TOEFL Integrated Writing

📖 Reading Passage

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 massive volcanic activity, specifically the Deccan Traps in present-day India, was the primary driver. First, the timing of the Deccan Traps eruptions coincides with the extinction event. Radiometric dating shows that the main phase of volcanic activity occurred around 66 million years ago, precisely when the dinosaurs disappeared. Second, volcanic emissions of sulfur dioxide and carbon dioxide would have caused severe climate change, including rapid global warming and ocean acidification, which could have disrupted ecosystems long before any impact. Third, the fossil record reveals a gradual decline in dinosaur diversity over the last few million years of the Cretaceous, consistent with a slow, prolonged stressor like volcanism, rather than a sudden catastrophic event. These points suggest that volcanic activity, not an asteroid, was the decisive factor.

🎧 Listening Lecture

The reading presents a compelling case for volcanism, but it overlooks critical evidence that strongly supports the asteroid impact hypothesis. First, while the Deccan Traps eruptions did occur near the extinction boundary, their most intense phase actually began about 300,000 years before the impact and lasted for over a million years. If volcanism were the cause, we would expect a gradual extinction pattern aligned with this prolonged activity. However, the fossil record shows a sudden and catastrophic extinction at the boundary, with over 75% of species disappearing in a geologically instantaneous moment. Second, the climate effects of volcanism, though significant, were not severe enough to cause a global extinction. For example, similar massive eruptions, such as the Siberian Traps at the end of the Permian, did lead to extinction, but the Deccan Traps’ sulfur and carbon emissions were far smaller in magnitude and spread over a longer period, allowing ecosystems to adapt. Third, the claim of a gradual dinosaur decline is based on incomplete fossil sampling. More recent high-resolution studies in North America show that dinosaur diversity remained stable until the very end of the Cretaceous, with no evidence of a long-term decline. Instead, the abrupt disappearance coincides exactly with the Chicxulub impact. Thus, the asteroid remains the most plausible cause.