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NSF PR 98-70 - October 29, 1998
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Scientists Propose Layered Model of Earth's Inner
Core
Earth's inner core is not a uniform iron crystal, but
is instead composed of two distinct layers, according
to researchers funded by the National Science Foundation
(NSF).
Seismologists Xiaodong Song from Columbia University's
Lamont-Doherty Earth Observatory and Don Helmberger
of the California Institute of Technology published
their study of the structure of the earth's core in
the October 30 issue of the journal Science.
The inner core of the earth is a 1,500-mile-wide sphere
comprised mainly of solid iron that rotates in an
outer core of molten iron. The scientists used seismic
data from earthquakes to infer that the inner core
has two distinct parts: a spherical lower part surrounded
by a thin, uneven upper layer of different material
properties. These findings are likely to affect the
current model of how the earth, and its magnetic field,
formed.
"This research provides new insights into the structure
of the earth's inner core," said Jim Whitcomb, director
of NSF's geophysics program which funded the research,
"and indicates that we still have much to learn about
the composition of our own planet."
Earthquakes generate seismic waves that can be measured
as they cross the globe. Song and Helmberger studied
seismic waves from earthquakes in the South Atlantic
Ocean that passed through the earth's core traveling
northward to seismographs in Alaska and Canada. These
waves moved faster than seismic waves that traveled
from those same southern earthquakes to seismographs
in Korea.
The waves moved faster because the earth's inner core
is anisotropic, it's iron crystals arranged in such
a way that waves move faster in one direction (North
to South) than in other directions (East to West).
Song and Helmberger noticed, however, that seismic
waves from particular earthquakes reached some northern
seismographs before getting to others -- and in the
journey became slightly altered. The pair narrowed
down the source of these alterations to the inner
core. There they found a point of transition 120 miles
into the inner core above which the core was no longer
anisotropic. It was, instead, isotropic, meaning that
seismic waves could travel at the same speed in any
direction.
"It is like placing a straw in a glass of water," said
Song. "When looking at it from the side of the glass,
the straw appears bent. The light is refracted, bent
a little as it passes through the water to the air.
In the same manner, the seismic wave is refracted
as it passes through the anisotropic layer of the
core and back to the isotropic layer that surrounds
it."
By measuring how far the seismic waves were refracted
and the energy that was reflected from the boundary,
the researchers were able to determine the shape and
thickness of the upper layer of the inner core. The
cause of such layering is unclear, but the researchers
propose that pressure, thermal and magnetic forces
in action at the center of the earth cause the upper
layer to change shape through time.
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