Findings offer new approach to understanding movements of continents
More than 40 years ago, pioneering tectonic geophysicist J. Tuzo Wilson published a paper in the journal Nature describing how ocean basins opened and closed along North America's eastern seaboard.
His observations, dubbed "The Wilson Tectonic Cycle," suggested the process occurred many times during Earth's long history, most recently causing the giant supercontinent Pangaea to split into today's seven continents.
Wilson's ideas were central to the so-called Plate Tectonic Revolution, the foundation of contemporary theories for processes underlying mountain-building and earthquakes.
Since his 1967 paper, additional studies have confirmed that large-scale deformation of continents repeatedly occurs in some regions but not others, though the reasons why remain poorly understood.
Now, new findings by Utah State University geophysicist Tony Lowry and colleague Marta Pérez-Gussinyé of Royal Holloway, University of London, shed surprising light on these restless rock cycles.
"It all begins with quartz," says Lowry, who published results of the team's recent study in the March 17 issue of Nature.
The scientists describe a new approach to measuring properties of the deep crust.
It reveals quartz's key role in initiating the churning chain of events that cause Earth's surface to crack, wrinkle, fold and stretch into mountains, plains and valleys.
"If you've ever traveled westward from the Midwest's Great Plains toward the Rocky Mountains, you may have wondered why the flat plains suddenly rise into steep peaks at a particular spot," Lowry says.
"It turns out that the crust beneath the plains has almost no quartz in it, whereas the Rockies are very quartz-rich."
He thinks that those belts of quartz could be the catalyst that sets the mountain-building rock cycle in motion.
"Earthquakes, mountain-building and other expressions of continental tectonics depend on how rocks flow in response to stress," says Lowry.
"We know that tectonics is a response to the effects of gravity, but we know less about rock flow properties and how they change from one location to another."
Wilson's theories provide an important clue, Lowry says, as scientists have long observed that mountain belts and rift zones have formed again and again at the same locations over long periods of time.
"Over the last few decades, we've learned that high temperatures, water and abundant quartz are all critical factors in making rocks flow more easily," Lowry says. "Until now, we haven't had the tools to measure these factors and answer long-standing questions."
Since 2002, the National Science Foundation (NSF)-funded Earthscope Transportable Array of seismic stations across the western United States has provided remote sensing data about the continent's rock properties.
"We've combined Earthscope data with other geophysical measurements of gravity and surface heat flow in an entirely new way, one that allows us to separate the effects of temperature, water and quartz in the crust," Lowry says.
Earthscope measurements enabled the team to estimate the thickness, along with the seismic velocity ratio, of continental crust in the American West.
"This intriguing study provides new insights into the processes driving large-scale continental deformation and dynamics," says Greg Anderson, NSF program director for EarthScope. "These are key to understanding the assembly and evolution of continents."
Seismic velocity describes how quickly sound waves and shear waves travel through rock, offering clues to its temperature and composition.
"Seismic velocities are sensitive to both temperature and rock type," Lowry says.
"But if the velocities are combined as a ratio, the temperature dependence drops out. We found that the velocity ratio was especially sensitive to quartz abundance."
Even after separating out the effects of temperature, the scientists found that a low seismic velocity ratio, indicating weak, quartz-rich crust, systematically occurred in the same place as high lower-crustal temperatures modeled independently from surface heat flow.
"That was a surprise," he says. "We think this indicates a feedback cycle, where quartz starts the ball rolling."
If temperature and water are the same, Lowry says, rock flow will focus where the quartz is located because that's the only weak link.
Once the flow starts, the movement of rock carries heat with it and that efficient movement of heat raises temperature, resulting in weakening of crust.
"Rock, when it warms up, is forced to release water that's otherwise chemically bound in crystals," he says.
Water further weakens the crust, which increasingly focuses the deformation in a specific area.