Magazine article Insight on the News

Geologists Now Know What Makes the World Go 'Round

Magazine article Insight on the News

Geologists Now Know What Makes the World Go 'Round

Article excerpt

Scientists have discovered that Earth's inner core rotates faster than the planet. Their findings could advance planetary science and unlock the secrets of Earth's fluctuating magnetic fields.

Two new studies indicate that Earth's iron inner core is spinning faster than the rest of the planet. The discovery may explain some of the mysteries about the planet's magnetic field, although at present it raises more questions than it answers.

Scientists at Columbia University's Lamont-Doherty Earth Observatory have been analyzing three decades of data from several seismic listening posts, including one in Antarctica and one in Alaska, plus data from formerly classified documents on underground nuclear testing in the Arctic by the Soviet Union. Irregularities in the core's structure conduct and interrupt seismic waves in a manner that remains consistent over time, much like a sonogram uses irregular responses to sound waves by a fetus to present its image inside a womb.

Over time, the Columbia team discovered "a small but systematic variation" in the travel times of seismic waves that traverse the inner core. The scientists concluded that the variation is caused by the core's more-rapid rotation "on the order of 1 degree per year faster than the daily rotation of the mantle and crust." (The findings were published in the July 18 issue of the journal Nature.)

According to the theory, Earth's solid inner core -- about 1,500 miles in diameter, or seven-tenths as wide as the moon -- is like the armature of a gigantic dynamo, spinning within a housing of liquid iron. This 1,400-mile-thick outer core has a low viscosity, or friction level, which allows the inner core to move freely.

There is some friction, however, especially at the boundary layer where the liquid iron in the outer core slowly "freezes" into the inner core's solid structure. Thus the inner core's rotation is gradually slowing just like the rest of the planet, which is imperceptibly lengthening its day in response to tidal forces generated by the moon.

Since some of the electromagnetic energy generated by the outer core is converted into torque, however, it helps propel the inner core and counteracts the tidal slowing. …

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