The extinction of the dinosaurs at the end of the Cretaceous period, about 66 million years ago, has long been attributed to a massive asteroid impact. However, a growing body of evidence suggests that intense volcanic activity, specifically the Deccan Traps in present-day India, was the primary driver of this mass extinction. First, volcanic eruptions released enormous quantities of sulfur dioxide and carbon dioxide over hundreds of thousands of years, leading to severe climate swings. These gases caused acid rain and global cooling followed by long-term warming, which would have stressed ecosystems before the asteroid even struck. Second, fossil records show a gradual decline in dinosaur diversity and abundance in the last few million years of the Cretaceous, consistent with a slow environmental deterioration caused by volcanism rather than a sudden catastrophic impact. Third, the timing of the main phase of Deccan eruptions coincides with the extinction event, and some geological layers at the extinction boundary contain volcanic ash and iridium anomalies that are more consistent with volcanic origin than impact debris. Therefore, while an asteroid may have contributed, the Deccan volcanism was the decisive factor in the dinosaurs' demise.
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The professor challenges the reading's claim that volcanic activity was the main cause of dinosaur extinction. While the Deccan Traps were indeed massive, their effects were likely not as severe as the reading suggests. First, the sulfur and carbon dioxide emissions from the Deccan eruptions occurred over many short pulses, not continuously. Modeling studies show that between pulses, the climate could recover, so the stress on ecosystems was not sustained enough to cause a global collapse. In contrast, the asteroid impact released energy equivalent to billions of nuclear bombs, causing an immediate and catastrophic 'impact winter' that blocked sunlight for months, halting photosynthesis—a sudden shock that matches the abrupt extinction pattern seen in the fossil record. Second, the reading's claim of a gradual decline in dinosaur diversity is based on incomplete fossil sampling. Recent high-resolution studies in North America show that dinosaur populations were thriving right up to the impact layer, with no evidence of a long-term decline. This supports a sudden, impact-induced extinction. Third, the iridium anomaly at the boundary is actually a strong signature of an asteroid, as iridium is rare in Earth's crust but abundant in meteorites. The volcanic hypothesis cannot explain the global iridium layer, nor the presence of shocked quartz and tektites—features only formed by high-velocity impacts. Thus, the asteroid impact remains the most compelling explanation.
📝 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.
