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  3. There are some earth phenomena you can count on, but the magnetic field, some say, is not one of them. It fluctuates in strength

There are some earth phenomena you can count on, but the magnetic field, some say, is not one of them. It fluctuates in strength

admin2012-09-10  7
问题 There are some earth phenomena you can count on, but the magnetic field, some say, is not one of them. It fluctuates in strength, drifts from its axis, and every few 100,000 years undergoes 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 metres of deep sea sediment (沉淀物) 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 short tern, 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 (熔化的) 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 identify 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 (陨星) 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 nest 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.
选项 A、Polarity Reversal :A Fantastic Phenomenon of Nature
B、Measurement of the Earth’s Magnetic-Field Intensity
C、Formation of the Two Poles of the Earth
D、A New Approach to the Study of Geophysics
答案D
解析 题目询问这篇作文最适当的标题是什么。通读全文就会发现作者主要介绍的是对地球磁极相互转化这一现象的研究。D项选择，地球物理研究的新方法，文章最后提到如果法国科学家的研究确实可行，那将为未来的研究指明方向。这才是作者主要写作意图。
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