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  3. One of the most catastrophic changes our planet has ever experienced was the K-T extinction event. Taking place over 65 million

One of the most catastrophic changes our planet has ever experienced was the K-T extinction event. Taking place over 65 million

admin2012-01-14  61
问题    One of the most catastrophic changes our planet has ever experienced was the K-T extinction event. Taking place over 65 million years ago, this mysterious happening resulted in the disappearance of thousands of plant and animal species, most famously the large reptiles known as dinosaurs. While many theories have been proposed for the cause of these extinctions, the majority of evidence suggests that responsibility lies with increased volcanic activity all over the world, including in the Deccan Traps, a region in present-day India.
   The first piece of evidence supporting the volcanism theory is the existence of a giant impact crater in the seafloor just off the coast of Mexico. The volcanic activity was so fierce that huge chunks of rock were ejected high into the atmosphere, and when they returned to Earth, they caused tremendous destruction. The Mexico crater records one of the biggest of these impacts, which likely caused an explosion severe enough to kill every living thing within a radius of thousands of miles.
   Volcanoes ejected massive amounts of sulfur, carbon dioxide, and dust into the atmosphere. These substances would have contributed to species extinctions in many ways, primarily by blocking sunlight from reaching Earth’s surface. With less sunlight, plants would have been unable to grow, and the entire food chain would have collapsed. Also, the increase in atmospheric carbon dioxide likely created an intense greenhouse effect, altering natural climate patterns and further disrupting the living conditions for plants and animals.
   Another clue that points to volcanism as the cause of the K-T extinctions is the timing of Deccan Trap activity. According to geological research, the increase in volcanic activity coincided closely with the disappearance of many of Earth’s species. The data prove that this is too unlikely to be a coincidence; the similar timing of both happenings proves that the K-T extinctions were brought about by a sudden, sharp rise in Deccan Trap and other volcanic activities.
   [L]ECTURE

   [R]ESPONSE
   TOEFL Writing
   Summarize the points made in the lecture, being sure to explain how they cast doubts on the points made in the reading passage.
   225 WORDS
选项
答案 [L] ECTURE SCRIPT [Professor] (female) People have been speculating as to the cause of the disappearance of the dinosaurs ever since these giant reptiles were discovered. Really, the mystery of the K-T extinctions is one of the most interesting and debated in all of geology. But there’s a pretty strong consensus among scientists that the base cause of this catastrophic event was a giant asteroid impacting the Earth. Let’s start with the impact crater that was found in 1980 just off the east coast of Mexico. Rather than evidence of volcanic eruptions, this crater appears to be the direct result of an asteroid strike. The size and shape of the crater correspond to what an asteroid collision would look like, and its age matches up almost exactly with the timing of the K-T extinctions. There’s little doubt among most scientists that this Mexican crater tells the story of the K-T event. Besides a tremendous initial explosion, an asteroid impact would’ve led to many other deadly conditions on Earth. Similar to what’s described in the volcanism theory, large amounts of toxic chemicals and asteroid debris would have been shot into the atmosphere, creating a veritable screen that blocked out sunlight and killed plants. There’s strong evidence in the geologic record that supports this...nameiy, a layer of iridium that can be found worldwide corresponding to the time of the impact and extinctions. You see, iridium is relatively rare in Earth’s crust but abundant in most asteroids. The existence of this iridium layer strongly suggests that an asteroid visited our planet at the K-T boundary. And finally...I know I’ve mentioned a bit about how the timing of both the proposed asteroid impact and the extinction event match up pretty closely. But doesn’t the timing of the Deccan Trap activity do that too? Well...not really. In fact, recent studies have concluded that the increase in volcanic eruptions began well before species started going extinct. This calls into question the validity of the volcanism theory, making the impact theory all the more plausible. [R]ESPONSE In both the reading and the lecture, the central theme is the cause of the K-T extinction event. The reading passage and the lecture present two theories about what caused the dinosaurs to die out 65 million years ago. While the reading passage defends the volcanism theory, the professor supports the asteroid impact theory. Firstly, the reading passage cites as evidence of the volcanism theory an enormous crater off the coast of Mexico, which was formed by debris from volcanic explosions. The professor refutes this idea, however, claiming that the crater was instead formed by an asteroid impact, indicating that the size and shape of the crater are consistent with the expected form of an asteroid crater. Second, the reading passage explains that the volcanism theory accounts for the mass extinctions, as volcanoes expelled huge amounts of dust into the air, blocking out sunlight and essentially destroying the food chain. The professor counters this argument, explaining that the toxic substances in the atmosphere were actually caused by the forceful impact of an asteroid. She points to the discovery of a layer of iridium in the Earth’s crust as evidence of the impact, as iridium is rarely found elsewhere on Earth. Finally, the reading passage proposes that the timing of Deccan Trap activity corresponds to the mass extinction of the dinosaurs so closely that it cannot be a coincidence. However, the professor counters this claim, suggesting that Deccan Trap activity began well before the extinction event. She says it corresponds perfectly with the supposed timing of the asteroid impact, proving this theory is actually correct.
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