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 prolonged volcanic activity, specifically the Deccan Traps in India, was the primary driver of this mass extinction. Three key arguments support this volcanic hypothesis. First, the timing of the Deccan Traps eruptions aligns closely with the extinction event. Radioisotopic dating places the main phase of volcanic activity at approximately 66 million years ago, coinciding with the Cretaceous-Paleogene boundary. This temporal correlation suggests a causal link. Second, volcanic emissions would have released vast quantities of sulfur dioxide and carbon dioxide, leading to severe climate perturbations. Sulfur aerosols could have caused rapid global cooling, while carbon dioxide would have induced long-term warming, creating a 'press-pulse' stress on ecosystems. These climate shifts would have disrupted food chains and habitats, driving many species to extinction. Third, the fossil record reveals a gradual decline in dinosaur diversity in the millions of years before the impact, indicating that dinosaurs were already stressed by environmental changes, likely volcanic in origin. This pre-existing vulnerability would have made them susceptible to extinction even without an asteroid.

🎧 リスニング講義

The reading presents a compelling case for volcanism, but it overlooks crucial evidence that strongly favors the asteroid impact hypothesis. First, while the Deccan Traps eruptions began around 66 million years ago, their main phase actually occurred about 300,000 years before the extinction event. The most intense volcanic activity had already subsided by the time of the mass extinction, whereas the Chicxulub impact occurred precisely at the boundary. This timing mismatch undermines the volcanic causality. Second, although volcanic gases can alter climate, the scale of climate change from the Deccan Traps was insufficient to cause a global catastrophe. In contrast, the asteroid impact would have ejected billions of tons of dust, soot, and sulfate aerosols into the stratosphere, blocking sunlight for months or even years, causing a sudden 'impact winter' that halted photosynthesis globally. This abrupt, severe cooling is consistent with the rapid extinction of dinosaurs and many marine species, which the more gradual volcanic effects cannot explain. Third, the claim of a gradual dinosaur decline is based on incomplete fossil records. High-resolution studies from the Hell Creek Formation show that dinosaur diversity was actually stable until the very end of the Cretaceous. The apparent decline is an artifact of sampling bias, as fewer fossil-bearing sediments exist from the latest Cretaceous. Thus, the dinosaurs were not already stressed, and the impact delivered a sudden, fatal blow.