how these simplified tectonic models work, what makes them change from one state to another, and how stable they are, I feel that will be a really important contribution.”

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