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 volcanic activity, specifically the Deccan Traps eruptions in present-day India, was the primary driver. First, the timing of the Deccan Traps eruptions coincides with the extinction event. Geochemical dating shows that the main phase of volcanic activity occurred around 66 million years ago, exactly when the dinosaurs disappeared. This temporal correlation is too precise to be coincidental. Second, volcanic emissions would have caused severe climate change. The Deccan Traps released vast amounts of sulfur dioxide and carbon dioxide, leading to acid rain, global cooling followed by greenhouse warming, and disruption of photosynthesis. Such rapid environmental shifts could have destroyed food chains and habitats, making it impossible for large reptiles to survive. Third, the asteroid impact theory has weaknesses. The Chicxulub crater, often cited as evidence for the impact, is dated to about 300,000 years before the extinction, meaning the impact may have occurred too early to be the direct cause. Additionally, many species had already been declining for millions of years before the impact, suggesting a gradual environmental stress, which is more consistent with prolonged volcanic activity than with a sudden catastrophe.

🎧 リスニング講義

The reading argues that volcanic eruptions were the main cause of dinosaur extinction, but this view is seriously flawed. First, the timing of the Deccan Traps is not as precise as claimed. While the main eruptions began around 66 million years ago, they actually spanned a period of over a million years, with many pulses occurring both before and after the extinction horizon. In contrast, the Chicxulub impact occurred exactly at the boundary layer, within a few thousand years of the extinction. This precise coincidence strongly supports the impact as the immediate trigger. Second, the climate effects of volcanoes were likely insufficient to cause mass extinction. Sulfur aerosols from the Deccan Traps would have caused some cooling, but the eruptions were intermittent, allowing the climate to recover between phases. In contrast, the asteroid impact would have injected dust and soot into the stratosphere, blocking sunlight for months and causing a global blackout, a much more severe and rapid disruption. Third, the reading's claim about the crater's age is misleading. Recent high-precision radiometric dating places the Chicxulub impact at 66.04 million years ago, virtually simultaneous with the extinction. Moreover, the pre-impact decline of species can be explained by volcanic stress, but it was the impact that delivered the final, fatal blow. Without the impact, dinosaurs might have survived the volcanic activity, as they had survived other volcanic episodes in Earth's history.