Core Spins Faster Than Earth, Scientists Find
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Earth's inner core is rotating faster than the planet itself, say National Science Foundation-funded scientists at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York. The motion of the inner core has never before been detected or measured.
The discovery indicates that the inner core--a solid iron crystal whose mass is comparable to the size of the moon--is spinning independently from the rest of the solid Earth. It is driven by magnetic and electrical effects within the near-frictionless liquid outer core that surrounds it.
The finding, reported in the July 18th issue of the journal Nature, will likely advance understanding of how the Earth's magnetic field is created and why it reverses periodically; how heat flows through the planet; and how the Earth's multi-layered interior has evolved. The research was conducted by Xiaodong Song and Paul Richards, seismologists at Lamont-Doherty.
"The inner core rotates in the same direction as the Earth but slightly faster," explains Jim Whitcomb, director of NSF's geophysics program. "Over the past 100 years that extra speed has gained the core a quarter-turn on the planet as a whole. Such motion is remarkably fast for geological movements--some 100,000 times faster than the drift of continents." The Lamont-Doherty scientists made their finding by measuring changes in the speed of earthquake-generated seismic waves that pass through the inner core.
"For decades, the motion of the inner core has been the realm of theoreticians," says Paul Richards. "For the first time, we now have a hard piece of observational evidence, an actual measurement, of what's happening down there." The discovery will spark new research to explain the pattern of changes in Earth's magnetic field, including the way the north and south poles have "wandered" and reversed periodically during Earth's history. It will yield new knowledge about temperatures at the center of the Earth, and the flow of planetary heat that ultimately drives the motions of tectonic plates at Earth's surface.
Song and Richards studied seismic waves from 38 earthquakes that occurred between 1967 and 1995 near the South Sandwich Islands at the bottom of the globe. They measured the speed of waves that traveled up through the inner core to receiving seismographs in Alaska and found that the waves arrived about 0.3 seconds sooner in the 1990s than they did in the 1960s. The scientists also measured travel times of seismic waves to Norway from earthquakes in the Kermadec Islands near New Zealand. These waves took longer to travel through the inner core in the 1990s than they did in the 1980s. Song and Richards calculated that over a year, the inner core rotates about one longitudinal degree more than the Earth's mantle and crust. The inner core makes a complete revolution inside the Earth in about 400 years.
The core was formed very early in Earth's history as heavier molten iron sank toward the center of the planet. As the planet cooled and dissipated its internal heat toward the surface, some molten iron began to solidify to create the dense, solid inner core. Enormous pressure keeps the inner core solid in a region with temperatures in the range of 7,000 degrees Fahrenheit and possibly much higher. Fluid iron in the outer core has continued to solidify at the boundary between the inner and outer cores, so that over a billion years, the inner core has grown steadily to its present diameter of 1,500 miles.
NOTE TO BROADCAST MEDIA: A minute-long, broadcast quality color video is available showing the motions of the inner core and the seismic waves used to make the discovery.
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