Yale scientists break ground on Earth’s mantle

Yale scientists are breaking ground on the effects of pressure on the crystalline structure of the Earth’s mantle.

In a report published in the Sept.24 edition of Science, lead author Lowell Miyagi, a post-doctoral assistant in the Department of Geology and Geophysics,performed experiments in conjunction with researchers at the University of California, Berkeley that were designed to simulate the pressure experienced in the lower subsection of the Earth’s mantle. By simulating pressure 1.8 million times that of the atmosphere, Miyagi was able to uncover the reason for the previously unexplained deformation of post-perovskite, a chemicalmaterial found in abundance in the lower mantle.

“For a long time, scientists have known about weird seismological behavior near the core-mantleboundary,” Miyagi said.

The crystalline structure of the materialbegins to deform when subjected to extremely high temperatures and pressures, Miyagi said, noting that this phenomenon may yield clues about the seismological origins of earthquakes.

It was this problem that inspired Miyagi to begin investigation on the internal dynamics of the core-mantleboundary more than four years ago, he said.

Kanini Lee, an assistant professor of geology andgeophysics and a co-author of the paper, said the research group tried to approach the problem from a different angle, using magnesium silicate as a replacement for post-perovskite because they thoughtthe compound’s chemistrywas morecomparable.

“This is the first study to have a relevant chemistry and use a substance without any molecular memory,” Lee explained.

By using a glass piece of magnesium silicate, Lee said that the new crystalline product was not impacted by its previous shape. Other studies, Lee said, used minerals which may run into this problem.

In the experiment, the researchers found that the magnesium silicate crystal split along a plane perpendicular to the longest plane instead of the longest plane as expected. Using the analogy of a horizontal stack of papers, the expected path of splitting would be horizontal, or between sheets, Lee said. Instead, the sheets seemed to split vertically.

The resulting deformation, Miyagi said, explains a lot about the oddities of the core-mantleregion.

“Former models didn’t quite explain the seismological movement across a plane,” Lee said. “It’s nice to be able to understand exactly how they occur.”

The authors of the study used a high-intensity laser and two geometrically opposed diamonds whose crystalline tips wereremoved to manipulate the pressure exerted on the post-perovskite, Miyagi said.

The deformation of the post-perovskite also offers an explanation for the motion of tectonic plates, added Miyagi.

ButMiyagi admits that these findings will not allow the prediction of future earthquakes.

“Convection is only related to plate movement peripherally,” he explained.

Though the study explores the notion of plate tectonics in a novel fashion, using a glass sheet and arealistic chemistry of post-perovskite, Dr. Miyagi believesthat more to explore exists.

“It’s not the end of the story,” Miyagi said. “We still need a more definite interpretation.”

Correction: October 7, 2010

An earlier version of this article incorrectly attributed the quote “Former models didn’t quite explain the seismological movement across a plane. It’s nice to be able to understand” to Department of Geology and Geophysics post-doctoral student Lowell Miyagi. It was in fact stated by Kanini Lee, an assistant professor in the department.

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