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 a subject of scientific debate. 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 eruptions lasted for roughly 30,000 years, releasing enormous amounts of sulfur dioxide and carbon dioxide. This prolonged volcanic activity would have caused severe global cooling, acid rain, and ocean acidification, creating a hostile environment for many species long before the asteroid struck. Second, the fossil record shows a gradual decline in dinosaur diversity and population sizes over the last few million years of the Cretaceous, which aligns with the timing of the eruptions rather than a sudden catastrophic event. Third, the asteroid impact site at Chicxulub in Mexico, while large, may not have been globally devastating on its own; its effects could have been amplified by the already stressed ecosystems caused by volcanism, but without the volcanic pre-conditioning, the impact alone might not have been sufficient to trigger mass extinction.

🎧 リスニング講義

The lecture challenges the volcanic explanation, arguing that the asteroid impact is the far more plausible cause. First, while the Deccan eruptions were long-lasting, their sulfur and carbon emissions were released gradually over tens of thousands of years. This slow release would have allowed the climate to buffer and recover, whereas the asteroid impact injected dust and aerosols into the atmosphere in a matter of hours, causing a sudden 'impact winter' that disrupted photosynthesis globally. Second, the claim of a gradual dinosaur decline is misleading. Recent high-resolution fossil data from the last 100,000 years of the Cretaceous show that dinosaur populations were stable and diverse right up to the impact. The apparent decline in older records is an artifact of incomplete sampling, as dinosaur fossils become rarer near the boundary not because of prior extinction but because of the way sediments were deposited during the impact event. Third, the asteroid impact at Chicxulub was far more powerful than earlier estimates suggested. It released energy equivalent to billions of Hiroshima bombs, triggering massive tsunamis, wildfires, and a global dust cloud that blocked sunlight for months. This sudden and catastrophic disruption would have been fatal to dinosaurs regardless of any volcanic activity. In contrast, the Deccan eruptions, even if stressful, did not cause a mass extinction until after the impact, suggesting they were a minor factor.