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 Deccan Traps eruptions in India, was the primary cause. First, the timing of the Deccan Traps eruptions aligns closely with the extinction event. Geochemical dating shows that the main phase of volcanic activity occurred around 66 million years ago, exactly when the dinosaurs disappeared. Second, volcanic eruptions released enormous quantities of sulfur dioxide and carbon dioxide, leading to severe climate change. Sulfur aerosols caused rapid global cooling, while CO2 triggered long-term warming, creating a highly unstable environment that stressed ecosystems worldwide. Third, the fossil record indicates a gradual decline in dinosaur diversity millions of years before the impact, which is more consistent with the prolonged, episodic stress of volcanism rather than a sudden catastrophe. Therefore, while an asteroid may have contributed, the Deccan Traps volcanism was the dominant driver of the end-Cretaceous mass extinction.

🎧 リスニング講義

The reading’s argument for volcanic activity as the main cause is flawed. First, although the Deccan Traps began erupting before the extinction, the most intense phase—responsible for over 80% of the lava—occurred after the extinction event, not at the same time. The critical extinction interval coincides precisely with the Chicxulub impact, not with peak volcanism. Second, the climate effects of the Deccan eruptions were real but not severe enough to cause a global mass extinction. Sulfur aerosols from volcanism cause cooling, but the magnitude was limited compared to the impact’s ejecta, which blocked sunlight for months, causing a sudden and drastic ‘impact winter’ that shut down photosynthesis. Third, the gradual decline in dinosaur fossils cited in the reading is misleading. This apparent decline is partly due to sampling bias—the fossil record is incomplete, and many dinosaur species persisted until the very end. High-resolution studies show that dinosaur ecosystems were thriving just before the impact, and their extinction was abrupt, not gradual. Thus, the asteroid impact remains the most plausible and immediate cause.