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 eruptions in the Deccan Traps of India were the primary cause. First, the timing of the Deccan eruptions overlaps with the extinction event. Geochemical dating shows that the main phase of volcanic activity began about 66 million years ago, just before the extinction, and continued for roughly 700,000 years. This prolonged period of volcanic activity would have released enormous amounts of sulfur dioxide and carbon dioxide, leading to severe global cooling followed by warming, which could have disrupted ecosystems over a long time. Second, volcanic eruptions produce iridium, an element found in high concentrations at the Cretaceous-Paleogene boundary. While iridium is often cited as evidence of an asteroid impact, it is also present in volcanic gases and lava, and the Deccan Traps alone could have supplied the observed iridium levels. Third, the fossil record shows a gradual decline in dinosaur diversity over the last few million years of the Cretaceous, not a sudden die-off. This pattern is more consistent with the long-term environmental stress caused by volcanism than with a sudden impact, which would have caused a rapid extinction.

🎧 Listening Lecture

The professor disagrees with the reading's claim that volcanic eruptions were the main cause of dinosaur extinction. She argues that the asteroid impact remains the more plausible explanation. First, while the Deccan eruptions overlapped in time with the extinction, the most intense phase of volcanism actually occurred about 300,000 years before the extinction event, not exactly at the boundary. The timing is therefore not as precise as the reading suggests, and the impact occurred exactly at the boundary layer, making it a more likely trigger. Second, iridium is not a reliable indicator of volcanism. The amount of iridium found at the boundary is far higher than what any known volcanic eruption could produce. In fact, iridium is extremely rare in Earth's crust and mantle, but it is abundant in asteroids. The iridium layer also contains shocked quartz and tektites, which are only formed under the extreme pressure of an impact, not by volcanic activity. Third, the gradual decline in dinosaur fossils may be an artifact of incomplete fossil records. Many dinosaur-bearing rock layers from the late Cretaceous are missing or poorly sampled, especially in the regions where dinosaurs were most diverse. When more complete sections are examined, the fossil record shows a sudden disappearance of dinosaurs at the boundary, consistent with an impact. Thus, the reading's arguments do not withstand scrutiny.