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 intense volcanic activity, specifically the Deccan Traps eruptions in present-day India, was the primary cause. Three main arguments support this volcanic hypothesis. First, the Deccan Traps eruptions began well before the asteroid impact, approximately 66.3 million years ago, and lasted for about 750,000 years. This prolonged period of volcanic activity would have released vast amounts of sulfur dioxide and carbon dioxide, leading to severe global cooling and ocean acidification long before any asteroid struck. Second, the timing of the Deccan Traps' most intense phase, which occurred around 66.0 million years ago, closely coincides with the dinosaur extinction horizon, whereas the Chicxulub impact occurred slightly later, at 66.04 million years ago, making the volcanic eruptions the more temporally plausible cause. Third, the fossil record shows a gradual decline in dinosaur diversity over the last few million years of the Cretaceous, consistent with the cumulative effects of volcanic stress, rather than a sudden catastrophic die-off that would be expected from an asteroid impact. Therefore, the Deccan Traps volcanism, not the asteroid, was the dominant driver of the end-Cretaceous mass extinction.

🎧 リスニング講義

The professor argues that the volcanic hypothesis presented in the reading is flawed, and that the asteroid impact remains the more convincing explanation. First, while the Deccan Traps eruptions were indeed long-lasting, the intensity of the early phase (before the impact) was relatively low, and climate models suggest that the environmental changes during that period were not severe enough to cause a global extinction. The most catastrophic volcanic phase, which the reading claims coincides with the extinction, actually occurred after the impact—likely triggered by the seismic shock of the asteroid—and thus its effects were superimposed on the impact's devastation, not a pre-existing cause. Second, the reading's timeline is misleading: high-precision radiometric dating shows that the Chicxulub impact occurred at 66.04 million years ago, while the extinction event is dated to 66.043 million years ago, meaning the impact happened within a few thousand years of the extinction, essentially simultaneous on geological timescales. In contrast, the main volcanic pulse did not begin until 66.02 million years ago, after the extinction. Third, the fossil record does show a decline in dinosaur diversity, but this decline is more likely due to long-term ecological changes, such as sea-level regression and habitat fragmentation, not volcanic stress. Moreover, the final extinction was abrupt, with no evidence of a gradual increase in volcanic-related stress (like iridium spikes or shocked quartz) in the last 100,000 years before the boundary. These markers are uniquely associated with the impact. Therefore, the asteroid impact is the primary cause, with volcanism playing only a secondary role.