Churning at the Core

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.

However, deep inside the planet’s bowels, 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,” says Edward Garnero, an ASU seismologist. “It is a super dynamic situation, probably the most exotic part of Earth’s interior. This area is where the mantle meets the core halfway to Earth’s center. It is about 2,900 kilometers below the 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 recently completed a study of Earth’s interior. They deciphered unusual layering deep inside the planet that may contain clues about how the interior churns, as well as the relationship between Earth’s interior and its ever-evolving surface.

The team probed a deep mantle region called D" (D double prime). The area is several hundred kilometers thick. It 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 recorded in North America from South American earthquakes. As part of their study, the team completed a detailed probing of D" beneath Central America and the Caribbean Ocean.

Garnero explains 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 says. “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.”

The ASU scientist says that in the deepest mantle there are incredible changes from place to place geographically over short distances. 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. It cools off through this stirring and internal mixing,” he adds.

Garnero says the findings are helping to reshape the contemporary view of the inner workings of Earth. The research is funded by the National Science Foundation.—Skip Derra