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  3. There are some earth phenomena you can count on, but the magnetic field, som

There are some earth phenomena you can count on, but the magnetic field, som

游客2025-04-12  26
问题     There are some earth phenomena you can count on, but the magnetic field, someday is not of them. It fluctuates in strength, drifts from its axis, and every few 100,000 years undergo a dramatic polarity reversal—a period when north pole becomes south pole and south pole becomes north pole. But how is the field generated, and why is it so unstable?
    Groundbreaking research by two French geophysicists promises to shed some light on the mystery. Using 80 meters of deep sea sediment core, they have obtained measurements of magnetic-field intensity that span 11 polarity reversals and four million years. The analysis reveals that intensity appears to fluctuate with a clear, well-defined rhythm. Although the strength of the magnetic field varies irregularly during the shout term, there seems to be an inevitable long term decline preceding each polarity reversal. When the poles flip—a process that takes several hundred thousand years—the magnetic field rapidly regains its strength and the cycle is repeated.
    The results have caused a stir among geophysicists. The magnetic field is thought to originate from molten iron in the outer core, 3,000 kilometers beneath the earth’s surface. By studying mineral grains found in material ranging from rocks to clay articles, previous researchers have already been able to i-dentify reversals dating back 170 million years, including the most recent switch 730,000 years age. How and why they occur, however, has been widely debated. Several theories link polarity flips to external disasters such as meteor impacts. But Peter Olson, a geophysicist at the Johns Hopkins University in Baltimore, says this is unlikely if the French researchers are right. In fact, Olson says intensity that predictably declines from one reversal to the next contradicts 90 percent of the models currently under study. If the results prove to be valid, geophysicists will have a new theory to guide them in their quest to understand the earth’s inner physics. It certainly points the direction for future research. [br] In Peter Oslo’s opinion, the French experiment____.
选项 A、is likely to direct further research in the inner physics of the earth
B、has successfully solved the mystery of polarity reversals
C、is certain to help predict external disasters
D、has caused great confusion among the world’s geophysicists
答案 D
解析 四个选项中，只有D项为正确答案，这可从文中的“In fact，Olson says intensity that predictablydeclines from one reversal to the next contradicts 90 percent of the models currently under study．”推知，即奥尔森说，实际上两次转换间预计的衰落强度同90％的现行研究模式相矛盾。
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