Earth's Inner Core Might Be on the Move

Earth's solid inner core may be continually inching eastward relative to its liquid outer core, renewing itself by shedding its front while solidifying its back, a team of French scientists suggests.

"Within less than 100 million years, everything that has been crystallized on the west will have melted on the east," said lead researcher Thierry Alboussiere of Université Joseph Fourier in Grenoble.

The idea counters traditional theory that the big ball at the center of the Earth stands still, growing uniformly in all directions as the planet cools. It could shed light on the nature of the core — such as its age, apparent seismic mismatches, and a mysterious coating of dense fluid on its surface.

Seismic clues

About a billion years ago, the middle of the Earth began slowly solidifying from the inside out. The planet is hottest at its center, possibly even hotter than the surface of the sun, yet the core's iron is thought to be solid because of the extreme pressure that has raised its melting temperature. As it freezes, according to theory, the inner core takes in more iron, sending lighter elements up through the liquid outer core. This movement is thought to drive Earth's magnetic field.

Since scientists can't journey the roughly 4,000 miles (6,400 kilometers) to the center of the Earth to see what's going on, insight of the 1,600-mile (2,570-km) wide innermost layer more or less stops there. But bits and pieces of indirect information come in all the time, through earthquakes.

"Whenever there is a big earthquake, seismic waves get sent out in all directions and seismographs around the globe record the wiggles," said Michael Bergman of Bard College at Simon's Rock, Mass., who was not involved with the study but wrote an accompanying commentary on it for the journal Nature.

Earthquakes help scientists learn about the inner depths of the planet because seismic waves change speed as they move through different mediums. Researchers can piece together this information to figure out what kinds of rocks and minerals are located in different parts of the Earth's interior.

"But earthquakes don't occur everywhere on the surface of Earth, so we don't have enough data to completely constrain the inner core," Bergman told OurAmazingPlanet. "We have to take what nature can give us."

Over the last couple of decades, nature has offered more questions than answers. Seismologists noticed that waves appear to travel faster through the inner core from north to south than from west to east. Seismic properties also seemed to vary between the Eastern and Western hemispheres of the globe.

These findings did not fit with the traditional picture of the inner core, and neither did the 124-mile (200-km) thick layer of dense material detected on its surface.

"How do you build this dense layer next to the inner core when you're only releasing light material?" Alboussiere said in an interview.

He and his colleagues had an idea that could explain some of the discrepancies. What if rather than just crystallizing over its entire surface, the inner core was simultaneously melting and solidifying on opposite sides? Then perhaps the melting side could inject dense material into the outer core.

Eastward shift

Through mathematical models and lab experiments, the team determined that this may, in fact, describe the dynamics of the inner core in a way that is compatible with seismology's recent discoveries.

They speculate that the inner core is shifted slightly off-center, just to the east. This would put more pressure on the western side, where it would be closer to the center of the planet, and less pressure on the eastern side. The result could be a perpetually denser Western hemisphere and a continual flow of dense fluid from the east that eventually spreads out atop the entire inner core. It might also inspire a re-evaluation of how the Earth's magnetic field is generated and a recalculation of the age of the inner core and its oldest piece.

"The inner core is basically regenerating itself. And superimposed on that is this overall cooling that makes the inner core bigger and bigger over time," Bergman said. "That's just cool to think about."

The study is detailed in the Aug. 4 edition of the journal Nature.

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This article was updated at 3:19 p.m. ET to reflect the correct diameter of Earth.