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  3. Older Volcanic Eruptions Volcanoes were more destructive in ancient history, not because they were bigger, but because the c

Older Volcanic Eruptions Volcanoes were more destructive in ancient history, not because they were bigger, but because the c

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问题                          Older Volcanic Eruptions
    Volcanoes were more destructive in ancient history, not because they were bigger, but because the carbon dioxide they released wiped out life with greater ease.
    Paul Wignall from the University of Leeds was investigating the link between volcanic eruptions and mass extinctions. Not all volcanic eruptions killed off large numbers of animals, but all the mass extinctions over the past 300 million years coincided with huge formations of volcanic rock. To his surprise, the older the massive volcanic eruptions were, the more damage they seemed to do. He calculated the " killing efficiency" for these volcanoes by comparing the proportion of life they killed off with the volume of lava(熔岩)that they produced. He found that size for size, older eruptions were at least 10 times as effective at wiping out life as their more recent rivals.
    The Permian(二叠纪)extinction, for example, which happened 250 million years ago, is marked by floods of volcanic rock in Siberia that cover an area roughly the size of western Europe. Those volcanoes are thought to have pumped out about 10 gigatonnes(十亿吨)of carbon as carbon dioxide. The global warming that followed wiped out 80 percent of all marine genera(种类)at the time, and it took 5 million years for the planet to recover. Yet 60 million years ago, there was another huge amount of volcanic activity and global wanning but no mass extinction. Some animals did disappear but things returned to normal within ten thousands of years. "The most recent ones hardly have an effect at all. " Wignall says. He ignored the extinction which wiped out the dinosaurs 65 million years ago, because many scientists believe it was primarily caused by the impact of an asteroid(小行星). He thinks that older volcanoes had more killing power because more recent life forms were better adapted to dealing with increased levels of CO2.     Vincent Courtillot, director of the Paris Geophysical Institute in France, says that Wignall’ s idea is provocative. But he says it is incredibly hard to do these sorts of calculations. He points out that the killing power of volcanic eruptions depends on how long they lasted. And it is impossible to tell whether the huge blasts lasted for thousands or millions of years. He also adds that it is difficult to estimate how much lava prehistoric volcanoes produced, and that lava volume may not necessarily correspond to carbon dioxide emissions.
"Killing efficiency" is calculated by comparing______.
选项 A、the span of time volcanoes lasted
B、the volume of lava volcanoes produced
C、the number of living things volcanoes killed off
D、the proportion of life wiped out with the volume of lava
答案D
解析 根据第二段第四句可知，杀伤效率是通过比较火山释放出的熔岩的体积与杀死生命的比例来计算出来的。
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