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 intense volcanic activity in the Deccan Traps of India was the primary cause. First, the timing of the Deccan eruptions aligns closely with the extinction event. Radioisotopic dating shows that the main phase of volcanic activity occurred just before and during the Cretaceous-Paleogene boundary, releasing vast amounts of lava over a relatively short geological interval. Second, volcanic emissions could have caused severe environmental stress. The Deccan Traps released enormous quantities of sulfur dioxide and carbon dioxide, leading to acid rain, ocean acidification, and rapid global cooling followed by long-term warming. These climate shifts would have disrupted food chains and habitats, making it difficult for many species, including non-avian dinosaurs, to survive. Third, the fossil record reveals a gradual decline in dinosaur diversity well before the asteroid strike. Studies of dinosaur fossils in North America show that species richness was already decreasing in the last few million years of the Cretaceous, suggesting that volcanic-induced environmental deterioration was already taking its toll. Therefore, while an asteroid impact may have delivered the final blow, the primary driver of the extinction was likely the prolonged volcanic activity.
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The professor challenges the volcanic hypothesis presented in the reading. First, although the Deccan eruptions began before the extinction, the most intense volcanic phase actually occurred about 300,000 years after the Cretaceous-Paleogene boundary. The reading claims a precise alignment, but the critical extinction event happened abruptly, while the bulk of lava was emitted later, so the timing does not support a causal link. Second, the environmental effects of volcanic gases, while significant, were likely not severe enough to cause a global mass extinction. Climate models indicate that the cooling and warming from the Deccan eruptions were moderate and occurred over long timescales, allowing species to adapt. In contrast, an asteroid impact would have caused a sudden “impact winter” with a dramatic drop in sunlight and photosynthesis, a mechanism far more lethal. Third, the claim of a gradual dinosaur decline is based on incomplete fossil sampling. More recent, high-resolution studies in the same regions show that dinosaur diversity remained stable until the very end of the Cretaceous, and apparent declines are artifacts of the Signor-Lipps effect, where the absence of fossils near the boundary is due to poor preservation. Therefore, the asteroid impact remains the most plausible cause of the mass 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.
