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 more nuanced view suggests that volcanic activity, specifically the Deccan Traps eruptions in India, was the primary driver. Proponents of the volcanic hypothesis offer three main arguments. First, the Deccan Traps eruptions began well before the asteroid impact, around 66 million years ago, and lasted for nearly a million years. This prolonged volcanic activity would have released vast amounts of sulfur dioxide and carbon dioxide, causing severe climate fluctuations, including rapid cooling (volcanic winter) and subsequent warming, which could have stressed ecosystems long before the impact. Second, the timing of the eruptions coincides with a gradual decline in dinosaur diversity observed in the fossil record. Fossil evidence from the late Cretaceous shows a reduction in dinosaur species richness, suggesting that dinosaurs were already struggling due to volcanic-induced environmental changes before the impact. Third, the asteroid impact hypothesis fails to explain why many species, including some dinosaurs, marine reptiles, and mammals, survived the initial impact but disappeared later. Volcanic activity, with its sustained and repeated eruptions, would have caused more prolonged and widespread environmental disruption, leading to a more gradual extinction pattern that matches the fossil evidence better than a single catastrophic event.

🎧 リスニング講義

The lecture challenges the volcanic hypothesis, arguing that the asteroid impact is the far more likely cause of dinosaur extinction. First, while the Deccan Traps eruptions were indeed long-lasting, their intensity was not constant. The most intense phase of volcanism actually occurred after the asteroid impact, not before. The initial eruptions were relatively mild, and the major climatic disruptions—such as massive sulfur emissions—happened in a short burst following the impact, which is more consistent with a sudden catastrophe. Second, the claim that dinosaur diversity declined gradually before the impact is not supported by high-resolution fossil data. Recent studies using advanced dating techniques show that dinosaur populations were stable and diverse up until the very end of the Cretaceous, with no evidence of a long-term decline. The apparent decline in some earlier studies was likely due to sampling biases or incomplete fossil records. Third, the gradual extinction pattern attributed to volcanism is actually better explained by the impact. The impact would have triggered global fires, a dust cloud blocking sunlight, and a subsequent collapse of food chains, leading to a rapid but staggered extinction—some species dying immediately, others succumbing over the following months to years. This matches the fossil record, which shows a sharp extinction horizon, not a slow fade-out. In contrast, volcanic eruptions would have allowed ecosystems to adapt over time, which is not what we observe.