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 sustained 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 began about 66.3 million years ago and lasted for roughly 750,000 years, which brackets the extinction horizon at 66 million years ago. This prolonged eruption would have released enormous quantities of sulfur dioxide and carbon dioxide, leading to severe global cooling, acid rain, and ocean acidification—conditions fatal to many species. Second, the fossil record indicates a gradual decline in dinosaur diversity well before the asteroid impact. In the last few million years of the Cretaceous, dinosaur fossils become increasingly rare, and many species disappeared from the record earlier than the impact layer. This pattern is more consistent with a slow, ongoing environmental stress caused by volcanism rather than a sudden catastrophic event. Third, the asteroid impact, while dramatic, was too short-lived to cause long-term ecological collapse. The Chicxulub impact released energy equivalent to billions of atomic bombs, but the resulting dust and aerosols would have settled within months to years. In contrast, the Deccan eruptions continued for hundreds of thousands of years, maintaining a persistent state of environmental crisis that would have overwhelmed ecosystems and driven species to extinction over time.
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The reading presents a compelling case for volcanism, but the evidence actually points to the asteroid impact as the decisive cause. First, the claim about timing is misleading. While the Deccan eruptions did begin before the extinction, the most intense phase—the one that released 80% of the total lava—occurred after the impact, not before. In fact, the impact may have triggered a surge in volcanic activity by causing seismic shocks that destabilized the magma chamber. Therefore, the eruptions were a consequence, not the cause, of the extinction. Second, the gradual decline in dinosaur diversity cited in the reading has been disputed by recent high-resolution studies. When researchers account for sampling biases, such as the fact that younger rocks are less likely to preserve fossils, the apparent decline disappears. Dinosaurs were thriving until the very end, and their sudden disappearance aligns precisely with the impact layer, not with the earlier volcanic phases. Third, the argument that the impact was too short-lived is flawed. The immediate effects—wildfires, tsunamis, and a dust cloud that blocked photosynthesis for months—were devastating enough. Moreover, the impact injected sulfur aerosols into the stratosphere, causing a global impact winter that lasted for years to decades, which would have collapsed food chains worldwide. This rapid and severe disruption, not the prolonged but less intense volcanic activity, matches the sudden extinction pattern seen in the fossil record.
📝 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.
