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New insights into center of the Earth (8/18/2008)
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| Model of seismic waves travelling through the Earth - Photo by James Wookey |
A new observation of the very deepest part of the Earth, the solid inner core, has been reported this week in Nature. The team from the University of Bristol also observed intriguing evidence of a 'texture' in the solid iron that may reflect the patterns left as the swirling liquid iron of the outer core freezes to form the inner core.
Researchers at the University of Bristol have measured 'PKJKP' - an elusive seismic wave which traverses the Earth's solid inner core - with greater precision than ever before. This was achieved using Hi-net, an array of over 750 seismometers which span the Japanese islands and are designed to provide earthquake warnings.
Dr James Wookey, lead author on the study, said: "The Earth's solid inner core started to form over a billion years ago when the liquid iron core that surrounds it became cool enough to begin solidifying. It has now grown to a radius of around 1200 km. But the difficulty in studying it is the thousands of kilometres of rock in the way. As a result, we know more about the surfaces of other planets than we do about the deep interior of our own. However, we have a powerful tool for studying the inner architecture of the Earth."
When large earthquakes or powerful explosions occur, vibrations (like sound waves) travel throughout the Earth and seismometers can detect these 'seismic waves' at great distances, even on the opposite side of the planet. By studying the characteristics of these waves (eg, how long they take to reach us, how strong they are) we can infer many things about the deep Earth.
The most recent advances in seismology have come from simultaneous observations of these waves using large numbers of instruments in dense networks. At present, the biggest of these arrays is the Japanese Hi-net array. This network, built for providing earthquake warnings, consists of over 750 seismometers across all of the Japanese islands, each installed in a borehole between 100m and 2km deep.
George Hefflich, Professor of Seismology at the University of Bristol, and also an author on the paper, said: "A side benefit of this network is that the data can also be used to study the deep Earth, looking for extremely faint vibrations passing through the very centre of the planet. One such subtle signal is the seismic wave called 'PKJKP'. The properties of this wave contain a wealth of information about the Earth's inner core".
As well as showing direct evidence of the solidity of the Earth's inner core, the team observed evidence of 'texturing' of the iron of the inner core. This may reflect the patterns left as the swirling liquid iron of the outer core freezes.
More observations like this will allow seismologists to start to understand these patterns, and what they imply for how the very deepest part of our planet has changed throughout the Earth's history.
The Earth has been cooling since its birth from the fiery debris of the early Solar System four-and-a-half billion years ago. Earthquakes, volcanoes, the movement of continents, and the Earth's magnetic field are all, ultimately, expressions of convection caused by heat being transported from the hot interior to the cooler exterior. At the very centre of the Earth is the most striking evidence of this cooling â€" the frozen inner core.
Note: This story has been adapted from a news release issued by Bristol University
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Comments:
| 1. |
Bob |
9/1/2008 9:21:12 AM MST |
I cannot understand how the frozen inner core can be "frozen" or cooler than the surrounding outer core of molten rock. Despite the inferred findings I would have to say there must be another possible interpretation of the scientists' findings which they have not discovered because of their prejudice toward their current interpretation of the observations. It wouldn't be the first time such a thing has occurred in science and won't be the last. For their interpretation of the observations to be true and given the "inferred" age of the earth the inner core would have had to have been below absolute zero to exist as a cooler solid now. In any event, since there can be no eyewitnesses to any of the subjects of this analysis any conclusions can only be hypotheticals at best and really of no consequence to be publicized except as fodder to cast doubt on scientific inferences through technological analysis when better technology or observations can be obtained in the future that may refute these conclusions, which has happened before and therefore heightens my skepticism of articles like this. |
| 2. |
Chris |
9/18/2008 10:52:13 PM MST |
The inner core doesn't have to be cooler to be solid. Most materials expand when they melt, including iron--if the temperature is exactly equal to the melting point, the solid bits will sink in the liquid (just the opposite of ice floating on water, since ice shrinks when it melts).
Additionally, large amounts of pressure will change the melting point of the material, so the iron closer to the surface has a slightly lower melting point than the iron in the very middle of the earth.
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| 3. |
Tim |
11/17/2008 6:07:47 PM MST |
http://www.sciencedaily.com/releases/2004/01/040122083025.htm
This site gives some info on the temperature-pressure relationships of iron in the core.
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