Discovering the Earth’s inner inner core

By Madeline Decker

The question of what lies at the center of the Earth has long captivated the public imagination. A recent discovery by University professor Xiaodong Song may transform the way that seismologists view the Earth.

Using a new seismology technique, Song and his team discovered that the core of the Earth contains an inner region that is smaller than the Earth’s moon at only about 600 kilometers in radius. Song discovered that within this inner inner core, where iron crystals have long been thought to align only in a north to south orientation, crystals in this central region are actually aligned east to west.

While Song’s team still awaits confirmation of research results, this discovery could transform the way scientists view the planet.

The Earth’s inner core is like the rings of the tree, according to Song.

By looking at the rings in a tree’s inner core, for example, one can detect its age or past environmental traumas. Much in the same way, the Earth’s inner core, which grows over time “from the solidification … of liquid iron in the outer core,” Song said, provides insights into the planet’s development.

“The Earth’s inner core has recorded a very long history of the Earth, over at least a billion years,” he said.

The north-south iron crystal orientation in the Earth’s core has flipped several times in the past. The east-west alignment of iron crystals in Earth’s inner inner core, however, suggests that “right around 600 million years ago … there was something different in the paleomagnetic field,” Song said, assuming that the Earth is approximately a billion years old.

In other words, the iron crystal alignment in the inner inner core could indicate changes in the Earth that occurred long ago — changes scientists may now seek to further explore.

Song made this discovery in partnership with a team of researchers at Nanjing University in China. The U.S. National Science Foundation and the National Science Foundation of China funded his work.

Curiosity, along with a powerful new technique in seismology, inspired Song’s research. Richard Weaver, University physics professor, was a pioneer in the development of the technique Song used.

Just as doctors use ultrasounds to explore the inside of the human body, Weaver explained, seismologists use the rate at which sound waves travel through the Earth to determine properties of the Earth’s interior. Seismologists have for many years analyzed seismic waves from earthquakes, but with a relatively limited scope.

About 16 years ago, however, Weaver — who specializes in ultrasonics, a discipline similar to seismology but on a smaller, more accessible scale — had a theory that the randomized waves produced by the reverberations of earthquakes, referred to as noise, could also reveal information about the Earth’s interior. He performed experiments in his laboratory to demonstrate that the technique would indeed work on a larger scale.

“It’s been a revolution in seismology ever since,” Weaver said.

This technique allowed Song to obtain a somewhat complete picture, by using the noise from any station across the globe, Weaver said.

Analyzing about 20 years worth of seismic records from around the world, Weaver said, Song determined “seismic waves travel faster in some directions through the (inner core of the) Earth than in others.”

Because the Earth is an oblate spheroid, it has long been understood that the speeds of waves traveling from north to south through the Earth’s core will differ from those of waves traveling along the equator, Weaver explained. Song discovered that wave speed also varies depending on the equatorial plane direction.

This led him to discover the inner inner core of the Earth and its properties — a finding both Song and Weaver hope will serve as a trajectory for further investigation of the planet’s interior.

“It’s hard to say if there’s a practical application,” Song said. “We have known, for example, from kindergarten, that the Earth has different layers and whatnot, and now we suddenly have a very striking, new layer, if you like, that we didn’t know (about) before. That’s something that I think is very cool.”

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