What evidence do we have to show that the iron inner core has flipped poles every 10 million years
The Cadre of the Earth May Be a Gigantic Crystal Made of Iron
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Apr 4, 1995
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NEW clues and theories are illuminating one of the most inaccessible and enigmatic parts of the planet, its inner core, suggesting that this mass of solid fe has a crystalline structure that may exist responsible for a serial of magnetic riddles observed at the earth'due south surface.
By weight, the earth is more often than not fe. But relatively little of the metallic is constitute at its surface, which is made principally of lighter elements like oxygen and silicon. Billions of years ago, during the planet'south fiery birth, most of the iron sank to the planet'southward deep interior because of its enormous weight and eventually formed a core there, or really two of them.
The outer core is molten fe. Its churnings are thought to be responsible for the earth'due south magnetic field.
Deeper yet, pressures and densities go so monumental that the atomic number 26 is solid despite temperatures believed to be in backlog of 7,000 degrees Fahrenheit. This solid inner cadre is nearly 1,500 miles broad and makes up less than 1 per centum of the earth's volume. Science has long considered it obscure, featureless and having little or no appreciable impact on the planet.
Merely new studies suggest otherwise. This mass of metal appears to have a texture or grain much as forest does. Going further, some experts say it may be a solid crystalline structure of gigantic proportions.
"My hypothesis is that information technology's like a diamond in the heart of the world, simply one unmarried crystal," Dr. Ronald E. Cohen, a geophysicist at the Carnegie Institution of Washington who is using supercomputers to model the core's structure, said in an interview.
The texture of the inner core is seen as explaining mysteries of the world's magnetic field, like the peculiar skewing of its field lines. Another insight concerns the beliefs of the magnetic poles, which every half million years or and then reverse positions and appear to linger mysteriously in certain parts of the earth during the transition.
"What's going on is quite interesting," said Dr. Jeroen Tromp, an earth scientist at Harvard University. "Parts of the puzzle are coming together. In that location'due south still lots of questions. Just what'south exciting is that so many aspects of the earth sciences" are pooling skills to try to fathom the core's structure and effects.
The principal way of studying this out-of-the-mode region is with sensors that option up faint vibrations in the ground, which allow scientists to map the paths and speeds of shock waves that radiate out from big earthquakes the fashion ripples cross a pond.
Recently, after a decade of study, geologists who rails such seismic waves take agreed that something odd is going on: waves traveling through the earth'south inner core along a north-due south path consistently get faster than those traveling east-westward. The deviation in speed is about four seconds.
Terminal year, a large earthquake 395 miles beneath Bolivia sent out a series of powerful shock waves deep into the world. And one time over again, analysis of the upheaval is confirming the disparity.
"We're seeing pretty stiff bear witness" of a speed difference, said Dr. T. Guy Masters, a seismologist at the University of California at San Diego who is analyzing such information.
Scientists believe the cause of the inconsistency lies in the makeup of the core. Such directional differences, known as anisotropies, are common in everyday materials and often involve such physical properties every bit elasticity or electrical conductivity. In the case of the world's inner core, the directional differences of elastic waves are seen every bit offering clues to its composition.
A paper in the electric current upshot of the journal Science provides the best window yet on the mystery. Dr. Cohen of the Carnegie Institution and a colleague, Dr. Lars Stixrude of the Georgia Institute of Technology, did a series of calculations in which they explored the behavior of various forms of iron that might be under the enormous temperatures and pressures found at the middle of the globe. Inner-cadre pressures are more than 3 million times greater than those at the earth's surface.
The squad, financed by the National Science Foundation, worked with a Cray supercomputer at the Pittsburgh Supercomputer Center. Their calculations were based on a physics tool known equally Density Functional Theory, which uses mathematics to model the electronic structure of materials and derive from them all kinds of observable properties.
