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 volcanic activity, specifically the Deccan Traps eruptions in India, was the primary cause. First, the timing of the Deccan Traps eruptions coincides closely with the extinction event. Radioisotopic dating indicates that the main phase of volcanic activity occurred around 66 million years ago, lasting for roughly 700,000 years, which brackets the extinction horizon. Second, volcanic emissions would have caused severe environmental stress. The eruptions released vast quantities of sulfur dioxide and carbon dioxide, leading to acid rain, global cooling followed by greenhouse warming, and ocean acidification—all of which could have disrupted ecosystems long before any impact. Third, the fossil record shows a gradual decline in dinosaur diversity in the millions of years before the impact, suggesting that volcanic-induced environmental changes were already pushing species toward extinction, making the impact merely the final blow rather than the sole cause.

🎧 Listening Lecture

The lecture challenges the idea that volcanic activity was the primary cause of dinosaur extinction, arguing that the asteroid impact was the decisive factor. First, while the Deccan Traps eruptions did begin before the extinction, their most intense phase actually occurred after the impact, not before. The largest volcanic pulses postdate the extinction event by about 200,000 years, so the timing does not support a volcanic kill mechanism. Second, the environmental effects of the eruptions, though significant, were not severe enough to cause a mass extinction. For example, similar large igneous province eruptions, such as the Siberian Traps, did correlate with extinction, but the Deccan Traps released gases over a much longer period, allowing the climate to recover between pulses. In contrast, the asteroid impact released energy equivalent to billions of nuclear bombs, causing immediate wildfires, tsunamis, and a global dust cloud that blocked photosynthesis for months. Third, the claim of a gradual decline in dinosaur fossils is misleading. The apparent decline is likely due to sampling biases, as dinosaur fossils are rare and become less common in younger rock layers. Recent high-resolution studies show that dinosaur diversity was stable until the very end of the Cretaceous, and the extinction was sudden, consistent with an impact, not a prolonged volcanic crisis.