The extinction of the dinosaurs at the end of the Cretaceous period (about 66 million years ago) 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. Three main arguments support this volcanic hypothesis. First, the timing of the Deccan eruptions, which lasted for roughly one million years, overlaps with the extinction event. Geochemical dating shows that the main phase of volcanic activity occurred just before and at the boundary layer, making volcanism a more plausible kill mechanism than a single, instantaneous impact. Second, volcanic emissions of sulfur dioxide and carbon dioxide would have caused severe climate swings—global cooling from sulfate aerosols followed by long-term warming from CO2—that could have stressed ecosystems over thousands of years, matching the gradual decline observed in many fossil records prior to the extinction. Third, the Deccan eruptions produced enormous quantities of toxic gases, such as hydrogen fluoride and mercury, which would have poisoned soils and water sources, directly harming plants and animals. Collectively, these long-term, sustained environmental disruptions align better with the fossil evidence of a prolonged extinction interval than the sudden, short-term effects of an asteroid strike.
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The reading presents a compelling case for volcanism, but its arguments are fundamentally flawed. First, while the Deccan eruptions did occur near the extinction, the most intense phase of volcanic activity actually began about 300,000 years before the extinction event, and the extinction itself was extremely sudden—within a few thousand years. This mismatch in timing undermines the idea that gradual volcanic stress caused the die-off. In contrast, the Chicxulub impact in Mexico occurred exactly at the boundary, within a narrow window of tens of thousands of years, making it a far more precise trigger. Second, the climate effects of volcanism would have been cyclical and perhaps survivable, as similar large igneous provinces in the past did not cause mass extinctions. However, the asteroid impact would have caused an immediate, catastrophic ‘impact winter’—blocking sunlight with dust and soot for months, halting photosynthesis, and collapsing food chains globally. This sudden shock is more consistent with the abrupt disappearance of many species at the boundary. Third, the claim about toxic gas poisoning is overstated. Mercury and other toxins from volcanism are found in the sediment record, but they are spread over hundreds of thousands of years, allowing ecosystems to adapt. In contrast, the impact released a cloud of vaporized rock and metals, including iridium, which is globally distributed in a thin, concentrated layer at the boundary—a signature that cannot be explained by volcanic emissions. Thus, the evidence overwhelmingly favors the asteroid impact as the principal cause.
📝 Writing Prompt Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
📊 TOEFL Integrated Writing Rubric
Content Accuracy: Accurately capture the relationship between lecture and reading
Information Completeness: Cover all key counter-arguments from the lecture
Structure & Coherence: Logical organization and progression
Language Use: Grammar, vocabulary, expression accuracy
Scored 0-30 based on holistic scoring (original 0-5, scaled to 30).
