The extinction of the dinosaurs at the end of the Cretaceous period has long been attributed to a massive asteroid impact. However, a closer examination of the fossil record and geological evidence suggests that volcanic activity, specifically the Deccan Traps in India, was the primary driver of this mass extinction. Three key lines of evidence support this view. First, the Deccan Traps eruptions began well before the asteroid impact, approximately 66.3 million years ago, and continued for nearly a million years. This prolonged volcanic activity would have released vast amounts of sulfur dioxide and carbon dioxide into the atmosphere, causing severe global cooling followed by warming, which could have stressed ecosystems long before the impact. Second, the timing of the extinction, as recorded in sedimentary layers, shows a gradual decline in dinosaur diversity over the last few hundred thousand years of the Cretaceous, rather than a sudden die-off consistent with an impact. This gradual pattern aligns with the cumulative effects of volcanic emissions, which would have disrupted plant life and food chains over time. Third, recent geochemical analyses of marine sediments reveal spikes in mercury, a volcanic tracer, that correlate with periods of environmental stress and extinction pulses, even in layers predating the impact. These mercury anomalies suggest that volcanic activity alone was sufficient to trigger biological crises, independent of any extraterrestrial event.
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The reading argues that volcanic activity was the main cause of dinosaur extinction, but this claim is flawed. While the Deccan Traps eruptions did begin before the impact, the most intense phase of volcanism occurred after the impact, not before. In fact, the impact at Chicxulub triggered a dramatic increase in volcanic activity, so the bulk of sulfur and carbon emissions occurred in the aftermath, meaning they could not have caused the initial extinction. Furthermore, the alleged gradual decline in dinosaur diversity is misleading. Recent high-resolution studies of fossil assemblages show that dinosaur populations were thriving until the very end of the Cretaceous, with no significant decline in the last 100,000 years. The apparent gradual decline in earlier studies was due to sampling biases and incomplete fossil records. Finally, the mercury spikes cited as volcanic tracers are not exclusive to volcanism; they can also be produced by impact-related processes, such as the vaporization of mercury-rich rocks at the impact site. In fact, the largest mercury anomaly coincides exactly with the impact layer, not with earlier volcanic phases. Thus, the evidence strongly points to the asteroid impact as the decisive cause, with volcanism playing only a minor, secondary role.
📝 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).
