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 played a more significant role. The Deccan Traps in India, a vast volcanic province, erupted for over a million years, releasing enormous amounts of sulfur dioxide and carbon dioxide. First, volcanic gases would have caused rapid global cooling followed by long-term warming, creating drastic climate swings that stressed ecosystems. Second, the timing of the eruptions coincides with the mass extinction event, with the most intense phase occurring just before the boundary layer. Third, volcanic emissions of sulfur aerosols could have caused acid rain, which would have damaged plant life and disrupted food chains, leading to a gradual decline of dinosaur populations before the impact. Thus, while an asteroid may have delivered the final blow, volcanic activity was the primary driver of the extinction.
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The theory that volcanoes alone caused dinosaur extinction is problematic. First, while volcanic gases can alter climate, the most recent high-precision dating shows that the main phase of Deccan Traps eruptions occurred about 250,000 years before the extinction, not at the boundary. The climate changes from those eruptions were significant but not severe enough to cause a global extinction; many species survived those fluctuations. Second, the impact of the Chicxulub asteroid, which struck at the exact boundary, released energy equivalent to billions of atomic bombs, triggering a sudden 'impact winter' that cut off sunlight for months, halting photosynthesis globally. This is a much more abrupt and catastrophic mechanism than gradual volcanic effects. Third, the acid rain from volcanoes would have been localized, not global, and many plants and animals had adapted to volcanic stress over time. In contrast, the asteroid impact’s effects were immediate and worldwide, leaving a layer of iridium and shocked quartz that is found in sediments globally. Therefore, the impact, not volcanism, is the compelling 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.
