NSF PR 96-38 - July 17, 1996
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Core Spins Faster than Earth, Scientists Find
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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