Core Spins Faster Than Earth, Lamont Scientists Find
at Columbia University's Lamont-Doherty Earth Observatory have
found that the Earth's inner core is rotating faster than the
The motion of the inner core has never before been detected or
measured. The finding, reported July 18 in 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
inner core rotates in the same direction as the Earth and slightly
faster, completing its once-a-day rotation about two-thirds of
a second faster than the entire Earth. Over the past 100 years
that extra speed has gained the core a quarter-turn on the planet
as a whole, the scientists found. Such motion is remarkably fast
for geological movements -- some 100,000 times faster than the
drift of continents, they noted. The scientists made their finding
by measuring changes in the speed of earthquake-generated seismic
waves that pass through the inner core.
The research was conducted by Xiaodong Song and Paul G. Richards,
seismologists at Lamont-Doherty, Columbia's earth sciences research
institute in Palisades, N.Y. Dr. Song is the Storke-Doherty Lecturer
and Dr. Richards the Mellon Professor of Natural Sciences at Columbia.
"For decades, the motion of the inner core has been the realm of
theoreticians," Dr. Richards said in an interview. "For the
first time, we
have a hard piece of observational evidence, an actual measurement,
of what's happening down there."
The discovery of such a fundamental property will advance planetary
understanding, the scientists said. It will spark new research
to explain the observed pattern of changes in Earth's magnetic
field, including the way the north and south poles have
and reversed periodically over 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 to create mountains and
oceans, split continents and cause earthquakes.
Lamont-Doherty researchers' findings culminate a period of rapid
discovery about the inner core. In the late 1980s and early 1990s,
several scientists, including Dr. Song when he worked with colleagues
at the California Institute of Technology, observed that seismic
waves traveling through the inner core along a roughly north-south
pathway moved faster than those traveling closer to the equator.
The scientists theorized that the enormous pressure in the Earth's
core, which is millions of times higher at the inner core surface
than the atmospheric pressure at the Earth's surface, has aligned
iron atoms in a distinctive crystalline structure that slows down
waves in some directions and creates "a fast track"
for seismic waves in a near north-south direction. More recent
studies also indicate that the "fast-track axis" is
not exactly north-south but is tilted slightly from the axis around
which the Earth is rotating. The core's "fast axis"
would emerge at an imaginary point on Earth's surface up to 10
degrees from the north pole, where the spin axis would emerge.
Earth and the core are rotating on the same spin axis, but because
the inner core rotates just a bit faster than the planet as a
whole, the "fast axis" through the core moves eastward.
Over the years, it traces a circular path around the north pole
and moves to different positions relative to the Earth's mantle
and crust. This basic feature allowed the Lamont scientists to
make their discovery.
and Dr. 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 at the top of the globe and found that the waves arrived
about 0.3 seconds sooner in the 1990s than they did in the 1960s.
speed of the waves steadily increased over the period because
the inner core's "fast axis" -- the fastest possible
route through the inner core -- was gradually becoming more closely
aligned with the actual pathway traveled by the waves between
South Sandwich and Alaska, the scientists said. The change in
the waves' speed showed that the fast axis was in motion relative
to the Earth, proving that the core is spinning faster than the
Earth, they said.
added proof, the Lamont-Doherty scientists measured travel times
of seismic waves from earthquakes at the Kermadec Islands near
New Zealand that were received in Norway. These waves took longer
to travel through the inner core in the 1990s than they did in
the 1980s because the core's fast axis had moved away from the
seismic wave pathway between Kermadec and Norway over that period.
research was funded by the National Science Foundation. The NSF
also has supported Lamont-Doherty's archive of long-term, worldwide
seismological records, which hold key data for measuring the inner
View an animation of the Earth (511K)
Song and Dr. Richards were intrigued by preliminary results last
fall from a theoretical computer model simulating the dynamo that
creates Earth's magnetic field. The model, created by Gary Glatzmaier
of the Los Alamos National Laboratory and Paul Roberts of the
University of California at Los Angeles, suggested that the Earth
and its inner core may rotate at different rates.
prove the theory, the Lamont-Doherty seismologists identified
a pathway that aligned closely with the inner core's fast axis.
On one end was a seismic monitoring station; on the other, a quake-prone
region that consistently generated many seismic waves to study.
They compared the arrival times of seismic waves that traveled
through the inner core and those that passed merely through the
outer core. The latter waves remained constant over the 28-year
span, but those that traversed the inner core arrived slightly
but steadily sooner over the three decades, they found.
Song and Dr. 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.
Earth's core was formed very early in Earth's history as heavier
molten iron sank toward the center of the planet. As the Earth
cooled and dissipated its internal heat toward the surface, some
molten iron began to solidify to create the dense, solid inner
core at the center of the planet. Enormous pressure keeps the
inner core solid in a region of temperatures in the range of 7,000
degrees Fahrenheit and possibly much higher.
iron in the outer core has continued to solidify at the boundary
between the two cores, so that over a billion years, the inner
core has grown steadily to its present diameter of 1,500 miles.
(The inner and outer core together are 4,350 miles wide and the
Earth's diameter is about 7,900 miles.) The electrical currents
and convecting movements within the Earth's liquid core generate
the planet's powerful magnetic field, which in turn drives the
inner core in the way that can now be observed.
a billion amps of current is flowing into and out of the inner
core, across the boundary between the inner and outer cores. This
current, in the presence of a magnetic field, results in forces
being applied to the inner core, which then rotates. Essentially,
the inner core rotates because it is part of a vast electric motor.
inner core has a diameter about three-quarters that of the moon,
and a mass density almost 13 times greater than the density of
water. The mass of the inner core is about one hundred million
million million tons -- which is about 30 per cent greater than
the mass of the moon.