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 in the Deccan Traps of India was the primary cause. First, volcanic eruptions release vast amounts of sulfur dioxide and carbon dioxide, which can cause rapid climate change. Studies of sediment layers from that time show a spike in sulfur and other volcanic gases precisely at the extinction boundary, indicating that volcanic emissions were strong enough to alter global temperatures and disrupt ecosystems. Second, the timing of the Deccan eruptions aligns closely with the extinction event. Radioisotopic dating of lava flows places the main phase of volcanism at about 66 million years ago, overlapping with the extinction horizon. This temporal correlation suggests a causal link, as the eruptions would have persisted for hundreds of thousands of years, causing prolonged environmental stress. Third, unlike a sudden impact, volcanic activity would have produced gradual but cumulative effects, such as ocean acidification and ozone depletion. Fossil records show a decline in biodiversity over the last 100,000 years before the extinction, consistent with a slow-burning crisis rather than an instantaneous catastrophe. Therefore, the Deccan Traps volcanism, not an asteroid, drove the dinosaurs to extinction.

🎧 リスニング講義

The reading presents a compelling case for volcanism, but it overlooks key evidence that strongly supports the asteroid impact theory. First, while volcanic gases can cause climate change, the sulfur spike at the extinction boundary is actually more consistent with an impact. Asteroid impacts vaporize sulfate-rich rocks, injecting sulfur into the stratosphere in a single, massive pulse. Such a pulse would cause a sudden, severe cooling—a 'impact winter'—that is far more devastating than the prolonged, moderate cooling from volcanism. Moreover, the timing argument is flawed. The Deccan eruptions began well before the extinction, but the main phase of volcanism actually ended about 200,000 years before the extinction horizon. In contrast, the Chicxulub impact crater in Mexico has been dated to exactly 66 million years ago, coinciding precisely with the extinction. Third, the gradual decline in biodiversity cited in the reading can be explained by the Deccan eruptions, but it was a pre-existing stress that did not cause extinction. The final blow came from the asteroid, which triggered a global firestorm, tsunamis, and a dust cloud that blocked photosynthesis for months. This catastrophic event, not the volcanic activity, was the tipping point that wiped out the dinosaurs and 75% of all species.