The extinction of the dinosaurs at the end of the Cretaceous period has long been attributed to a massive asteroid impact. However, a growing body of evidence suggests that sustained volcanic activity in the Deccan Traps of India was the primary cause. First, the timing of the Deccan eruptions aligns closely with the extinction event. Radioisotopic dating places the main phase of volcanism at approximately 66 million years ago, coinciding with the Cretaceous-Paleogene boundary, whereas the Chicxulub impact occurred about 300,000 years earlier, a gap that weakens the impact hypothesis. Second, volcanic emissions would have caused severe environmental stress. The Deccan Traps released enormous quantities of sulfur dioxide and carbon dioxide, leading to acid rain, ocean acidification, and global cooling followed by intense warming. These climate fluctuations would have disrupted food chains and habitats long before any impact. Third, the gradual nature of volcanic effects is more consistent with the fossil record. Paleontological studies show a decline in dinosaur diversity over the last few million years of the Cretaceous, suggesting a slow, ongoing deterioration rather than a sudden catastrophe. In contrast, an asteroid impact would produce an abrupt mass extinction, which does not match the observed gradual pattern. Thus, volcanism, not impact, was the decisive factor.
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Now listen to a lecture on the same topic. The professor challenges the volcanic hypothesis presented in the reading. First, while the timing of the Deccan eruptions is close, the most intense phase actually occurred about 200,000 years after the extinction event. The initial eruptions were relatively mild, and the major climatic disruption happened after the dinosaurs were already gone. Thus, volcanism cannot explain the mass extinction. Second, the environmental effects of the Deccan Traps, though significant, were not extreme enough to cause a global catastrophe. The sulfur and carbon dioxide emissions would have caused some climate change, but the oceans would have partially buffered the acidification, and many species, including crocodiles and birds, survived the event. An asteroid impact, in contrast, would have produced a sudden nuclear winter, global fires, and a shock wave, which better explains the rapid and selective extinction of large terrestrial animals. Third, the gradual decline in dinosaur diversity is not universally accepted. Many recent studies show that dinosaur populations were stable and diverse right up to the boundary, and the apparent decline in the fossil record may be due to sampling biases or incomplete preservation. Finally, the Chicxulub impact, though earlier, could have triggered a series of aftershocks and secondary effects, including enhanced volcanic activity, that culminated at the boundary. In fact, the impact may have triggered the most violent phase of Deccan volcanism, linking the two events. Therefore, the asteroid impact remains the more plausible primary cause.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