Fe can class crystals of three known structures. 1 that the squad investigated is known as trunk-centered cubic, a simple form plant at the globe's surface in which a single atomic number 26 atom is surrounded by eight others to make up a cube. It was apace eliminated when the team institute that it was mechanically unstable at high pressure level.
Another form is confront-centered cubic, a type in which a single iron atom is surrounded past 12 neighbors and endlessly repeats this unit in a cubic motif. The structure was found to be stable at the enormous pressures and to have directionality in the transmission of elastic waves. Simply the magnitude of directionality failed to fit the seismic data.
The final form is close-packed hexagonal. Information technology too has a single iron atom surrounded by a dozen neighbors, just in this instance the unit design is a hexagonal prism.
The team found that the elastic discrepancies of this structure fit the seismic bear witness quite nicely.
A surprise, yet, was that to conform to the observed information the close-packed structures would accept to exist nearly perfectly aligned in the largest such alignment ever found in nature. "The very strong texturing indicated by our results suggests the possibility that the inner core is a very big unmarried crystal," the pair wrote in the Science article.
The crusade of such a massive alignment remains a mystery, although the team speculated that the high temperatures and ho-hum growth rates of the inner core "may prove ideal for growing single crystals much larger than those observed in the near-surface environment."
The single-crystal theory is aiding enquiry on how a textured inner core might generate magnetic furnishings at the earth's surface. Early this year, Dr. Bradford Thousand. Cloudless of Florida International University in Miami joined forces with Dr. Stixrude to lay out a comprehensive theory in Earth and Planetary Science Letters, a scholarly journal.
The pair assumed that directional differences in the core's elasticity mirrored variations in its electrical conductivity and electromagnetic behavior, including its magnetic susceptibility. Such clusters of anisotropies are not uncommon. Mica and graphite, for instance, showroom them.
If the powerful magnetic fields of the outer cadre take magnetized the inner one, the scientists theorized, then the inner core might take its own magnetic field, probably with a different management from that of the outer one. And the inner core'south magnetic field, they say, may exist the hidden hand backside a series of surface effects that have long baffled scientists.
One enigma concerns the lines of force of the earth'south magnetic field, both today and in the afar by. If the earth were a perfect bar magnet, sending out its fields in long arcs, so the field lines at the magnetic equator should parallel the earth's surface. Simply they do non. They show a persistent four-degree tilt.
Dr. Stixrude and Dr. Clement suggest that the inner-cadre field is oriented in such a way as to throw off the main field from its predicted country.
Another riddle and possible explanation concerns the reversal of the earth'south north and due south magnetic poles, which has occurred repeatedly over the ages. During some reversals, the wandering poles seem to linger for thousands of years near the southern cease of South America and around Western Australia.
The scientists propose that during the reversal, the main magnetic field is weakened and that this weakening allows the inner core's field to take on new prominence. The odd lingering results when the inner field captures the outer 1 and forces it to correspond with its ain poles.
"It hangs together very neatly," Dr. Cloudless said of the new theory. "It shows that i very uncomplicated process can explain longstanding problems we've had with the behavior of the earth'southward magnetic field."
The rush of new theories and calculations makes some experts charily optimistic that the inner core is finally surrendering its secrets, while other experts are but cautious. They say more than theoretical crude edges volition have to be smoothed out and more seismic evidence gathered earlier the new piece of work gains wide acceptance.
Simply Dr. Stixrude noted in an interview that further evidence was already being gathered as seismologists searched for new clues. The theory predicts that stupor waves transmitted through a big inner crystal should have a subtle repercussion in the course of a particular blazon of faint moving ridge measured at the earth'southward surface.
"A lot of people are looking for it," said Dr. Stixrude. "We've gotten a lot of seismologists excited. It's a relatively weak stage, which is why they hadn't seen information technology before. But in principle it's observable."
If the faint wave were eventually institute, he added, "that would be a very strong confirmation" that something resembling a giant crystal lies buried at the heart of the globe.
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