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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.
An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Asteroids and other large bodies from space have occasionally struck Earth, causing mass destruction and extinction.
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Answer Choices
(A) The large impactor that crashed in what is now the Arizona desert exploded with the force of a 20-megaton hydrogen bomb.
(B) Although the crash of a large impactor would have killed all living things near the site of impact, most animals living farther away adapted to changes in the environment.
(C) Scientists believe the reason the impactor that struck Earth 65 million years ago was so deadly was because it contained many toxic compounds, such as iridium.
(D) A worldwide sediment layer of iridium suggests that an asteroid or comet impacted Earth 65 million years ago, possibly causing the extinction of the dinosaurs.
(E) The large impact that caused the Yucatan crater probably produced both immediate natural disasters and a long-lasting layer of dust in the atmosphere.
(F) Despite the destructive effects of the large impact 65 million years ago, around 25 percent of Earth’s species survived and mammals came to be dominant.
选项
答案D,E,F
解析 本题属于文章总结题。D项“含有铱元素的沉积层遍布全球的情况说明，在6,500万年前，有小行星或彗星撞击过地球，而这可能是导致恐龙灭绝的原因”符合第2段的中心思想，即阿尔瓦雷茨父子根据铱元素广泛分布的情况和时间推测…恐龙灭绝的原因。E项“造成尤卡坦陨石坑的大型撞击可能引发了随即而来的自然灾害，导致了尘土在大气中长期存在”符合第3段的内容描述，其中，immediate natural disasters可体现在熔化岩基、引发碎片海、淹没1,000多千米内陆，以及很多北美洲物种在撞击后不久便灭绝等情况上；a long—lasting layer of dust体现在大量尘土在平流层停留数年、阻挡阳光、导致动植物灭绝等情况上。F项“尽管6,500万年前的大型撞击产生了毁灭性的影响，但是地球上仍有约25％的物种存活了下来，哺乳动物也占据了统治地位”是第4段的整合，为文章重要观点。A项“这个当时撞击了现在亚利桑那州沙漠的巨大天体产生了相当于20兆吨氢弹爆炸的威力”符合首段第2句的说法，该句作用是证明前面一句话给出的细节信息，并非文章的主要观点。B项“尽管大的撞击天体所产生的冲击足以杀死撞击点附近地区所有的生物，但大多数生活得较远的动物都作出了调整以适应环境变化”和C项“科学家们认为，6,500万年前撞击地球的天体如此致命足因为它里面有很多像铱元素这样的有毒成分”在原文均未被提及。
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