Until the 1980s, most scientists believed that noncatastrophic geological processes caused the extinction of dinosaurs that occurred approximately 66 million years ago, at the end of the Cretaceous period. Geologists argued that a dramatic drop in sea level coincided with the extinction of the dinosaurs and could have caused the climatic changes that resulted in this extinction as well as the extinction of many ocean species.
This view was seriously challenged in the 1980s by the discovery of large amounts of iridium in a layer of clay deposited at the end of the Cretaceous period. Because iridium is extremely rare in rocks on the Earth’s surface but common in meteorites, researchers theorized that it was the impact of a large meteorite that dramatically changed the earth’s climate and thus triggered the extinction of the dinosaurs.
Currently available evidence, however, offers more support for a new theory, the volcanic-eruption theory. A vast eruption of lava in India coincided with the extinctions that occurred at the end of the Cretaceous period, and the release of carbon dioxide from this episode of volcanism could have caused the climatic change responsible for the demise of the dinosaurs. Such outpourings of lava are caused by instability in the lowest layer of the Earth’s mantle, located just above the Earth’s core. As the rock that constitutes this layer is heated by the Earth’s core, it becomes less dense and portions of it eventually escape upward as blobs or molten rock, called “diapirs,” that can, under certain circumstances, erupt violently through the Earth’s crust.
Moreover, the volcanic-eruption theory, like the impact theory, accounts for the presence of iridium in sedimentary deposits; it also explains matters that the meteorite-impact theory does not. Although iridium is extremely rare on the Earth’s surface, the lower regions of the Earth’s mantle have roughly the same composition as meteorites and contain large amounts of iridium, which in the case of a diapir eruption would probably be emitted as iridium hexafluoride, a gas that would disperse more uniformly in the atmosphere than the iridium-containing matter thrown out from a meteorite impact. In addition, the volcanic-eruption theory may explain why the end of the Cretaceous period was marked by a gradual change in sea level. Fossil records indicate that for several hundred thousand years prior to the relatively sudden disappearance of the dinosaurs, the level of the sea gradually fell, causing many marine organisms to die out. This change in sea level might well have been the result of a distortion in the Earth’s surface that resulted from the movement of diapirs upward toward the Earth’s crust, and the more cataclysmic extinction of the dinosaurs could have resulted from the explosive volcanism that occurred as material from the diapirs erupted onto the Earth’s surface.
1. The passage suggests that during the 1980s researchers found meteorite impact a convincing explanation for the extinction of dinosaurs, in part because
(A) earlier theories had failed to account for the gradual extinction of many ocean species at the end of the Cretaceous period
(B) geologists had, up until that time, underestimated the amount of carbon dioxide that would be released during an episode of explosive volcanism
(C) a meteorite could have served as a source of the iridium found in a layer of clay deposited at the end of the Cretaceous period
(D) no theory relying on purely geological processes had, up until that time, explained the cause of the precipitous drop in sea level that occurred at the end of the Cretaceous period
(E) the impact of a large meteorite could have resulted in the release of enough carbon dioxide to cause global climatic change
2. According to the passage, the lower regions of the Earth’s mantle are characterized by
(A) a composition similar to that of meteorites
(B) the absence of elements found in rocks on the Earth’s crust
(C) a greater stability than that of the upper regions
(D) the presence of large amounts of carbon dioxide
(E) a uniformly lower density than that of the upper regions
3. It can be inferred from the passage that which one of the following was true of the lava that erupted in India at the end of the Cretaceous period?
(A) It contained less carbon dioxide than did the meteorites that were striking the Earth’s surface during that period.
(B) It was more dense than the molten rock, located just above the Earth’s core.
(C) It released enough iridium hexafluoride into the atmosphere to change the Earth’s climate dramatically.
(D) It was richer in iridium than rocks usually found on the Earth’s surface.
(E) It was richer in iridium than were the meteorites that were striking the Earth’s surface during that period.
4. In the passage, the author is primarily concerned with doing which one of the following?
(A) describing three theories and explaining why the latest of these appears to be the best of the three
(B) attacking the assumptions inherent in theories that until the 1980s had been largely accepted by geologists
(C) outlining the inadequacies of three different explanations of the same phenomenon
(D) providing concrete examples in support of the more general assertion that theories must often be revised in light of new evidence
(E) citing evidence that appears to confirm the skepticism of geologists regarding a view held prior to the 1980s
5. The author implies that if the theory described in the third paragraph is true, which one of the following would have been true of iridium in the atmosphere at the end of the Cretaceous period?
(A) Its level of concentration in the Earth’s atmosphere would have been high due to a slow but steady increase in the atmospheric iridium that began in the early Cretaceous period.
(B) Its concentration in the Earth’s atmosphere would have increased due to the dramatic decrease in sea level that occurred during the Cretaceous period.
(C) It would have been directly responsible for the extinction of many ocean species.
(D) It would have been more uniformly dispersed than iridium whose source had been the impact of a meteorite on the Earth’s surface.
(E) It would have been more uniformly dispersed than indium released into the atmosphere as a result of normal geological processes that occur on Earth.
6. The passage supports which one of the following claims about the volcanic-eruption theory?
(A) It does not rely on assumptions concerning the temperature of molten rock at the lowest pan of the Earth’s mantle.
(B) It may explain what caused the gradual fall in sea level that occurred for hundreds of thousands of years prior to the more sudden disappearance of the dinosaurs.
(C) It bases its explanation on the occurrence of periods of increased volcanic activity similar to those shown to have caused earlier mass extinctions.
(D) It may explain the relative scarcity of iridium in rocks on the Earth’s surface compared to its abundance in meteorites.
(E) It accounts for the relatively uneven distribution of iridium in the layer of clay deposited at the end of the Cretaceous period.
7. Which one of the following, if true, would cast the most doubt on the theory described in the last paragraph of the passage?
(A) Fragments of meteorites that have struck the Earth are examined and found to have only minuscule amounts of iridium hexafluoride trapped inside of them.
(B) Most diapir eruptions in the geological history of the Earth have been similar in size to the one that occurred in India at the end of the Cretaceous period and have not been succeeded by periods of climatic change.
(C) There have been several periods in the geological history of the Earth, before and after the Cretaceous period, during which large numbers of marine species have perished.
(D) The frequency with which meteorites struck the Earth was higher at the end of the Cretaceous period than at the beginning of the period.
(E) Marine species tend to be much more vulnerable to extinction when exposed to a dramatic and relatively sudden change in sea level than when they are exposed to a gradual change in sea level similar to the one that preceded the extinction of the dinosaurs.
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