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, 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, may have been the primary driver. First, volcanic eruptions released enormous quantities of sulfur dioxide and carbon dioxide, leading to rapid climate swings—cooling from sulfate aerosols and subsequent warming from CO2—that would have stressed ecosystems over hundreds of thousands of years. Second, the timing of the Deccan Traps eruptions aligns closely with the extinction event, with the main phase of activity occurring just before the boundary layer where fossils disappear. Third, the fossil record shows a gradual decline in dinosaur diversity and other species in the millions of years preceding the impact, consistent with prolonged volcanic stress rather than a sudden catastrophe. Thus, while an asteroid may have delivered a final blow, volcanic activity likely weakened ecosystems to the point of collapse.

🎧 リスニング講義

The lecture challenges the volcanic hypothesis, arguing that the asteroid impact was the sole and sufficient cause. First, the climate effects of Deccan volcanism were not as severe as claimed; recent models show that the eruptions released gases gradually, allowing the planet to buffer changes, whereas the asteroid impact injected dust and aerosols into the stratosphere in a single pulse, causing a sudden 'impact winter' that shut down photosynthesis globally. Second, precise radiometric dating now places the main phase of Deccan eruptions about 300,000 years before the extinction, with only minor activity at the boundary—too early to be a direct cause. Third, paleontological data reveal that dinosaur diversity was actually stable until the very end of the Cretaceous, with no gradual decline; fossils from the last 100,000 years show a thriving population, contradicting the idea of prolonged stress. Therefore, the lecture concludes that the asteroid impact, not volcanism, was the decisive factor.