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Earth's Moving, Melting Core
4 August 2010 3:27 pm
Strange forces are at work 5000 kilometers below Earth's surface. The inner core is acting in ways that scientists can't explain. Theoretically, the core should be drawing iron from its molten surroundings and crystallizing it into solid metal. But that alone doesn't account for a number of odd observations—unless, as a few scientists speculate, the core is also melting.
The inner core, a solid ball of mostly iron 1200 kilometers across, formed hundreds of millions of years ago when Earth's blazing interior began to cool and the heaviest elements started to sink. It's been growing slowly ever since. Scientists speculate that the inner core gets bigger as it absorbs and hardens molten iron from the liquid outer core. But that fails to explain the dense liquid at the boundary between the inner and outer core. The liquid should be lighter if the inner core really is pulling heavier elements from the outer core. The hypothesis also doesn't explain why seismic waves from earthquakes—some of the only tools scientists can use to see that far beneath the surface—travel faster on the eastern side of the inner core than on the western side.
The answer to these riddles, according to a new model published in the 5 August issue of Nature, is that Earth's core is much more dynamic than scientists thought. Lead author Thierry Alboussière, a geophysicist at the University of Lyon in France, suggests that the inner core crystallizes in the west and melts in the east. That is, the eastern edge is slowly breaking down, while the western edge keeps building up more iron. The process is a bit like a treadmill: the solid iron "moves" eastward at a pace of roughly 1.5 centimeters per year, melting when it reaches the eastern edge. At this pace, Alboussière says, it takes 100 million years for the inner core to recycle itself.
The team's model would explain the thick liquid layer at the boundary between the inner and outer cores. If it is fluid that just melted out of the dense inner core, it would be heavier than the rest of the outer core. And if the inner core is crystallizing on its west end and melting on its east, that would also account for the different seismic wave speeds on both sides.
If the model is correct, Alboussière says, the inner core is still growing, but it has to absorb iron much faster to make up for all the iron that's melting away. And because movement in the inner core affects the outer core, which generates Earth's magnetic field, the lopsided melting would force scientists to rethink how Earth's magnetism works as well.
"There's a lot of unknowns, but it's definitely plausible," says Peter Olson, a geophysicist at Johns Hopkins University in Baltimore, Maryland. He notes that another model has tried using the mantle, the layer between the crust and the outer core, to make sense of the core's oddities. That hypothesis speculates that circulation in the mantle affects circulation in the outer core, which in turn affects the inner core in something like a chain reaction. The fact that Alboussière's model manages to explain things more simply is a plus, Olson says. "It's an Occam's razor. It has fewer moving parts."
The two hypotheses aren't mutually exclusive, geophysicist Michael Bergman of Bard College at Simon's Rock in Great Barrington, Massachusetts, points out. The inner core could be under the mantle's influence and be melting at the same time. Still, he says, "I think it's at this point the most plausible explanation we have."