托福阅读真题+题目+答案：Causes of Glacial Ages

托福阅读真题+题目+答案：Causes of Glacial Ages

Any theory that attempts to explain the causes of glacial ages must successfully address two basic questions.First,what causes the onset of glacial conditions?For ice sheets to have formed,average temperatures must have been somewhat lower than at present and perhaps substantially lower than throughout much of geologic time. For that reason,a successful explanation would have to account for the gradual cooling that finally leads to glacial conditions.The second question is:What caused the alternation of glacial and interglacial stages that have been documented for the Pleistocene epoch (1.6 million to 10,000 years ago)?Whereas the first question deals with long-term trends in temperature that occur on a scale of millions of years,this second question relates to much shorter-term changes.

1.According to paragraph 1,a theory of past glacial ages must explain why the Earth was much cooler than it is today and

O what caused the Pleistocene epoch to end 10,000 years ago

O why during the Pleistocene epoch glaciers reoccurred at relatively short intervals

O how glaciers affect the long-term trend of warming on Earth

O how the Pleistocene epoch differed from previous glacial periods

Probably the most attractive proposal for explaining why extensive glaciations have occurred only a few times in the geologic past comes from the theory of plate tectonics.Not only does this theory provide geologists with explanations about many previously misunderstood processes and features,it also provides a possible explanation for some previously unexplainable climatic changes,including the onset of glacial conditions.Since glaciers can form only on land,we know that landmasses must exist somewhere in the higher latitudes before an ice age can commence.Many believe that ice ages have only occurred when Earth's shifting crustal plates carried the continents from tropical latitudes to more poleward positions.

2.It can be inferred from paragraph 2 that the theory of plate tectonics includes the proposition that

O climatic changes are caused by the movement of glaciers

O continents change their position on Earth's surface over time

O some climatic changes cannot be explained

O glaciers can form wherever there are landmasses

Glacial features in present-day Africa,Australia,South America, and India indicate that these regions experienced an ice age near the end of the Paleozoic era,about 250 million years ago.For many years this puzzled scientists.Was the climate in these relatively tropical latitudes once like it is today in Greenland and Antarctica? Why did glaciers not form in North America and Eurasia?Until the plate tectonics theory was formulated and proven,there was no reasonable explanation.⬛Today scientists realize that the areas containing these ancient glacial features were joined together as a single supercontinent called Pangaea that was located at high latitudes far to the south of their present positions.⬛Later,this landmass broke apart and its pieces,each moving on a different plate,drifted toward their present locations.⬛It is now believed that during the geologic past,plate movements accounted for many dramatic climatic changes as landmasses shifted in relation to one another and moved to different latitudinal positions.⬛Changes in oceanic circulation also must have occurred,altering the transport of heat and moisture and,consequently,the climate as well.Since the rate of plate movement is very slow,on the order of a few centimeters per year,appreciable changes in the positions of the continents occur only over great spans of geologic time.

3.Why does the author refer to“Greenland and Antarctica”in paragraph 3

A They are examples of landmasses that show indications of glacial activity from the Paleozoic era.

B They are examples of landmasses moving from tropical positions to high latitudes.

C They are examples of landmasses that in today's climate are occupied by glaciers.

D In the Paleozoic era,they had climates similar to present-day Africa,Australia,South America and India.

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