Impacts and Mass Extinctions

Paragraph 1：Meteorites and impact craters bear witness to the fact that large impacts occasionally occur on Earth. Meteor Crater in the northern Arizona desert of the United States formed about 50,000 years ago when a metallic impactor roughly 50 meters across crashed to Earth with the explosive power of a 20-megaton hydrogen bomb. Although the crater is only slightly more than one kilometer across, an area covering hundreds of square kilometers was probably battered by the blast and ejecta—the debris ejected or displaced during the formation of an impact crater. Far bigger impacts have occurred, sometimes with catastrophic consequences for life on Earth. 

 

 

 

Paragraph 2：While collecting geological samples in Italy in 1978, the father-son team of Luis and Walter Alvarez discovered a thin layer of dark sediment that had apparently been deposited 65 million years ago—at about the same time that the dinosaurs and many other organiSms suddenly became extinct. Subsequent studies found similar sediment deposited at the same time at many sites around the world. Careful analysis showed this worldwide sediment layer to be rich in iridium, an element that is rare on Earth’s surface. But iridium is common in primitive meteorites, which led the Alvarezes to a stunning conclusion: the extinction of the dinosaurs was caused by the impact of an asteroid or comet. This conclusion was not immediately accepted and still generates some controversy, but it now seems clear that a major impact coincided with the death of the dinosaurs. While the dinosaurs were the most famous victims of this mass extinction, it seems that up to 99 percent of all living things were killed and that 75 percent of all species living on Earth were wiped out at that time. 

 

 

1. In paragraph 2, the author discusses the presence of iridium in the sediment layer discovered by the Alvarezes because 

 

O it supported their claim that toxic levels of iridium could have killed the dinosaurs 

 

O it led some scientists to question the validity of the Alvarezes’ study 

 

O it led the Alvarezes to conclude that a space impactor killed the dinosaurs

 

O it showed that new discoveries were still being made about the extinction of the dinosaurs 

 

 

 

2. Which of the following statements about scientists’ theories on the extinction of the dinosaurs can be inferred from paragraph 2? 

 

O Some scientists do not believe that an asteroid or comet impact killed the dinosaurs.

 

O Some scientists think that iridium was abundant on Earth before the extinction of the dinosaurs. 

 

O Some scientists doubt that all of the dinosaurs died out 65 million years ago. 

 

O Some scientists believe that the dinosaurs became extinct only after the plants and animals they ate disappeared. 

 

 

 

3. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. 

 

O A large percentage of Earth’s living things were killed along with the dinosaurs. 

 

O Seventy-five percent of all species, including 75 percent of dinosaur species, were victims of a single mass extinction. 

 

O In the extinction that eliminated the dinosaurs, up to 99 percent of all living things and 75 percent of all species were wiped out.

 

O Dinosaurs were the most famous victims of a mass extinction, but there have also been mass extinctions that wiped out most of Earth’s other species. 

 

 

 

 

 

Paragraph 3：How could an impact lead to mass extinction? The amount of iridium deposited worldwide suggests that the impactor must have been about 10 kilometers across. After a decade-long search, scientists identified what appears to be the impact crater from the event. Located off the coast of Mexico’s Yucatan peninsula, it is 200 kilometers across, which is close to what one would expect for a 10-kilometer impactor, and dates to 65 million years ago. Further evidence that the Yucatan crater is the right one comes from the distribution of Small glassy spheres that formed when the molten impact ejecta solidified as it rained back to Earth. More of these glassy spheres are found in regions near the crater, and careful study of their distribution suggests that the impactor crashed to Earth at a slight angle. These pieces of once molten rock are evidence of an explosion powerful enough to instantly melt bedrock and propel it far from its origin. The impact almost immediately sent a shower of debris raining across much of North and South America and generated huge waves that may have sloshed more than 1,000 kilometers inland. Many North American species thus may have been wiped out shortly after impact. For the rest of the world, death may have come more slowly. Heat from the impact and returning ejecta probably ignited wildfires in forests around the world. Evidence of wildfires is found in the large amount of soot (a black powdery form of carbon produced when coal, wood, or oil is burned) that is also present in the indium-rich sediment from 65 million years ago. The impact also sent huge quantities of dust high into the stratosphere, where it remained for several years, blocking out sunlight, cooling the surface, and affecting atmospheric chemistry. Plants died for lack of sunlight, and effects propagated throughout the food chain. 

 

 

 

4. According to paragraph 3, what information did scientists use to predict the size of the impactor that they believed struck Earth 65 million years ago? 

