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 eruptions occurred over a period of several hundred thousand years, coinciding with the end of the Cretaceous. The release of vast amounts of sulfur dioxide and carbon dioxide would have caused severe acid rain and a rapid cooling followed by long-term global warming, creating environmental stress that gradually decimated dinosaur populations. Second, the fossil record shows a decline in dinosaur diversity in the few million years before the impact, which is consistent with the gradual effects of volcanic activity rather than a sudden catastrophe. Third, recent high-precision dating of Deccan lava flows indicates that the most intense phase of eruptions began slightly before the asteroid impact, suggesting that volcanic effects had already weakened ecosystems, making dinosaurs vulnerable to extinction even without the impact.

🎧 リスニング講義

The lecture challenges the volcanic theory, arguing that the asteroid impact remains the most plausible explanation. First, while the Deccan eruptions were long-lasting, their climatic effects were not as severe as claimed. Studies of ancient sediments and climate models show that the warming and cooling caused by the eruptions were within tolerable ranges for large vertebrates, and many species, including crocodiles and birds, survived. The impact, however, generated a global dust cloud that blocked sunlight for months, causing a sudden collapse of photosynthesis and food chains—a far more lethal mechanism. Second, the apparent decline in dinosaur diversity before the impact is likely a sampling artifact. Fossil beds from that period are rare and often incomplete, and newer finds in some regions show that dinosaurs were thriving until the very end. Third, the timing of the most intense volcanic phase is actually after the impact, not before. High-precision dating shows that the peak eruptions occurred roughly 200,000 years after the asteroid struck, meaning volcanism could not have weakened dinosaurs prior to the event. The impact was the decisive blow.