The extinction of the dinosaurs at the end of the Cretaceous period, about 66 million years ago, has long fascinated scientists. While the asteroid impact theory is widely accepted, a competing hypothesis suggests that massive volcanic eruptions in the Deccan Traps of India were the primary cause. Proponents of the volcanic theory offer three main arguments. First, the Deccan eruptions were prolonged and massive, releasing enormous amounts of sulfur dioxide and carbon dioxide over hundreds of thousands of years. This would have caused severe climate cooling (due to sulfur aerosols) followed by warming (due to CO2), creating stress that could have gradually destabilized ecosystems long before the asteroid impact. Second, fossil records show that dinosaur diversity was already declining in the few million years before the impact, suggesting that volcanic activity, not the asteroid, was the driving force of their demise. Third, the timing of the Deccan eruptions, which peaked around the Cretaceous-Paleogene boundary, closely matches the extinction event, whereas the Chicxulub impact occurred only 300,000 years before the boundary, leaving a gap that weakens the asteroid's causal link. Thus, the volcanic hypothesis argues that the dinosaurs were already on the brink of extinction due to Deccan volcanism, and the asteroid merely delivered the final blow.
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The lecture challenges the volcanic theory presented in the reading. While the professor acknowledges that Deccan volcanism was significant, she argues that it cannot explain the suddenness and selectivity of the extinction. First, she points out that volcanic gases are released gradually, and while they could cause long-term climate stress, the fossil record shows a very abrupt extinction event—over just a few thousand years—which is more consistent with an asteroid impact that would have caused immediate darkness, global fires, and a rapid temperature drop. Second, the claim of declining dinosaur diversity is misleading. The professor notes that the apparent decline in fossil diversity may be due to sampling bias: the rock layers from that period are incomplete, and when corrected for preservation gaps, dinosaur diversity appears stable until the impact. Third, she argues that the timing of Deccan volcanism does not match the extinction. The most intense phase of eruptions occurred about 300,000 years before the boundary, and the majority of volcanic activity was over before the mass extinction. In contrast, the Chicxulub impact precisely coincides with the extinction layer, and the impact's effects, such as iridium anomalies and shocked quartz, are found globally at the boundary. Therefore, the lecture concludes that the asteroid impact, not volcanism, was the primary cause of the dinosaurs' 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.
