The extinction of the dinosaurs at the end of the Cretaceous period, about 66 million years ago, has long fascinated scientists. While the asteroid impact theory is widely accepted, a competing hypothesis suggests that massive volcanic eruptions in the Deccan Traps of India were the primary cause. This volcanic theory rests on three main arguments. First, the Deccan eruptions were prolonged and massive, releasing enormous amounts of sulfur dioxide and carbon dioxide over hundreds of thousands of years. Such emissions could have caused severe global cooling (from sulfate aerosols) followed by long-term warming (from CO2), creating drastic climate instability that stressed ecosystems long before the asteroid struck. Second, paleontological evidence shows a gradual decline in dinosaur diversity in the few million years leading up to the extinction, consistent with the cumulative effects of volcanic stress rather than a sudden catastrophe. Third, the timing of the main pulse of Deccan volcanism, dated to just before the boundary layer, overlaps with the extinction event, suggesting that volcanic activity, not the asteroid, delivered the final blow. Therefore, the volcanic hypothesis argues that the dinosaurs were already on the brink of extinction due to environmental deterioration from the Deccan traps, and the asteroid was merely a footnote.
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The reading presents the volcanic hypothesis, but recent evidence strongly favors the asteroid impact as the sole or dominant cause. First, while volcanic gases can affect climate, the magnitude and rapidity of the asteroid impact’s effects were far more severe. The impact ejected dust and soot into the stratosphere, blocking sunlight for months to years, causing a sudden global darkness and a sharp drop in temperature known as an impact winter. In contrast, volcanic emissions were spread over millennia, allowing ecosystems to partially adapt. Second, the claim of gradual dinosaur decline is misleading. High-resolution fossil records from the latest Cretaceous show that dinosaurs were thriving until the very end, with no significant diversity loss. The apparent decline in older studies was due to sampling biases, such as incomplete fossil preservation. When corrected, the data reveal a sudden extinction at the boundary. Third, the timing argument is flawed. Recent radiometric dating places the main Deccan phase after the extinction event, not before. The most intense volcanic activity occurred within 250,000 years after the impact, meaning volcanism could not have been the immediate cause. Furthermore, the impact site, Chicxulub, is dated precisely to the extinction moment. Thus, the asteroid impact is the only event capable of causing the rapid, global extinction observed.
📝 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.
