Word: earthly
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Dates: during 1980-1989
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...earth science began in 1981, when scientists learned that planet-wide vibrations resulting from earthquakes deep within the earth are split into a complex system of overlapping "tones." The implication: there is something going on in the core that no one had previously suspected. Recalls John Woodhouse, a colleague of Dziewonski's at Harvard: "It was the beginning of a new wave of attention to the core...
...surface. Explains Dziewonski: "If it's a faulty lens, you're going to have a wrong image." By 1984 the Harvard group had assembled the first detailed map of the mantle ever published. Their data consisted of the patterns of earthquake-generated pressure waves that passed through the solid earth, moving faster through cooler regions of the mantle and more slowly through warmer areas...
...Harvard group found, for example, that pressure waves travel more quickly when moving parallel to the earth's axis than when they are perpendicular. That could be explained if the solid inner core were a crystal, in which waves would travel at different speeds along different axes, but molten iron is hardly crystalline. Instead, Don Anderson and his colleagues at Caltech's seismological lab postulated the existence of iron rain. Their theory: the polar regions of the core are slightly flattened and tend to be cooler than the equatorial regions. The heat exchange between the two areas may then result...
That scenario could illuminate more than just the confusing differences in the velocity of pressure waves; it could also help explain the mystery of the origin of the earth's magnetic field, as well as its unexplained reversals. About once every million years, magnetic north and south inexplicably exchange places. Scientists do not understand whether this phenomenon comes about gradually, say, over thousands of years, or all at once. One idea, advanced in recent years, is that turbulent eddies within the core-mantle boundary somehow give rise to electromagnetic disturbances that trigger the reversals. A rain of iron particles...
...have advanced enough to give scientists a reasonably consistent idea of how the overall picture fits together. Says Subir Banerjee, a geophysicist at the University of Minnesota: "In every discipline, our measuring capabilities have gone up so much that we are at last able to home in on the earth's core by a number of techniques." For scientists who have long struggled to penetrate the mysteries at the center of the earth, solving the puzzle now seems within reach...