2015年職稱英語考試綜合類：閱讀材料六

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　　遠古火山噴發(fā)

　　古代的火山更具殺傷力，不是因為它們更大，而是因為它們釋放出來的二氧化碳更容易把人殺死。

　　來自里茲大學的Paul Wignall正在調查火山爆發(fā)和大規(guī)模物種滅絕之間的聯(lián)系。不是所有的火山都會造成大量動物死亡。但在過去的三億年間，巨大的火山巖石的形成都伴隨著大規(guī)模種族滅絕。讓他驚奇的是，遠古的火山造成的破壞更大。他通過比較火山釋放出的熔巖的體積與殺死生命的比例計算這些火山的殺傷效力。他發(fā)現(xiàn)，體積相同的情況下，老遠古火山要比近期火山的殺傷力大十倍。

　　例如，發(fā)生在兩億五千年前的二疊紀物種滅絕就與西伯利亞的火山爆發(fā)發(fā)生在同一時期。當時的火山巖大約覆蓋了西歐那么大的面積。當時火山釋放出了大約100億噸梯恩梯的二氧化碳，隨之而來的全球變暖殺死了80%的海洋生物。500萬年以后，地球才恢復到原來的狀態(tài)。然而，6000年前，火山大規(guī)模爆發(fā)，以及而后的全球變暖并沒有帶來種族滅絕。一些動物確實消失了，但幾萬年以后，地球又恢復到以前的水平。“最近的一次火山爆發(fā)基本上沒帶來什么影響?！盬ignall稱。他并沒有將650萬年前的恐龍滅絕計算在內，因為大部分的科學家認為那是有小行星撞擊地球所引起的。他認為，遠古火山之所以殺傷力大是因為近期的生物能夠更好的適應二氧化碳含量上升。

　　法國巴黎地理研究所的所長Vincent Courtillot稱Wignall的觀點很具爭議性，并且很難做出這樣的計算。他指出火山爆發(fā)的殺傷力是由其持續(xù)長度所決定的。并且很難知道是否大規(guī)模爆發(fā)會持續(xù)上百萬年。他補充道，很難計算遠古火山灰噴發(fā)多少火山巖，并且火山巖的數(shù)量也不一定和二氧化碳釋放數(shù)量相一致。

　　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 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 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 of carbon as carbon dioxide. The global warming that followed wiped out 80 per cent of all marine genera 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 warming 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. 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.

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