托福阅读真题+题目+答案：Causes of Ice Ages

托福阅读真题+题目+答案：Causes of Ice Ages

The term “ice age” is used by most people to refer to a time in the relatively recent geologic past when extensive continental glaciers covered millions more square kilometers of area than they now do. ▋There were many cycles of advance and retreat of the ice sheets during this epoch, known to geologists as the Pleistocene, which spanned the time from about 2 million to 10 thousand years ago.▊ Pleistocene glaciation can be studied in some detail because it was so recent, and the evidence left behind is thus fairly well preserved. It is not, however, the only large-scale continental glaciation in Earth’s past.▋ There have been at least half a dozen ice ages over Earth’s history, going back a billion years or more.▊

An underlying question is what kinds of factors might be involved in creating conditions conducive to the formation of such immense ice masses. The proposed causes of ice ages fall into two groups: those that involve events external to Earth and those for which the changes arise entirely on Earth.

1.What role does paragraph 2 play in the passage?

A.It summarizes the information presented in the first paragraph.

B.It explains why the basic question raised by the first paragraph is an important one.

C.It discusses the two causes that brought about the ice ages.

D.It presents the classification of proposed causes of ice ages that will be used in the rest of the passage.

One possible external cause would be a significant change in the Sun’s energy output. Present cycles of sunspot activity cause variations in sunlight intensity, which should logically result in temperature fluctuations worldwide, but the variations in solar-energy output would have to be about ten times as large as they are in the modern sunspot cycle to account even for shout-term temperature fluctuations observed on Earth. To cause an ice age of major proportions and of thousands of years duration, any cooling trend would have to last much longer than the eleven years of the present sunspot cycle. Another problem with linking solar-activity fluctuations with past ice ages is simply lack of evidence. Although means exist for estimating temperatures on Earth in the past, scientists have yet to conceive of a way to determine the pattern of solar activity in ancient times. Thus, there is no way to test the theory, to prove or disprove it.

2.According to paragraph 3, one problem with the idea that sunspot cycles like those observed in modern times could cause glaciation is that these cycles

A.would have to repeat over thousands of years

B.have not yet been studied enough

C.vary too much in length

D.are not long enough

3.According to paragraph 3, scientists cannot test the theory that changes in the Sun’s energy activity are responsible for ice ages because they do not know

A.how long the earliest ice ages lasted

B.how long a cooling trend would have to last to result in glaciation

C.what temperatures were like on the Earth in the distant past

D.what the pattern of solar-energy activity was in the distant past

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