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  3. IMPACTS AND MASS EXTINCTIONS (1) Meteorites and impact craters bear witness to the fact that large impacts occasionally occu

IMPACTS AND MASS EXTINCTIONS (1) Meteorites and impact craters bear witness to the fact that large impacts occasionally occu

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问题                                         IMPACTS AND MASS EXTINCTIONS
    (1) Meteorites and impact craters bear witness to the fact that large impacts occasionally occur on Earth. Barringer Meteor Crater in the northern Arizona desert of the United States formed about 50,000 years ago when a metallic impactor roughly 50 meters across crashed to Earth with the explosive power of a 20-megaton hydrogen bomb. Although the crater is only slightly more than one kilometer across, an area covering hundreds of square kilometers was probably battered by the blast and ejecta—the debris ejected or displaced during the formation of an impact crater. Far bigger impacts have occurred, sometimes with catastrophic consequences for life on Earth.
    (2) While collecting geological samples in Italy in 1978, the father-son team of Luis and Walter Alvarez discovered a thin layer of dark sediment that had apparently been deposited 65 million years ago—at about the same time that the dinosaurs and many other organisms suddenly became extinct. Subsequent studies found similar sediment deposited at the same time at many sites around the world. Careful analysis showed this worldwide sediment layer to be rich in iridium, an element that is rare on Earth’s surface. But iridium is common in primitive meteorites, which led the Alvarezes to a stunning conclusion: the extinction of the dinosaurs was caused by the impact of an asteroid or comet. This conclusion was not immediately accepted and still generates some controversy, but it now seems clear that a major impact coincided with the death of the dinosaurs. While the dinosaurs were the most famous victims of this mass extinction, it seems that up to 99 percent of all living things were killed and that 75 percent of all species living on Earth were wiped out at that time.
    (3) How could an impact lead to mass extinction? The amount of iridium deposited worldwide suggests that the impactor must have been about 10 kilometers across. After a decade-long search, scientists identified what appears to be the impact crater from the event. Located off the coast of Mexico’s Yucatan peninsula, it is 200 kilometers across, which is close to what one would expect for a 10-kilometer impactor, and dates to 65 million years ago. Further evidence that the Yucatan crater is the right one comes from the distribution of small glassy spheres that formed when the molten impact ejecta solidified as it rained back to Earth. [A] More of these glassy spheres are found in regions near the crater, and careful study of their distribution suggests that the impactor crashed to Earth at a slight angle. [B] These pieces of once molten rock are evidence of an explosion powerful enough to instantly melt bedrock and propel it far from its origin. [C] The impact almost immediately sent a shower of debris raining across much of North and South America and generated huge waves that may have sloshed more than 1,000 kilometers inland. [D] Many North American species thus may have been wiped out shortly after impact. For the rest of the world, death may have come more slowly. Heat from the impact and returning ejecta probably ignited wildfires in forests around the world. Evidence of wildfires is found in the large amount of soot (a black powdery form of carbon produced when coal, wood, or oil is burned) that is also present in the iridium-rich sediment from 65 million years ago. The impact also sent huge quantities of dust high into the stratosphere, where it remained for several years, blocking out sunlight, cooling the surface, and affecting atmospheric chemistry. Plants died for lack of sunlight, and effects propagated throughout the food chain.
    (4) Perhaps the most, astonishing fact is not that 75 percent of all species died, but that 25 percent survived. Among the survivors were a few small, rodent-like mammals. These mammals may have survived because they lived in underground burrows and managed to store enough food to outlast the long spell of cold, dark days. Small mammals had first arisen at about the same time as the dinosaurs, more than 100 million years earlier. But the sudden disappearance of the dominant dinosaurs made these mammals dominant.
In paragraph 2, the author discusses the presence of iridium in the sediment layer discovered by the Alvarezes because ________.
选项 A、it supported their claim that toxic levels of iridium could have killed the dinosaurs
B、it led some scientists to question the validity of the Alvarezes’ study
C、it led the Alvarezes to conclude that a space impactor killed the dinosaurs
D、it showed that new discoveries were still being made about the extinction of the dinosaurs
答案C
解析 本题属于修辞目的题，问在第2段中，作者为什么要提到阿尔瓦雷茨父子发现沉积层中存在铱元素。原文第2段第3、4句提到，阿尔瓦雷茨父子发现沉积层中富含铱元素，而该元素在原始陨石中很常见，于是他们得出结论：小行星或彗星的撞击导致了恐龙的灭绝。由此推断，作者提到他们发现铱元素是为了说明他们是如何得出这个结论的，故选C项。A项“这支持了阿尔瓦雷茨父子关于铱的毒素水平导致恐龙灭绝的观点”没有依据。B项“这使得一些科学家质疑阿尔瓦雷茨父子研究的可靠性”，第2段没有提到关于阿尔瓦雷茨父子研究的争议与铱元素相关。D项“这表明仍不断有关于恐龙灭绝的新发现出现”，文章一直在谈论的就是恐龙或许因为天体撞击而灭绝这个发现。作者提到阿尔瓦雷茨父子的研究也只是为了引入这个话题。并且，本段前后都没有再提到其他关于恐龙灭绝的新发现。因此，该项的逻辑不合理，不是作者的修辞目的。
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