 

O The size of the debris found in geological samples from that time 

 

O The distance inland that traces of tidal waves were found 

 

O The amount of iridium found in sediment from that time

 

O The number of species that became extinct at that time 

 

 

 

5. According to paragraph 3, the shower of debris created by the impact suggests that the impactor 

 

O was 200 kilometers across 

 

O landed about 1,000 kilometers inland 

 

O had a molten core that solidified immediately as the impactor hit Earth 

 

O was probably responsible for the immediate extinction of many North American species

 

 

 

6. In paragraph 3, why does the author include the information that a large amount of soot is found in sediment from 65 million years ago? 

 

O To identify the largest component of the dust sent into the atmosphere by the impactor’s crash 

 

O To support the idea that the impactor’s crash probably caused widespread wildfires

 

O To emphasize the large variety of substances found in the sediment 

 

O To cast doubt on the claim that the impact affected only the area near the crash site 

 

 

 

7. Which of the following is NOT mentioned in paragraph 3 as an effect of the dust raised by the impact 65 million years ago? 

 

O It cooled the surface of Earth. 

 

O It released toxic chemicals into the stratosphere.

 

O It caused changes in the chemical composition of the atmosphere. 

 

O It prevented sunlight from reaching the surface of Earth. 

 

 

Paragraph 4：Perhaps the most astonishing fact is not that 75 percent of all species died, but that 25 percent survived. Among the survivors were a few Small, rodent-like mammals. These mammals may have survived because they lived in underground burrows and managed to store enough food to outlast the long spell of cold, dark days. Small mammals had first arisen at about the same time as the dinosaurs, more than 100 million years earlier. But the sudden disappearance of the dominant dinosaurs made these mammals dominant. 

 

 

 

8. According to paragraph 4, all of the following are true of the animals that survived the impact EXCEPT: 

 

O They probably had a lot of food stored up. 

 

O Many lived underground. 

 

O Most were relatively new species.

 

O They tended to be Small. 

 

 

 

 

 

Paragraph 3：How could an impact lead to mass extinction? The amount of iridium deposited worldwide suggests that the impactor must have been about 10 kilometers across. After a decade-long search, scientists identified what appears to be the impact crater from the event. Located off the coast of Mexico’s Yucatan peninsula, it is 200 kilometers across, which is close to what one would expect for a 10-kilometer impactor, and dates to 65 million years ago. Further evidence that the Yucatan crater is the right one comes from the distribution of Small glassy spheres that formed when the molten impact ejecta solidified as it rained back to Earth. ■More of these glassy spheres are found in regions near the crater, and careful study of their distribution suggests that the impactor crashed to Earth at a slight angle. ■These pieces of once molten rock are evidence of an explosion powerful enough to instantly melt bedrock and propel it far from its origin. ■The impact almost immediately sent a shower of debris raining across much of North and South America and generated huge waves that may have sloshed more than 1,000 kilometers inland. ■Many North American species thus may have been wiped out shortly after impact. For the rest of the world, death may have come more slowly. Heat from the impact and returning ejecta probably ignited wildfires in forests around the world. Evidence of wildfires is found in the large amount of soot (a black powdery form of carbon produced when coal, wood, or oil is burned) that is also present in the indium-rich sediment from 65 million years ago. The impact also sent huge quantities of dust high into the stratosphere, where it remained for several years, blocking out sunlight, cooling the surface, and affecting atmospheric chemistry. Plants died for lack of sunlight, and effects propagated throughout the food chain. 

 

 

 

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

 

As a result, molten material was pushed out toward the north with Small glass spheres found more than 2,500 kilometers to the northwest.

 

Where would the sentence best fit? Click on a square [■] to add the sentence to the passage. 

 

 

 

 

 

10. 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 answer choices 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. Drag your choices to the spaces where they belong. To review the passage, click on View Text. 

 

Asteroids and other large bodies from space have occasionally struck Earth, causing mass destruction and extinction.

 

 

 

Answer Choices 

 

O The large impactor that crashed in what is now the Arizona desert exploded with the force of a 20-megaton hydrogen bomb. 

 

O Although the crash of a large impactor would have killed all living things near the site of impact, most animals living farther away adapted to changes in the environment. 

 

O Scientists believe the reason the impactor that struck Earth 65 million years ago was so deadly was because it contained many toxic compounds, such as iridium. 

 

O A worldwide sediment layer of iridium suggests that an asteroid or comet impacted Earth 65 million years ago, possibly causing the extinction of the dinosaurs.

 

O The large impact that caused the Yucatan crater probably produced both immediate natural disasters and a long-lasting layer of dust in the atmosphere.

 

O Despite the destructive effects of the large impact 65 million years ago, around 25 percent of Earth’s species survived and mammals came to be dominant. 

 

 

 

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