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Passage III:
Nothing is sure but death and taxes, and of course that north is north and south is south, and thus it has always been, so they say. But they'd be wrong. You can perhaps be sure about death and taxes, but you might want to reconsider the rest of it. In fact, at many times in our planet's history, north has become south and south has become north, in a process called magnetic reversal. 

Paleogeologists have discovered the existence of these mysterious phenomena(in a field study known as paleomagnetism) by investigating rocks. When rocks are being formed from magmas, atoms within their crystals respond to the earth's magnetic field by "pointing" towards the magnetic north people. By age dating the rocks and nothing their magnetic alignment, scientists can determine where on earth the north pole was located at that time because as the rocks solidified, they trapped that information within them. The study of ancient lava flows has revealed that at certain periods in the earth's history magnetic north was directly opposite its presentlocation. In fact, it has been determined that the north/south reversal has occurred on average every 500,000 years and that the last reversal took place about 700,000 years ago. Scientists call those periods of "normal" polarity (the magnetic orientation of our modern era) and “reversed” polarity (the magnetic orientation of reverse situation) by the name "magnetic chrons." 

Although the fact of such reversals is clear, why and how they happen and their effects on the planet are subjects of considerable debate. Because no one knows precisely how the earth's magnetic field is produced, it becomes difficult to say how it might be reversed. Among explanations proposed are a reversal of the direction of convection currents in the liquid outer core of the earth and a collision between the earth and a meteorite or comet. And while the precise effects of a reversal are not known, there can belittle doubt that the earth would receive during the process a great deal more damaging ultraviolet radiation than it now does and that such occurrences have been correlated with the extinction of certain species in the geologic past.

Question 130

The main purpose of the passageis to


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