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 India, was the primary driver. First, the timing of the Deccan Traps eruptions aligns closely with the extinction event. Radioisotopic dating indicates that the main phase of volcanic activity occurred over a relatively short period, approximately 66 million years ago, coinciding with the Cretaceous-Paleogene boundary. This temporal correlation suggests a causal link. Second, volcanic emissions would have caused severe environmental stress. The Deccan Traps released vast quantities of sulfur dioxide and carbon dioxide, leading to acid rain, global cooling followed by warming, and ocean acidification. These changes could have disrupted food chains and caused habitat loss, sufficient to trigger a mass extinction without an impact. Third, the fossil record shows a gradual decline in dinosaur diversity in the few million years before the boundary, which is more consistent with a prolonged volcanic stressor than a sudden impact. Paleontological studies in North America and India reveal that dinosaur populations were already dwindling, with fewer species and reduced population sizes, indicating that they were vulnerable before any potential asteroid strike. Thus, while an impact may have occurred, the volcanic hypothesis better explains the pattern and timing of the extinction.
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The reading argues that volcanic activity alone caused the dinosaur extinction, but this view is contradicted by several lines of evidence. First, the timing argument is flawed. Although the Deccan Traps began erupting around 66 million years ago, the most intense phase of volcanic activity actually occurred about 200,000 to 300,000 years *before* the impact layer that marks the extinction. In contrast, the Chicxulub impact in Mexico happened precisely at the boundary, within a few thousand years of the extinction. This sudden, catastrophic event matches the abrupt fossil disappearance, whereas the volcanic emissions were spread over a longer period. Second, the environmental effects of the Deccan Traps, while significant, were not severe enough to cause a global extinction. Climate models suggest that volcanic sulfur and CO2 caused a temperature change of only a few degrees, which many species could survive. However, the impact would have generated a global firestorm, tsunamis, and a dust cloud that blocked sunlight for months, halting photosynthesis and collapsing the food chain—a far more lethal scenario. Third, the claim of a gradual dinosaur decline is based on incomplete fossil sampling. In many regions, dinosaur fossils are rare in the last million years before the boundary, but this may reflect poor preservation conditions rather than actual decline. In fact, recent finds in the Hell Creek Formation show that dinosaur diversity remained high until the very end, with no evidence of a multi-million-year decline. Therefore, the impact remains the sole plausible cause for the sudden extinction.
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
