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 prolonged volcanic activity in the Deccan Traps of India was the primary cause. First, the timing of volcanic eruptions aligns closely with the extinction event. Radioisotopic dating of Deccan lava flows indicates that the most intense phase of eruptions occurred just before and during the mass extinction, spanning roughly 66.0 to 65.5 million years ago. This near-synchrony challenges the idea that a single impact event was responsible. Second, volcanic emissions of sulfur dioxide and carbon dioxide would have caused drastic climate changes. Sulfur aerosols could have triggered a rapid global cooling, while carbon dioxide would have led to long-term warming, creating stress on ecosystems that may have already been vulnerable. Third, the fossil record reveals a gradual decline in dinosaur diversity over the last few million years of the Cretaceous, consistent with the cumulative effects of volcanic activity, rather than a sudden catastrophic kill. Thus, the Deccan Traps volcanism, not an asteroid, likely drove the dinosaurs to extinction.
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The reading presents an intriguing case for volcanism, but it overlooks critical counterevidence. First, while the Deccan eruptions overlapped with the extinction, they were not a single pulse. In fact, the most intense volcanic phase occurred about 300,000 years before the extinction, and the later, smaller eruptions were insufficient to cause a global catastrophe. In contrast, the Chicxulub impact happened exactly at the extinction boundary, as evidenced by a global iridium layer. Second, the climate effects of volcanism were likely less severe than proposed. Sulfur aerosols from the Deccan eruptions may have caused only moderate, short-term cooling, not the prolonged stress needed to kill off dinosaurs. Meanwhile, the impact would have injected dust and soot that blocked sunlight for months, causing a sudden collapse in photosynthesis. Third, the claim of a gradual dinosaur decline is misleading. High-resolution fossil studies from the latest Cretaceous show that dinosaur populations were stable and diverse until the very end, and their disappearance is abrupt in the stratigraphic record. Therefore, the impact event remains the most plausible trigger for the mass extinction.
📝 Writing Prompt 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).
