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 hypothesis is widely accepted, a competing theory suggests that massive volcanic eruptions in the Deccan Traps of India were the primary cause. Proponents of the volcanic theory offer three main arguments. First, the Deccan Traps eruptions, which lasted for nearly a million years, released enormous quantities of sulfur dioxide and carbon dioxide into the atmosphere. These gases would have caused severe global cooling followed by warming, creating dramatic climate stress that could have disrupted ecosystems long before the asteroid struck. Second, sedimentary layers from the Cretaceous-Paleogene boundary show a gradual decline in dinosaur diversity in the millions of years leading up to the extinction event, suggesting that dinosaurs were already struggling due to volcanic-induced environmental changes. Third, the timing of the main phase of Deccan volcanism appears to coincide with the extinction horizon, with recent high-precision dating placing the most intense eruptions within a few hundred thousand years of the event. This close temporal correlation, argue volcanists, implies a causal link that does not require an asteroid impact to explain the mass dying.
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The lecture challenges the volcanic theory, asserting that the asteroid impact remains the most plausible explanation. First, while volcanic gases can alter climate, their effects would have been gradual and spread over hundreds of thousands of years, allowing many species to adapt. In contrast, the asteroid impact would have triggered an immediate and catastrophic cooling, blocking sunlight with dust and causing a sudden collapse in photosynthesis. This rapid change is more consistent with the abrupt extinction pattern seen in the fossil record. Second, the claim of declining dinosaur diversity before the impact is based on incomplete data. The fossil record is biased because sedimentary layers from that time are sparse in many regions, and recent discoveries show that dinosaur species were actually thriving until the very end of the Cretaceous. Third, the timing argument is flawed: the most intense Deccan eruptions occurred about 250,000 years before the impact, not at the extinction boundary. Moreover, the extinction event itself is marked by a layer of iridium, a rare metal in Earth's crust but common in asteroids, which is definitive evidence of a large impact. The volcanic theory cannot explain this iridium anomaly, whereas the asteroid hypothesis does so elegantly.
📝 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.
