The extinction of the dinosaurs at the end of the Cretaceous period has long been attributed to a massive asteroid impact. However, recent evidence suggests that volcanic activity, specifically the Deccan Traps in India, may have been the primary cause. First, the timing of the Deccan Traps eruptions coincides with the extinction event. Radiometric dating shows that the main phase of volcanic activity occurred around 66 million years ago, exactly when the dinosaurs disappeared. Second, volcanic eruptions release vast amounts of sulfur dioxide and carbon dioxide, which can cause severe climate change. Sulfur aerosols block sunlight, leading to a global cooling, while CO2 traps heat, causing long-term warming. Such rapid temperature fluctuations would have stressed ecosystems, making it difficult for large reptiles to survive. Third, the fossil record indicates a gradual decline in dinosaur diversity well before the asteroid impact. Studies of dinosaur fossils in North America show a decrease in species richness over the last few million years of the Cretaceous, suggesting that environmental stress from volcanism was already taking a toll. Therefore, the Deccan Traps volcanism, not the asteroid, is the more plausible driver of the mass extinction.
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The reading argues that volcanic activity caused the dinosaur extinction, but this view is deeply flawed. First, while the Deccan Traps eruptions began before the extinction, their most intense phase actually occurred about 300,000 years after the impact. The catastrophic event at the end of the Cretaceous is precisely dated to a single moment, and the volcanic activity at that exact time was relatively mild. Second, the climate effects of volcanic gases are not comparable to those of an asteroid impact. The impact vaporized sulfate-rich rock and injected dust into the stratosphere, causing a sudden and catastrophic darkness and cooling that halted photosynthesis for months. In contrast, volcanic emissions are gradual and would have allowed ecosystems to adapt. Third, the claimed decline in dinosaur diversity is misleading. The fossil record is incomplete, and many apparent declines are due to sampling biases, such as the scarcity of sedimentary layers from that period. In fact, when corrected for these biases, dinosaur diversity appears stable until the very end. Thus, the asteroid impact remains the most compelling explanation for the sudden and global extinction.
📝 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.
