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

游客2024-03-04  5
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?
Research by two French geophysicists promises to shed some light on the mystery. Using 80 meters of deep sea sediment (沉淀物), 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 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 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 ago. 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 that if the French researchers are right. In fact, Olson says intensity that predictably declines from one reversal to the next contradictions 90 percent of the model 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.
Passage Two
选项 A、regularly changes its strength and axis
B、is relatively more difficult to conduct research on
C、is dynamic and constantly changing
D、is usually analyzed by deep sea sediment
答案 C
解析 本题问“磁场是一种怎样的地理现象?”文章第一句谈到，There are some earth phenomena you can count on，but the magnetic field，some say，is not one of them. 也就是说，许多地球现象你都可以认为是可依赖的稳定现象，但是磁场不是其中之一。随后说到两位法国学者发现了其长期的规律性，但是磁场始终是变化的。故[C]“是不稳定而且不断变化的”正确。[A]“有规律地改变强度和两极”。文中第二段说法国学者发现了长期规律性，但短期内仍没有规律，题项说法过于绝对。[B]“相对来说很难进行研究”。文章并没有提到对磁场做研究的难度，本项用conduct on一词迷惑考生，考生需要清楚该词的含义。[D]“经常用深海沉淀物来研究”。文中第二段第二句说两位学者用深诲沉淀物来做研究，但文中并没有提所有有关磁场的研究都用深海沉淀物。本项属于过度引申。
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