托福阅读真题+题目+答案：Iridium and the Terminal Cretaceous Event

托福阅读真题+题目+答案：Iridium and the Terminal Cretaceous Event

There have been a number of natural catastrophes in Earth's past that wiped out huge numbers of plant and animal species- -in effect,"resetting the clock" and requiring a new start for much of life on Earth. One such catastrophe took place at the end of the Permian era, some 251 million years ago. But perhaps the most famous one occurred about 65 million years ago at the end of the Cretaceous, causing the dinosaurs to go extinct (except for a small number that evolved into modern birds). Evidence that the terminal Cretaceous event was triggered by the impact of an asteroid with Earth began to emerge in the early 1980s, when Luis and Walter Alvarez discovered a thin layer of iridium in rock strata (layers) 65 million years old at widely separated sites around the world. Iridium is very rare on Earth's surface but is much more common in some kinds of meteorite. The two scientists calculated that the impact of such an object, about 10 kilometers across, could have spread the right amount of iridium around the globe in the cloud of debris (fragments) blasted out by the impact. The idea was greeted with skepticism at first, but more traces of debris from an impact occurring at the time of the death of the dinosaurs began to turn up as geologists searched for evidence for or against the hypothesis. To most people, the most decisive evidence came in the early 1990s, when a geological feature at Chicxulub in the Yucatan Peninsula of Mexico was identified as the remains of an impact crater in exactly the right place and with exactly the right age to connect it to the terminal Cretaceous event.

1.Why does the author include the information that"Iridium is very rare on Earth's surface but is much more common in some kinds of meteorite”?

O To explain why Luis and Walter Alvarez put forward the idea that a meteorite caused the terminal Cretaceous event

O To question whether what Luis and Walter Alvarez had discovered was actually a thin layer of iridium in very old rock strata

O To indicate why scientists other than the Alvarez brothers did not believe that an asteroid caused the terminal Cretaceous event

O To argue that not all types of meteorite could have caused the terminal Cretaceous event

2.The word "decisive" in the passage is closest in meaning to

O interesting

O conclusive

O recent

O surprising

3.According to paragraph 1, which of the following statements about the terminal Cretaceous event is true?

O It destroyed large numbers of plant and animal species.

O It took place about 251 million years ago.

O It is not as well-known as the catastrophe at the end of the Permian era.

O It was the first catastrophe in Earth's history to be triggered by an impact

4.According to paragraph 1, which of the following findings provided support to Luis and Walter Alvarez' theory?

O Signs of past dinosaur activity near an impact crater

O Evidence that iridium was more common on Earth's surface earlier than 65 million years ago

O Evidence of the right amount of iridium in an impact crater

O Debris from an impact that occurred at the time when dinosaurs went extinct

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