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 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 Traps eruptions were prolonged, lasting over a million years, and released enormous quantities of sulfur dioxide and carbon dioxide. These gases would have caused severe climate fluctuations, including rapid cooling and warming, which could have stressed dinosaur populations and led to their gradual decline. Second, the fossil record shows that dinosaur diversity was already decreasing in the few million years before the asteroid impact, indicating that environmental stress from volcanism was already taking its toll. Third, the timing of the eruptions coincides with the extinction event, and some scientists argue that the asteroid impact merely delivered the final blow to an already vulnerable ecosystem. Thus, volcanic activity, not the asteroid, was the dominant factor in the dinosaurs' demise.

🎧 リスニング講義

The lecture challenges the volcanic theory, arguing that the asteroid impact is a far more plausible explanation. First, while the Deccan Traps eruptions were indeed massive, their effects were spread over hundreds of thousands of years, allowing species to adapt gradually. In contrast, the asteroid impact caused a sudden, catastrophic change—blocking sunlight, triggering wildfires, and causing a global winter—that would have been lethal within months. Second, the claim of declining dinosaur diversity before the impact is based on incomplete fossil sampling. Recent studies in North America, where the fossil record is most detailed, show that dinosaur populations were stable until the very end of the Cretaceous, just before the impact. Third, the timing argument is flawed: the main phase of Deccan volcanism occurred about 300,000 years before the impact, and the dinosaurs did not go extinct then. However, the impact occurred precisely at the iridium layer, which marks the extinction horizon. Therefore, the lecture concludes that the asteroid impact, not volcanism, was the primary cause of dinosaur extinction.