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 Published by the American Geological Institute
January 2001
Newsmagazine of the Earth Sciences

News Notes

 Scientists from the United States and Canada are proposing a new layer in the “onion” model of Earth, essentially a zone of material with properties of both the outer core and lower mantle. Their idea suggests that the boundary between core and mantle may not be sharp. It proposes a conducting layer at the core-mantle boundary that could explain Earth’s wobble and, potentially, the zones of low seismic velocity found in the lower mantle. Surprisingly, their theory takes a page out of oceanic sedimentation models.

The Earth’s rotation is subject to periodic variations called nutations — a wobble along Earth’s axis. This wobble is ascribed to the Sun and Moon’s gravitational pull, which tugs on Earth’s magnetic field. The pulling sloshes around Earth’s liquid interior layers. The planet’s magnetic field, which originates in the solid iron core, could be carried through a conducting layer at the core-mantle boundary, the scientists say. The magnetic field could then link the core and mantle magnetically, and pull on the solid mantle to make the planet nutate, or wobble, slightly.
According to current theories about the formation of Earth, the solid inner core has been cooling and solidifying for billions of years. As it cools, the lighter iron alloys separate from the pure iron and move outward to the liquid outer core. The liquid layer must then chemically equilibrate with the lower mantle. Bruce Buffett, a geophysicist from the University of British Columbia in Vancouver, seismologist Edward Garnero of Arizona State University in Tempe, and mineral physicist Raymond Jeanloz of the University of California, Berkeley, reported in the Nov. 17 Science that the light iron alloys will react with the lower silicate mantle to make iron silicates. A slow inverted rain of iron silicate “sediments” moves out from the core because the sediments are less dense than the alloys. The sediments rise and collect unevenly at the boundary between the outer liquid core and lower mantle. However, some liquid iron is still found in spaces between the sediments. This is the key to Buffett’s theory. Under the pressure of accumulating sediments, some of the iron will be squeezed out. The remaining iron, however, would provide enough conductivity in the new layer to explain Earth’s wobble.
A layer of high conductivity could account for another geophysical phenomenon: areas of low seismic velocity found at the core-mantle boundary. Most information about the inner Earth is inferred from the study of how seismic waves pass through the planet. Within a few tens of kilometers of the core-mantle boundary, seismic waves slow down, possibly more than 10 percent relative to the average velocity of waves in the lower mantle. Historically, these ultralow-velocity zones (ULVZs) were attributed to partial melting of the lower mantle. However, the presence of liquid iron in the sediments could significantly slow down seismic waves and possibly explain the ULVZs, Buffett says.
One test of the theory would be to correlate the location of ULVZs with topographic maps of the core-mantle boundary. However, better maps will be needed, says Quentin Williams of the University of California, Santa Cruz, who believes that while core sedimentation could explain the ULVZs, it is still “a speculative story that’s going to be difficult to verify.” Either way, the seismic techniques will have to be improved to better distinguish core and mantle sediments, just as they help to distinguish between oceanic and continental sediments.

Audrey Slesinger
Geotimes contributing writer

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