Mass extinction（据说是托福tpo，求做过的同学进来验证）

SWens

Mass Extinctions+ S5 q  |  |2 e
Cases in which many speciesbecome extinct within a geologically short interval of time are called mass extinctions.There was one such event at the end of theCretaceous period (around 70 million years ago). There was another, evenlarger, mass extinction at the end of thePermian period (around 250 million years ago). The Permian event hasattracted much less attention than other mass extinctions because mostly unfamiliarspecies perished at that time.

The fossil record shows atleast five mass extinctions in which many families of marine organisms diedout. The rates of extinction happening today are as great as the rates duringthese mass extinctions. Many scientists have therefore concluded that a sixthgreat mass extinction is currently in progress.

What could cause such highrates of extinction? There are several hypotheses, including warming or coolingof Earth, changes in seasonal fluctuations or ocean currents, and changingpositions of the continents. Biological hypotheses include ecological changesbrought about by the evolution of cooperation between insects and floweringplants or of bottom-feeding predators in the oceans. Some of the proposedmechanisms required a very brief period during which all extinctions suddenlytook place; other mechanisms would be more likely to have taken place moregradually, over an extended period, or at different times on differentcontinents. Some hypotheses fad to account for simultaneous extinctions on landand in the seas. Each mass extinction may have had a different cause. Evidencepoints to hunting by humans and habitat destruction as the likely causes forthe current mass extinction.
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American paleontologists David Raup and JohnSepkoski, who have studied extinction rates in a number of fossil groups,suggest that episodes of increased extinction have recurred periodically,approximately every 26 million years since the mid-Cretaceous period. The lateCretaceous extinction of the dinosaurs and am monoids was just one of the moredrastic in a whole series of such recurrent extinction episodes. Thepossibility that mass extinctions may recur periodically has given rise to suchhypotheses as that of a companion star with a long-period orbit deflectingother bodies from their normal orbits, making some of them fall to Earth asmeteors and causing widespread devastation upon impact.' L& S8 `. ]) J; K  p& }
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Of the various hypothesesattempting to account for the late Cretaceous extinctions, the one that hasattracted the most attention in recent years is the asteroid-impact hypothesisfirst suggested by Luis and Walter Alvarez. According to this hypothesis, Earthcollided with an asteroid with an estimated diameter of 10 kilometers, or withseveral asteroids, the combined mass of which was comparable. The force ofcollision spewed large amounts of debris into the atmosphere, darkening theskies for several years before the finer particles settled. The reduced levelof photosynthesis led to a massive decline in plant life of all kinds, and thiscaused massive starvation first of herbivores and subsequently of carnivores.The mass extinction would have occurred very suddenly under this hypothesis.

One interesting test of theAlvarez hypothesis is based on the presence of the rare-earth element iridium(Ir). Earth' s crust contains very little of this element, but most asteroidscontain a lot more. Debris thrown into the atmosphere by an asteroid collisionwould presumably contain large amounts of iridium, and atmospheric currentswould carry this material all over the globe. A search of sedimentary depositsthat span the boundary between the Cretaceous and Tertiary periods shows thatthere is a dramatic increase in the abundance of iridium briefly and preciselyat this boundary. This iridium anomaly offers strong support for the Alvarez hypothesiseven though no asteroid itself has ever been recovered.





这个是网上百度了一下摘抄下来的，据说应该是这个的缩略版本，大家可以参考一下。。。

790

说是“跟托福tpo里面一篇阅读几乎一样，就是缩减版”. 能把托福那篇找出来吗?

SWens

说是“跟托福tpo里面一篇阅读几乎一样，就是缩减版”. 能把托福那篇找出来吗?

-- by 会员 790 (2012/6/12 12:27:13)

不知道这个是不是就是托福tpo的那篇，我找的时候是按照那篇找的。。。
但是还是求做过的同学给个验证·····

秋晨小仔

TPO15原文粘给你哈~今天正好瞄见~
                                                                             Meteorite Impact and Dinosaur Extinction
   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 an ecological 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 mass of more than a trillion tons and a diameter of at least 10 kilometers. Scientists first identified this 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 component of 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 vast quantities of nitric acid and melted rock that sprayed out over much of Earth, starting widespread fires that must have consumed most terrestrial forests and grassland. Presumably, those environmental disasters could have been responsible for the mass extinction, 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 such specific 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 sudden evolutionary 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, the majority of all extinctions of species may be due to such impacts. Such a perspective fundamentally changes our view of biological evolution. The standard criterion for the survival of 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 random global 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 to investigate 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.

秋晨小仔

TPO原文粘给你哇~

安然向人

快来狗主验证呀~~~~~~~~~~~~~··