TOEFL統合ライティング

📖 読解文章

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 eruption of the Deccan Traps in India, played a more significant role. Three key arguments support this volcanic hypothesis. First, the Deccan Traps eruptions were prolonged, lasting over a million years, and would have released vast quantities of sulfur dioxide and carbon dioxide, causing severe climate disruption, including acid rain and global cooling, long before the asteroid impact. Second, the fossil record shows a gradual decline in dinosaur diversity in the last few million years of the Cretaceous, consistent with a slow environmental deterioration caused by volcanism, rather than a sudden catastrophic event. Third, recent geochronological dating indicates that the main phase of Deccan volcanism began about 300,000 years before the asteroid impact, and its effects would have already weakened ecosystems, making dinosaurs vulnerable to the later impact. Thus, while the asteroid may have delivered the final blow, volcanic activity was the primary driver of the extinction.

🎧 リスニング講義

The professor challenges the volcanic hypothesis presented in the reading. While the Deccan Traps were indeed massive, their role in dinosaur extinction is overstated. First, the climate effects of the Deccan eruptions were likely not as severe as claimed. Recent climate models suggest that the sulfur dioxide emissions, although substantial, were released over a long period, allowing the Earth's systems to buffer against drastic changes. In contrast, the asteroid impact injected dust and aerosols into the stratosphere in a matter of hours, causing a sudden 'impact winter' that blocked photosynthesis for months. Second, the alleged gradual decline in dinosaur diversity is not supported by high-resolution fossil data. Many studies show that dinosaur populations were stable and diverse right up to the very end of the Cretaceous, with no evidence of a prolonged decline. The apparent pattern in older data was due to sampling biases, as fewer sedimentary layers from that period have been preserved. Third, even if the main phase of Deccan volcanism preceded the impact, the most intense volcanic pulses actually occurred after the impact, not before. Moreover, the impact itself may have triggered even greater volcanic activity by causing seismic shocks. Therefore, the asteroid impact remains the most plausible and dominant cause of the extinction, with volcanism playing only a minor, secondary role.