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 in present-day India, was the primary driver of this mass extinction. First, the timing of the Deccan Traps eruptions coincides with the extinction event. Radiometric dating places the main phase of volcanic activity at approximately 66 million years ago, exactly when the dinosaurs disappeared. Second, volcanic eruptions released enormous quantities of sulfur dioxide and carbon dioxide, leading to severe climate change. Sulfur aerosols caused rapid global cooling, while long-term CO2 emissions triggered greenhouse warming, creating unstable conditions that stressed ecosystems. Third, the fossil record shows a gradual decline in dinosaur diversity in the million years before the impact, which aligns with the onset of intense volcanic activity rather than a sudden asteroid strike. This gradual decline suggests that volcanic stress had already weakened dinosaur populations, making them vulnerable to any final perturbation, whether volcanic or impact-related.
🎧 听力讲座
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The lecture challenges the volcanic hypothesis presented in the reading. While the Deccan Traps were indeed active around 66 million years ago, the timing does not prove causation. The most catastrophic phase of volcanic eruptions occurred about 200,000 years before the extinction event, and the dinosaurs showed no significant decline during that period. In contrast, the asteroid impact at Chicxulub occurred precisely at the extinction boundary, and its effects were immediate and devastating. The lecture also argues that the climate changes from volcanism were not as severe as claimed. Geological evidence indicates that volcanic gases may have caused some cooling, but the overall temperature fluctuations were within ranges that dinosaurs had survived earlier. Moreover, the claimed gradual decline in dinosaur diversity is misleading. The apparent decline in the fossil record is likely due to sampling bias—fewer sedimentary deposits from that time have been preserved. When corrected for sampling, dinosaur populations appear stable until the very end of the Cretaceous. Therefore, the abrupt, catastrophic asteroid impact remains the most convincing explanation for the extinction.
📝 작문 지시 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
📊 TOEFL Integrated Writing Rubric
Content Accuracy: Accurately capture the relationship between lecture and reading
Information Completeness: Cover all key counter-arguments from the lecture
Structure & Coherence: Logical organization and progression
Language Use: Grammar, vocabulary, expression accuracy
Scored 0-30 based on holistic scoring (original 0-5, scaled to 30).
