托福阅读常考话题及练习题推荐四：恐龙大灭绝（3）

1托福阅读常考话题：恐龙大灭绝练习题

Paragraph 5: Of the various hypotheses attempting to account for the late Cretaceous extinctions, the one that has attracted the most attention in recent years is the asteroid-impact hypothesis first suggested by Luis and Walter Alvarez. According to this hypothesis, Earth collided with an asteroid with an estimated diameter of 10 kilometers, or with several asteroids, the combined mass of which was comparable. The force of collision spewed large amounts of debris into the atmosphere, darkening the skies for several years before the finer particles settled. The reduced level of photosynthesis led to a massive decline in plant life of all kinds, and this caused massive starvation first of herbivores and subsequently of carnivores. The mass extinction would have occurred very suddenly under this hypothesis.

8. The phrase account for in the passage is closest in meaning to

A. describe

B. challenge

C. explain

D. test

Paragraph 6: One interesting test of the Alvarez hypothesis is based on the presence of the rare-earth element iridium (Ir). Earth’s crust contains very little of this element, but most asteroids contain a lot more. Debris thrown into the atmosphere by an asteroid collision would presumably contain large amounts of iridium, and atmospheric currents would carry this material all over the globe. A search of sedimentary deposits that span the boundary between the Cretaceous and Tertiary periods shows that there is a dramatic increase in the abundance of iridium briefly and precisely at this boundary. This iridium anomaly offers strong support for the Alvarez hypothesis even though no asteroid itself has ever been recovered.

9. According to paragraph 6, what made iridium a useful test of the Alvarez hypothesis?

A. Its occurrence in a few locations on Earth against several locations on other planets

B. Its occurrence in limited quantities on Earth against its abundance in asteroids

C. Its ability to remain solid at extremely high temperatures

D. Its ease of detection even in very small amounts

10. In stating that no asteroid itself has ever been recovered, the author emphasizes which of the following?

A. The importance of the indirect evidence for a large asteroid

B. The fact that no evidence supports the asteroid‑impact hypothesis

C. The reason many researchers reject the Alvarez hypothesis

D. The responsibility of scientists for not making the effort to discover the asteroid itself

Paragraph 7: An asteroid of this size would be expected to leave an immense crater, even if the asteroid itself was disintegrated by the impact. The intense heat of the impact would produce heat-shocked quartz in many types of rock. Also, large blocks thrown aside by the impact would form secondary craters surrounding the main crater. To date, several such secondary craters have been found along Mexico’s Yucatan Peninsula, and heat-shocked quartz has been found both in Mexico and in Haiti. A location called Chicxulub, along the Yucatan coast, has been suggested as the primary impact site.

11. The word intense in the passage is closest in meaning to

A. sudden

B. unusual

C. immediate

D. extreme

12. What is the purpose of paragraph 7 in the passage?

A. It proposes a decisive new test of the Alvarez hypothesis.

B. It presents additional supporting evidence for the Alvarez hypothesis.

C. It explains why evidence relating to the Alvarez hypothesis is hard to find.

D. It shows how recent evidence has raised doubts about the Alvarez hypothesis.

Paragraph 1: ■Cases in which many species become extinct within a geologically short interval of time are called mass extinctions. ■There was one such event at the end of the Cretaceous period (around 70 million years ago). ■There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago). ■The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.

13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.

In general, it is believed that these two extinctions resulted from drastic environmental changes that followed meteorite impacts or massive volcanic eruptions.

Where would the sentence best fit?

14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.There have been many attempts to explain the causes of mass extinctions.

Answer Choices

A. Asteroid impacts, evolutionary developments, and changes in Earth's climate and in the positions of the continents have all been proposed as possible causes of mass extinctions.

B. Researchers have observed 26-million-year cycles in extinction rates of a number of fossil groups that could all be attributed to the same cause.

C. According to the Alvarez hypothesis, much of the iridium originally present on Earth was thrown into the atmosphere as a result of an asteroid impact that also caused a mass extinction.

D. The unusual distribution of iridium on Earth and the presence of craters and heat-shocked quartz are central to the theory that an asteroid impact caused the late Cretaceous event.

E. The collision between Earth and a large asteroid resulted in massive damage and generated enough heat to cause irreversible changes in Earth's atmosphere.

F. There was a particularly large mass extinction that occurred around 250 million years ago at the end of the Permian period, whose cause could not be determined.