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 volcanic activity, specifically the Deccan Traps in India, was the primary driver. First, the timing of the Deccan Traps eruptions aligns closely with the mass extinction event, occurring over a span of 500,000 years around the Cretaceous-Paleogene boundary. This prolonged volcanic activity would have released vast amounts of sulfur dioxide and carbon dioxide, leading to global cooling and ocean acidification. Second, volcanic eruptions produce toxic gases such as mercury and sulfur, which have been found in sediment layers at the extinction boundary, directly linking volcanic emissions to environmental stress. Third, the asteroid impact, while catastrophic, was a short-lived event that could not have caused the long-term ecological disruption necessary for such a selective extinction. In contrast, the Deccan Traps' sustained emissions would have gradually destroyed habitats and disrupted food chains, explaining why many species, including dinosaurs, went extinct over an extended period.
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The lecture challenges the volcanic hypothesis, arguing that the asteroid impact remains the most plausible cause. First, while the Deccan Traps eruptions were extensive, their peak activity occurred about 300,000 years before the mass extinction, and by the time of the extinction, volcanic emissions had already subsided. This temporal mismatch weakens the claim that volcanoes were the direct trigger. Second, the presence of mercury in extinction layers is not unique to volcanic activity; asteroid impacts can also release mercury from heated crustal rocks. Moreover, the spike in iridium, a rare element characteristic of asteroids, is found precisely at the extinction boundary, providing stronger evidence for an impact. Third, the selectivity of the extinction actually supports the impact theory: an impact would cause rapid climate change, such as a nuclear winter and a brief but intense greenhouse effect, which would disproportionately affect large, cold-blooded animals like dinosaurs, while small, warm-blooded mammals with higher reproductive rates could survive. In contrast, volcanic eruptions, which release gases slowly, would have allowed species to adapt or migrate, making a sudden, catastrophic extinction more consistent with the impact scenario.
📝 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.
