Scientists have wondered for some time why certain seismic waves travel more quickly through the core-mantle boundary, a thin layer of the Earth’s interior that lies between about 1675 and 1800 miles below the surface. Now a new study by Yale University and the University of California, Berkeley sheds light on the mystery by showing how this region behaves under the extreme conditions found so deep in the Earth. The findings, which appear in the Sept. 24 issue of the journal Science, have important implications for understanding how the Earth’s internal heating and cooling processes work.
Geologists believe that most of the Earth’s mantle—an almost 1800-mile-thick layer between the crust and the core that makes up more than three quarters of the planet’s volume—is mostly made up of a mineral called magnesium silicate perovskite MgSiO3, or “perovskite” for short. Below this, the 125-mile-thick core-mantle boundary is composed in large part of a high-pressure phase of perovskite known as post-perovskite.
Because post-perovskite is created only under the extremely high temperatures and pressures that exist so deep in the Earth’s interior, it is not found on the planet’s surface. As a result, understanding the physics of this unique substance, and therefore the physics of the core-mantle boundary, has proven difficult.
Now a team led by Yale researcher Lowell Miyagi has managed to heat and compress post-perovskite to the conditions found at the core-mantle boundary, where temperatures soar to nearly 6000 degrees Fahrenheit and pressures are more than one million times the ambient pressure at the surface of the Earth.
Once Miyagi had formed the post-perovskite and squeezed it to these extreme pressures using a vise-like device that crushes substances between the tips of two diamonds, he discovered that the mineral behaved in a surprising way. “The preferred orientation of the post-perovskite’s crystal structure was very counterintuitive,” he said. “Post-perovskite has a layered crystal structure, but instead of deforming along the layers when compressed, like almost every other layered structure does, it deforms on a plane cutting across the layers.” Knowing more about the structure of the material that makes up much of the core-mantle boundary will help scientists understand how seismic waves travel through this region of the Earth’s interior.
Based on their experimental results, the team found that their model for mantle mixing correlates well with seismic observations. “The findings could explain why seismic waves tend to travel faster in certain directions near the core-mantle boundary,” said Kanani Lee, assistant professor of geology and geophysics at Yale and one of the study’s co-authors. “The alignment of post-perovskite’s crystal structure likely determines in which direction seismic waves travel fastest in that region. Understanding this structure gives us much more insight into the extreme physics taking place 1800 miles below the surface.”
It will also provide clues as to how Earth’s internal convection works there. After descending from the ocean floor, cool tectonic plates pass through the mantle and approach the dense, liquid-iron outer core, where they heat up and begin moving upward again in a repeated cycle of mantle mixing.
“Understanding how post-perovskite behaves is a good start to understanding what’s happening near the mantle’s lower reaches,” Miyagi said. “We can now begin to interpret flow patterns in this deep layer in the Earth.”
Other authors of the paper include Waruntorn Kanitpanyacharoen, Pamela Kaercher and Hans-Rudolf Wenk (University of California, Berkeley).