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Research hints at a very active inner Earth area as depicted by these drawings. The image on the left is of Earth. The center image shows a section of Earth and its main divisions (solid inner core, liquid outer core, and the lower mantle, including the D" zone). The image on the right is a close-up of the D" region, where researchers found strong topographical variations here. Image courtesy of E. Garnero

October 11, 2004

Seismology research reveals the dynamic inner workings of Earth

By Skip Derra, Arizona State University

At the surface of Earth, life on a geologic scale is calm and peaceful save the occasional earthquake caused by the rub and slip of Earth's tectonic plates. But below Earth's surface, scientists are beginning to find a far more dynamic and tumultuous region than previously thought.

Deep inside Earth, where the mantle meets the molten iron core, researchers are finding telltale signs of what could be a highly active area filled with exotic forms and substances.

Seismologist Thorne Lay, professor of Earth sciences and director of the Institute of Geophysics and Planetary Physics at UCSC, is part of a team of scientists studying the structure of the mineralogical fabric near the base of Earth's mantle. They reported their latest findings in the October 8 issue of the journal Science.

"This study demonstrates that the material at the base of the mantle is deforming in ways that are much more complicated than the simple structures we had inferred from earlier work," Lay said.

The lead author on the paper is Arizona State University seismologist Edward Garnero. The other coauthors are Valerie Maupin of the University of Oslo, Norway, and Matthew Fouch of Arizona State.

The researchers deciphered unusual layering in Earth's deep interior that may contain clues about how the interior churns and convects, and the relationship between Earth's interior and its ever-evolving surface.

"Over very long periods of time, the Earth's rocks behave fluidly," Lay said. "If you picked up a chunk of the mantle it would feel like hard, dense rock. But over tens of millions of years, hard rock can flow like putty, and that's what is happening in the deep mantle. It's flowing in order to cool off, with hot rock rising to the surface and cooling off, and cold rock sinking back down."

According to Garnero, the base of the mantle is probably the most exotic part of Earth's interior. "In this area, where the mantle meets the core halfway to Earth's center (2,900 km below Earth's surface), the change in density is several times greater than what we find at Earth's surface, as represented by air and rock," Garnero said.

The deep mantle region the team probed is a several-hundred-kilometer-thick zone called D" (D double prime), which is where the silicate rock lower mantle meets Earth's liquid iron outer core. The researchers used seismic waves generated by earthquakes to probe this region.

They measured unique directional vibrations of seismic waves produced by South American earthquakes and recorded in North America, permitting a detailed probing of D" beneath Central America and the Caribbean Sea. The scientists found unexpected wave vibration directions from these waves and showed the deepest mantle to be the source of these wave-motion alignment changes.

These observations can be explained by tilting of the once horizontal rock fabric in the lower mantle by 20 degrees, resulting in contortions of the mineralogical fabric that must vary over relatively short distances (hundreds of kilometers). The seismic readings indicate a complex area that churns and chugs as the liquid iron core roils at the bottom of the rocklike mantle.

"We were detecting changes in the directional dependency over a relatively small size scale of a few hundred kilometers," Garnero said. "We think there must be currents and turbulence over geologic-type timescales that are really quite vigorous and which are occurring at short lengths in order to stir things in such a way as to give this preferred alignment of the material."

The seismic waves may be detecting areas where there are dramatic differences in the types of materials inside Earth.

"What hasn't been appreciated is that in the deepest mantle there are incredible changes from place to place geographically over short distances," Garnero said. "These changes represent a very dynamic mantle system."

"The center of the planet is thought to be as hot as the surface of the Sun, so this is a planet that is going to take some time to cool off," he added. "It cools off through this stirring and internal mixing."

According to Lay, a massive new seismic monitoring project funded by the National Science Foundation (NSF) promises to give scientists an even better view of Earth's deepest layers. Among other things, the NSF EarthScope project will add 2,000 new seismic monitoring sites across the United States.

"Our study used the relatively sparse data presently available, and in the future we expect to have much better resolution," Lay said. "It will be like a giant telescope looking down into the Earth that we can use to study processes 1,700 miles deep and understand how the mantle is deforming and flowing."

Garnero said this research is helping reshape the contemporary view of the inner workings of Earth.

"In the past 10 to 15 years we have come to appreciate the importance of deciphering the lowest couple of hundred kilometers of the mantle. Doing so is critical in understanding how the interior of Earth actually turns and convects, and drives these motions that we see at the surface," Garnero said. "This research supports a new view of the deepest mantle, where the evolution and dynamics of Earth as a whole cannot be understood without first deciphering the D" layer."

UCSC science writer Tim Stephens contributed to this article.

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