GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 7AO443P8FVS5P407K

Roughly 250 million years ago, in the worst series of mass extinction in Earth's history, almost all species of life simply vanished. The traditionally held view has been that these extinctions occurred gradually, as changing environmental conditions – global cooling, global warming, dropping sea levels, or some combination of such factors – made life increasingly difficult during the last stages of the Permian period (284-245 million years ago). As Permian life-forms declined, they reached a fatal threshold and species succumbed to mass extinction. Supporters of the traditionalist view note that species begin disappearing from the fossil record well before the end of the Permian period. Yet, while such a pattern could have resulted from a slowly rising extinction rate, some scientists argue that just because a species disappears from the fossil record doesn't necessarily mean it became extinct at that point. Moreover, Paul Wignall and others have found sites where marine fossil deposits reached a peak of diversity at the very end of the Permian period. Thus, Wignall believes late-Permian marine life was thriving until it was decimated suddenly by some catastrophe. Some geoscientists think that this catastrophe involved a meteor striking the earth. MichaelRampino, for instance, claims that a giant meteor crashed into Gondwanaland (the southern portion of the supercontinent Pangea, which covered about 30 percent of the globe in Permiantimes), setting off intense volcanism. Airborne soot and dirt from the impact blanketed Earth, inhibiting photosynthesis and triggering mass extinction of species. This view is widely disputed by traditionalists, however, Grant Young, for example, holds that the gradual breakup of the Pangean supercontinent triggered widespread climatic change and glaciation, causing the Permian mass extinctions to occur over millions of years. Glaciation, Young claims, has occurred throughout Earth's history, and there is evidence that it occurred toward the end of the Permian period. Yet Rampino and others question one of the traditionalists' basic assumption: the periodic occurrence of glaciation throughout Earth's history. Crucial evidence includes the rock deposits known as tillites, because they resemble the unstratified drift produced by modern glaciers, geologists have long considered tillites to be signposts of glaciation. When trying to determine whether particular sedimentary layers represent tillite deposits, geologists look for stones with faceted shapes, scratched surfaces, and other features that presumably resulted from glacial action. The problem with ascribing all tillites to glaciation, however, is that tillite deposits are widespread in Precambrian sediments. Yet temperatures during the Precambrian (3.8 billion to 540 million years ago) are thought to have been warmer than those in today's non-glacial world. Thus, Rampino thinks that certain tillites were produced by meteor impacts, not glaciers. The first compelling evidence that meteor impacts might have produced tillites came from Moon rocks that showed the same features as earthly tillites. Since no one believes the Moon ever had glaciers that leaves but one possible cause: impacts.