Scientists Propose Layered Model of Earth's Inner Core
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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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