Earth's Inner Core Not A Monolithic Iron Crystal, Say UC Berkeley Seismologists

November 08, 1996

Berkeley -- A new analysis of seismic waves traveling through the center of the Earth disproves a neat hypothesis that many geophysicists had secretly hoped was true, that the planet's inner core is a perfectly aligned mass of iron crystals nearly 1,500 miles across.

The analysis of recent earthquakes by seismologists at the University of California at Berkeley suggests that the iron crystals in the core are not neatly aligned so as to behave like a single crystal, but instead align themselves like boats following the flow in a circular eddy.

The scientific team led by Barbara Romanowicz, professor of geology and director of the UC Berkeley Seismographic Station, report their findings in this week's issue of Science.

The analysis comes on the heels of another surprising discovery about the Earth's inner core -- that it spins slightly faster than the rest of the planet. This result, reported in the July 18 issue of Nature, came from charting long-term variations in how long it takes seismic waves to traverse the core.

Romanowicz and postdoctoral researchers Xiang- Dong Li and Joseph Durek used a subset of this same data to determine how seismic waves travel through the entire core, essentially obtaining a three-dimensional picture of the core's interior.

What they saw did not fit with the picture of iron crystals neatly stacked in alignment with the magnetic field of the Earth, as many geophysicists had argued, but rather a more complex alignment of crystals suggesting they are moving about in a slow-motion dance driven by the rise of hotter iron toward the surface of the core.

"We aren't seeing the flow itself, since the motion is very slow in the nearly solid inner core," said Romanowicz. "We are seeing a snapshot of the alignment of iron crystals, like little boats in a river."

The core of the Earth, about 4,200 miles across, is thought to be divided into an outer core of liquid iron and a nearly solid iron inner core about 1,500 miles in diameter. For comparison the moon is 2,200 miles across.

Though nearly pure iron, the outer core is thought to have the consistency of water, and convection in the outer core is thought to create the Earth's magnetic field. At the bottom of this ocean of iron, however, iron crystals settle out onto the inner core, causing it to grow at a slow rate of perhaps an inch every 50 years.

Geophysicists thought the inner core might be a mass of uniformly aligned crystals of iron because seismic waves travel at different speeds through the core, depending on whether they travel parallel to the Earth's rotation axis or perpendicular to it. The speeds in fact matched the speeds of sound parallel and perpendicular to the axis of iron crystals in the laboratory.

Romanowicz and her colleagues found that the variation of speed throughout the inner core was more complicated than would be expected if all of the iron crystals were uniformly aligned.

Another argument for a pure crystalline core was that when earthquakes force the core to ring like a bell it vibrates not at a pure frequency but at a mixture of frequencies, as if the core were somehow nonuniform.

The UC Berkeley team analyzed vibrations of the core following two major quakes in 1994 -- an 8.2 moment magnitude quake in Bolivia and an 8.3 magnitude quake in Russia's Kurile Islands -- and created a 3-D model which they then matched to seismic wave travel time data from the past several years. The model that best fit the data involves a simple convective flow in the inner core, a slow-motion cycling centered on the rotation axis.

"The pattern is very simple -- hotter iron rises at the center along the rotation axis and falls back along the surface," Romanowicz said. "We have made some simplifying assumptions, but that is the basic pattern."

The inner core is malleable and can flow like honey, she said, because it is just barely solidified, having properties of a solid just below the melting point at high pressure.

The findings will have implications for other studies of the core, she said, including modeling how the inner core spins.

Romanowicz notes that while geologists have been studying seismic waves traveling through the Earth for years to glean details of the interior, only recently have good quality seismic data become available from state-of-the-art seismometers. These seismometers cover the surface the Earth and allow scientists to reconstruct the interior, similar to the way doctors use sound to image the interior of the body as sonograms.
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University of California - Berkeley

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