TOEFL Integrated Writing

📖 Reading Passage

The extinction of the dinosaurs at the end of the Cretaceous period, about 66 million years ago, 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, the Deccan eruptions released enormous quantities of sulfur dioxide and carbon dioxide over hundreds of thousands of years, leading to severe global cooling and ocean acidification. These environmental stresses likely weakened ecosystems long before the impact, causing gradual declines in many species. Second, the fossil record shows a pattern of extinction that correlates more closely with the timing of volcanic phases than with the single impact event. For example, marine microfossils indicate a stepwise extinction pattern beginning about 300,000 years before the impact, coinciding with the main pulse of Deccan volcanism. Third, the impact hypothesis fails to explain why certain groups, such as crocodiles and turtles, survived despite the presumed global catastrophe. If the impact had been truly catastrophic, these hardy species would have suffered equally. In contrast, volcanic effects, which were more regional and prolonged, could have allowed some species to adapt. Therefore, the Deccan volcanism, not the asteroid, was the dominant driver of the end-Cretaceous mass extinction.

🎧 Listening Lecture

The lecture challenges the volcanic hypothesis presented in the reading. While the Deccan eruptions were indeed massive, their role in the extinction is overstated. First, the sulfur dioxide and carbon dioxide released by the Deccan volcanoes may have caused climate change, but the timing does not align with the observed extinction. High-precision radioisotopic dating shows that the main phase of Deccan volcanism ended about 200,000 years before the impact, and the most severe extinctions occurred within a very short period right after the asteroid hit. Second, the stepwise extinction pattern cited by the reading can be explained by the impact itself, which would have triggered immediate but also delayed effects, such as a prolonged 'impact winter' and subsequent greenhouse warming. These effects could produce a stepwise pattern over tens of thousands of years, not necessarily requiring volcanic activity. Third, the survival of crocodiles and turtles is not evidence against an impact. These species are ectothermic and have lower metabolic rates, making them more resilient to rapid environmental changes. Moreover, they often inhabit burrows or aquatic environments that could have buffered them from the worst of the impact's effects. In contrast, the volcanic theory struggles to explain the sudden, simultaneous extinction of many terrestrial and marine species, which is a hallmark of a catastrophic impact. Thus, the asteroid impact remains the most parsimonious explanation for the mass extinction.