A Huge Marble Of Metal

A dizzying 70-year cycle of spin and counterspin at Earth's core.

Imagine Earth's inner core - the dense center of our planet - as a metal ballerina, capable of pirouetting at ever changing speeds.

Seismologists have reported in the journal Nature Geoscience that the inner core appears to be changing the direction of its spin, relative to the motion of Earth's surface.

This may sound like a setup for a world-wrecking blockbuster movie. But fret not : Precisely nothing apocalyptic is expected to result from this planetary spin cycle, which may have been happening for eons.

In 1936, a Danish seismologist discovered that Earth's liquid outer core envelops an enormous, solid metal marble. Later research, based on seismic waves, has found that inner core spins at varying speeds, pulled by currents created by the Earth's magnetic field, restrained by the Earth's gravity.

It speeds up, then slows down, in a cycle that appears to run about 70 years. It was last stable, relative to the surface, about 2010 and is expected to be so again in the 2040s.

''It's weird that there's a solid iron ball kind of floating in the middle of the Earth,'' said John Vidale, a seismologist at the University of Southern California who was not involved with the study. [ Robin George Andrews ].


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