Perilous
Times
Earth's Core Rotating Faster Than Rest Of The Planet
Although the inner core is 5200km beneath our feet, the effect of
its presence is especially important on the Earth's surface. In
particular, as the inner core grows, the heat released during
solidification drives convection in the fluid in the outer core.
by Staff Writers
Cambridge UK (SPX) Feb 24, 2011
New research gives the first accurate estimate of how much faster
the Earth's core is rotating compared to the rest of the planet.
Previous research had shown that the Earth's core rotates faster
than the rest of the planet. However, scientists from the
University of Cambridge have discovered that earlier estimates of
1 degree every year were inaccurate and that the core is actually
moving much slower than previously believed - approximately 1
degree every million years. Their findings were published 20
February, in the journal Nature Geoscience.
The inner core grows very slowly over time as material from the
fluid outer core solidifies onto its surface. During this process,
an east-west hemispherical difference in velocity is frozen into
the structure of the inner core.
"The faster rotation rates are incompatible with the observed
hemispheres in the inner core because it would not allow enough
time for the differences to freeze into the structure," said
Lauren Waszek, first author on the paper and a PhD student from
the University of Cambridge's Department of Earth Sciences.
"This has previously been a major problem, as the two properties
cannot coexist. However, we derived the rotation rates from the
evolution of the hemispherical structure, and thus our study is
the first in which the hemispheres and rotation are inherently
compatible."
For the research, the scientists used seismic body waves which
pass through the inner core - 5200km beneath the surface of the
Earth - and compared their travel time to waves which reflect from
the inner core surface. The difference between the travel times of
these waves provided them with the velocity structure of the
uppermost 90 km of the inner core.
They then had to reconcile this information with the differences
in velocity for the east and west hemispheres of the inner core.
First, they observed the east and west hemispherical differences
in velocity. They then constrained the two boundaries which
separate the hemispheres and found that they both shifted
consistently eastward with depth.
Because the inner core grows over time the deeper structure is
therefore older, and the shift in the boundaries between the two
hemispheres results in the inner core rotating with time. The
rotation rate is therefore calculated from the shift of the
boundaries and the growth rate of the inner core.
Although the inner core is 5200km beneath our feet, the effect of
its presence is especially important on the Earth's surface. In
particular, as the inner core grows, the heat released during
solidification drives convection in the fluid in the outer core.
This convection generates the Earth's geomagnetic field. Without
our magnetic field, the surface would not be protected from solar
radiation, and life on Earth would not be able to exist.
"This result is the first observation of such a slow inner core
rotation rate," said Waszek "It therefore provides a confirmed
value which can now be used in simulations to model the convection
of the Earth's fluid outer core, giving us additional insight into
the evolution of our magnetic field."