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  3. The collapse of the Earth’s magnetic field—which guards the planet and guides many of its creatures—appears to have started seri

The collapse of the Earth’s magnetic field—which guards the planet and guides many of its creatures—appears to have started seri

admin2012-02-27  45
问题     The collapse of the Earth’s magnetic field—which guards the planet and guides many of its creatures—appears to have started seriously about 150 years ago, the New York Times reported last week.
    The field’s strength has decreased by 10 or 15 per cent so far and this has increased the debate over whether it signals a reversal of the planet’s lines of magnetic force.
    During a reversal, the main field weakens, almost vanishes, and reappears with opposite polarity. The transition would take thousands of years. Once completed, compass needles that had pointed north would point south.  A reversal could cause problems for both man and animals.  Astronauts and satellites would have difficulties. Birds, fish and animals that rely on the magnetic field for navigation would find migration confusing. But experts said the effects would not be a big disaster, despite claims of doom and vague evidence of links between past field reversals and species extinctions.
    Although a total transition may be hundreds or thousands of years away, the rapid decline in magnetic strength is already affecting satellites. Last month, the European Space Agency approved the world’s largest effort at tracking the field’s shifts. A group of new satellites, called Swarm, is to monitor the collapsing field with far greater precision. "We want to get some idea of how this would evolve in the near future, just like people trying to predict the weather," said Gauthier Hulot, a French geophysicist working on the satellite plan. "I’m personally quite convinced we should be able to work out the first predictions by the end of the mission."
    No matter what the new findings, the public has no reason to panic. Even if a transition is coming on its way, it might take 2 000 years to mature. The last one took place 780 000 years ago, when early humans were learning how to make stone tools. Deep inside the Earth flow hot currents of melted iron. This mechanical energy creates electromagnetism. This process is known as the geophysical generator. In a car’s generator, the same principle turns mechanical energy into electricity.
    No one knows precisely why the field periodically reverses.  But scientists say the responsibility probably lies with changes in the disorderly flows of melted iron, which they see as similar to the gases that make up the clouds of Jupiter.
The transition of the Earth’s magnetic field is possibly caused by ______.
选项 A、the flows of melted iron inside the Earth
B、the periodical movement of the Earth
C、the mechanical energy of the solar system
D、the force corning from outer space
答案A
解析 文章最后一段：谁也不能准确地知道为什么地球磁场会周期性转换，但是科学家说其原因可能在于地壳中融化了的铁的无序流动，它们看起来就像木星上组成云的气体一样。
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