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, particularly the Deccan Traps eruptions in India, was the primary cause. First, the timing of the Deccan Traps eruptions aligns closely with the extinction event. Radioisotopic dating indicates that the main phase of volcanic activity began about 66 million years ago and lasted for roughly 750,000 years, overlapping with the extinction horizon. This prolonged period of volcanic activity would have released enormous amounts of sulfur dioxide and carbon dioxide, leading to severe global cooling and acid rain, followed by long-term greenhouse warming—conditions that could disrupt ecosystems worldwide. Second, the Deccan Traps eruptions produced lava flows that covered an area of over 1.5 million square kilometers, with a volume exceeding one million cubic kilometers. Such massive outpourings would have emitted toxic gases, including sulfur dioxide and hydrogen sulfide, which could have poisoned the atmosphere and oceans, causing widespread die-offs in both terrestrial and marine life. Third, the fossil record shows a gradual decline in dinosaur diversity leading up to the extinction, rather than a sudden catastrophe. Paleontologists have documented a reduction in the number of dinosaur species in the last few million years of the Cretaceous, which is more consistent with a slow, stress-induced extinction caused by volcanic activity than with an instantaneous impact event.

🎧 リスニング講義

The reading presents volcanic activity as the main cause of dinosaur extinction, but this argument fails to consider several critical points. First, while the Deccan Traps eruptions did occur around the same time as the extinction, the most intense phase of volcanic activity actually began about 300,000 years before the extinction event and had largely subsided by the time of the impact. In contrast, the asteroid impact at Chicxulub occurred precisely at the extinction boundary, and its effects were immediate and catastrophic. Second, the volume of lava alone does not determine the extinction mechanism. The Deccan Traps released gases over a long period, allowing ecosystems to adapt gradually. However, the impact vaporized rock and released sulfate aerosols and soot into the stratosphere, blocking sunlight for months and causing a sudden drop in temperature—conditions that would have been lethal to many species. Third, the supposed decline in dinosaur diversity is misleading. Recent studies using statistical models that correct for sampling biases show that dinosaur populations were actually thriving until the very end of the Cretaceous. The apparent decline in the fossil record is due to incomplete preservation and collection bias, not a real decrease in species. Therefore, the impact hypothesis remains the most compelling explanation for the mass extinction.