New ASU research reveals the dynamic inner workings of Earth

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. “This layer is far more complex than what we thought 10 years ago,” said Arizona State University seismologist Edward Garnero. “It is a super dynamic situation, probably the most exotic part of Earth’s interior. 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 and a team of seismologists (Valerie Maupin, of the University of Oslo, Norway; Thorne Lay of the University of California, Santa Cruz; and Matthew Fouch of ASU) recently completed a study of Earth’s interior. They report their findings in the Oct. 8 issue of Science magazine. In “Variable Azimuthal Anisotropy in Earth’s Lowermost Mantle,” the ASU researchers decipher 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.

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, those generated by earthquakes, to probe this region. They measured unique directional vibrations of seismic waves recorded in North America from South American earthquakes, permitting a detailed probing of D” beneath Central America and the Caribbean Ocean. Garnero and his colleagues found unexpected wave vibration directions from these waves and showed the deepest mantle to be the source of these wave motion alignment changes.

Tilting of the once horizontal rock fabric in the lower mantle by 20 degrees explains the observation, where the fabric contortions 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 rock-like mantle, Garnero said. “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 time scales 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.

Garnero explained that what the seismic waves may be detecting are areas where there are dramatic differences in the types of materials inside Earth. “At the core mantle boundary layer there’s a huge contrast in density,” Garnero said. “You go from silicate-based rock (the mantle) to a liquid iron material really rapidly. The environment has all of the markings of something that may be more complex than what we see at the surface.”

“What hasn’t been appreciated is that in the deepest mantle there are incredible changes from place to place geographically over short distances,” he added. “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,” Garnero explained. “It cools off through this stirring and internal mixing.”

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.”