TPO阅读单词题有excavate的是那篇TPO

TPO阅读单词题有excavate的是那篇TPO
TPO阅读单词题有excavate的是那篇TPO

TPO阅读单词题有excavate的是那篇TPO
TPO23
Meteorite Impact and Dinosaur Extinction陨石撞击和恐龙的灭绝
There is increasing evidence that the impacts of meteorites have had important effects on Earth, particularly in the field of biological evolution. Such impacts continue to pose a natural hazard to life on Earth. Twice in the twentieth century, large meteorite objects are known to have collided with Earth.
If an impact is large enough, it can disturb the environment of the entire Earth and cause anecological catastrophe. The best-documented such impact took place 65 million years ago at the end of the Cretaceous period of geological history. This break in Earth's history is marked by a mass extinction, when as many as half the species on the planet became extinct. While there are a dozen or more mass extinctions in the geological record, the Cretaceous mass extinction has always intrigued paleontologists because it marks the end of the age of the dinosaurs. For tens of millions of years, those great creatures had flourished. Then, suddenly, they disappeared.
The body that impacted Earth at the end of the Cretaceous period was a meteorite with a massof more than a trillion tons and a diameter of at least 10 kilometers. Scientists first identifiedthis impact in 1980 from the worldwide layer of sediment deposited from the dust cloud that enveloped the planet after the impact. This sediment layer is enriched in the rare metal iridium and other elements that are relatively abundant in a meteorite but very rare in the crust of Earth. Even diluted by the terrestrial material excavated from the crater, this componentof meteorites is easily identified. By 1990 geologists had located the impact site itself in the Yucat region of Mexico. The crater, now deeply buried in sediment, was originally about 200 kilometers in diameter.
This impact released an enormous amount of energy, excavating a crater about twice as large as the lunar crater Tycho. The explosion lifted about 100 trillion tons of dust into the atmosphere, as can be determined by measuring the thickness of the sediment layer formed when this dust settled to the surface. Such a quantity of material would have blocked the sunlight completely from reaching the surface, plunging Earth into a period of cold and darkness that lasted at least several months. The explosion is also calculated to have produced vastquantities of nitric acid and melted rock that sprayed out over much of Earth, startingwidespread fires that must have consumed most terrestrial forests and grassland. Presumably, those environmental disasters could have been responsible for the massextinction, including the death of the dinosaurs.
Several other mass extinctions in the geological record have been tentatively identified with large impacts, but none is so dramatic as the Cretaceous event. But even without suchspecific documentation, it is clear that impacts of this size do occur and that their results can be catastrophic. What is a catastrophe for one group of living things, however, may create opportunities for another group. Following each mass extinction, there is a suddenevolutionary burst as new species develop to fill the ecological niches opened by the event.
Impacts by meteorites represent one mechanism that could cause global catastrophes and seriously influence the evolution of life all over the planet. According to some estimates, themajority of all extinctions of species may be due to such impacts. Such a perspectivefundamentally changes our view of biological evolution. The standard criterion for the survivalof a species is its success in competing with other species and adapting to slowly changing environments. Yet an equally important criterion is the ability of a species to survive randomglobal ecological catastrophes due to impacts.
Earth is a target in a cosmic shooting gallery, subject to random violent events that were unsuspected a few decades ago. In 1991 the United States Congress asked NASA toinvestigate the hazard posed today by large impacts on Earth. The group conducting the study concluded from a detailed analysis that impacts from meteorites can indeed be hazardous. Although there is always some risk that a large impact could occur, careful study shows that this risk is quite small.
Paragraph 1: There is increasing evidence that the impacts of meteorites have had important effects on Earth, particularly in the field of biological evolution. Such impacts continue to pose a natural hazard to life on Earth. Twice in the twentieth century, large meteorite objects are known to have collided with Earth.
参考译文
陨石撞击和恐龙的灭绝
越来越多的证据表明陨石撞击对地球造成的影响特别严重,尤其是生物进化领域.这种影响对地球上的生物来说,仍然是一种自然威胁.在二十世纪,被人们所知悉的就有两次大型陨石撞地球的事件发生.
当撞击力度特别大时,陨石和地球的相撞将扰乱整个地球的环境,并引发一场生态灾难.记载最完整的这种灾难曾发生在6500万年前,在地质历史阶段的白垩纪末期.地球历史上的那次重大灾难造成了大量生物死亡,当时多达一半的物种灭绝.根据地质史料记载,地球上有十几次甚至更多大型的物种灭绝,古生物学家一直对白垩纪大灭绝非常感兴趣,因为它标志着恐龙时代的结束.数千万年以来,那些庞大的生物繁盛活跃着；然后突然间,它们消失了.
白垩纪末期,撞击地球主体是一块巨大陨石,它的质量超过万亿吨,直径至少在10公里.1980年科学家通过研究撞击后形成的、覆盖地球表面的尘雾沉积层,首次确认了这次撞击.该沉积层富含稀有金属铱和其他在陨石中含量较丰富但在地壳中却很稀有的元素.即便这些物质从陨石坑被挖掘出来后受到地球物质的稀释,其中的成分仍然很容易被鉴定出来.1990年,地质学家已经将那次撞击的地点定位于墨西哥的尤卡地区.现在被深埋于沉积物中的陨石坑,最初的直径为200公里左右.
这次撞击释放出了巨大的能量,凿出了一个特别大的陨石坑,约为月球第谷山口面积的两倍大.那场爆炸将大量尘埃扬起至大气层中,尘埃降至地面后逐渐形成沉积岩,通过测量沉积岩的厚度,我们可以判断当时的大气中的尘埃约有100万亿吨.如此大量的物质存在于大气层中,将会完全阻隔阳光的照射,于是地球一瞬间进入了一个持续数月之久的寒冷黑暗时期.据估计,爆炸过程还产生了大量的硝酸和被溶解的岩浆并喷出地球外,造成了大面积火灾,大部分的森林和草原被燃尽.这些环境灾难很可能就是包括恐龙在内的大规模物种灭绝的原因.
根据地质学记载,一些其他大批生物的灭绝也被试验性地认为与类似的撞击有关,但都没有白垩纪的那次触目惊心.不过即便是没有具体史料依据,这种规模的撞击显然发生过,并且带来了灾难性的后果.然而对于一个生物种群而言算是大灾难,却有可能给另一个种群创造了机会.每一次大灭绝过后,都会有新的物种爆炸式地进化去填补由灭绝造成的物种空缺.
陨石撞击描绘了一个能够制造全球性的灾难的途径,这种灾难会对整个星球的生命体的进化带来重大影响.据估计,已经灭绝的所有物中,绝大部分都是由于这些撞击造成的.这种观点从根本上改变了我们对于生物进化的看法.一个物种生存的标准准则就是它成功地与其他物种对抗,并适应缓慢变化的环境.然而还有一个同等重要的准则就是,它们可以从随机的、由天体撞击造成的全球生态灾难中幸存.
几十年前,地球是宇宙射击场的一个靶子,容易受到未知的随机暴力事件的攻击.1991年美国国会要求美国国家航空航天局调查大型撞击对地球造成的危害.指挥这项研究的团队通过详细的分析得出这样一个结论：陨石撞击确实是危险的.尽管大型撞击发生的风险依然存在,但是谨慎的研究这门也认为这种风险的出现几率非常小.
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