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Listen to part of the lecture in an Earth Science class. The class has been discussing volcanoes.
Okay. We know the Earth's surface, the crust, is made up of tectonic plates, and these huge slabs of rocky crust are slowly sliding over or under or past each other, and we said that most of the world's volcanoes occur at the boundaries of these tectonic plates where you have hot molten rock squeezing up through gaps between the plates.
But some volcanoes occur not at the edges, but in the middle of a continental or oceanic plate. The Hawaiian Islands, for example, are thousands of kilometers away from any plate boundary, and yet you have vast amounts of magma, molten rock or lava, flowing up through the earth's crust, which means, of course, that volcanic activity there can't be explained simply by plate tectonics.
So, how do we explain these volcanic anomalies, these exceptions to the general rule? Well, back in 1963, a geophysicist by the name of Wilson came up with a hot-spot theory to explain how this particular type of volcanic activity can occur, and can go on for maybe tens or even hundreds of millions of years. Wilson's theory was that: hot spots exist below tectonic plates, and they're the cause of these volcanoes.
But what causes the hot spots? Hmm, well, the most popular theory that's been proposed is the plume hypothesis. According to this hypothesis, plumes, uh, basically columns of extremely hot magma. These plumes well up from deep inside the planet's interior, maybe even as deep as its core, and rise all the way up to melt through the Earth's crust. Imagine a burning candle, and imagine moving a sheet of heavy paper slowly over the flame of the candle. You're gonna get a series of burned spots in the paper, well, that's just like what's happening with the Hawaiian Islands, but instead of a sheet of paper, you've got a tectonic plate, and it's moving over this plume of intensely hot magma, and rather than a series of burned spots in the paper. You're getting a chain of volcanic islands where the hot plume melts through the crust under the Pacific Ocean at one point after another with active volcanoes on the younger islands that are now just above the plume, and the other islands, well, the farther away from the plume they are now, the older they are, and the longer ago their volcanoes went dormant or extinct.
Incidentally, volcanic islands may seem small, but the island known as the Big Island Hawaii is one of the tallest topographic features on the planet, more than five kilometers from the sea floor to the ocean surface, and almost that much again, up to its highest peak. That's nearly ten kilometers from ocean floor to the highest point on the island, which makes it taller even than Mount Everest. So, you can imagine the huge amounts of magma, or lava, they've flowed up to form even just this one island, much less the whole chain of islands.
Now, the Plume Hypothesis provides a pretty elegant explanation for a volcanic anomaly, like the Hawaiian Islands. But, while it's hypothetically attractive, there's very little direct evidence to support the theory, because so far, no one's been able to actually observe what's happening that far beneath the Earth's crust.
Some studies have been done, seismographic, geochemical, where the data's consistent with the model, but they aren't definitive proof. Even the model supporters are uncomfortable claiming that it explains every volcanic anomaly.
And like any popular theory, I suppose, it has some determined critics. These critics have put forth a number of alternative theories, all unproven so far. But one well-regarded theory is the crack hypothesis, which assumes that hot spots are created when a piece of the crust gets stretched thinner and thinner and the resulting stress causes small cracks to open up at weak spots in the crust, and it's through these cracks that magma pushes up to form volcanoes. Proponents of the crack hypothesis consider this a widespread phenomenon and believe that magma's not coming up from deep within the Earth's interior, but rather from just beneath the surface crust. This hypothesis is attractive; because it fits with what we already know about plate tectonics and it fits what we know about some secondary smaller hot spots, but how well does it explain the Hawaiian Islands? Could a series of random cracks produce that same particular string of Islands that's sequenced so neatly from old to young? You know, it worries me when a theory depends on coincidence to produce results.
TPO48托福听力Lecture 2题目+解析
1.What is the lecture mainly about?
A.Ways to determine the ages of volcanic islands
B.Criteria for classifying various types of volcanoes
C.Explanations for why volcanoes occur in some locations
D.Methods for measuring magma produced by volcanoes
Q1
正确答案：C
定位原文：
…And we said that most of the world's volcanoes occur at the boundaries of these tectonic plates…
So, how do we explain these volcanic anomalies, these exceptions to the general rule?
But what causes the hot spots? ...
…the plume hypothesis. According to this hypothesis…
… But one well-regarded theory is the crack hypothesis, which assumes that hot spots are created…
解析：全文按顺序在探讨：1.火山的一般成因;2. 违背一般成因的特例解释;含一个理论解释;3. 另一解释火山形成的流行理论。所以基本问题全是围绕火山爆发的成因，各种解释理论和原因。对应选项，只能选择C选项。
2.According to the professor, why was the hot spot theory originally proposed?
A.To explain prolonged volcanic activity far from plate boundaries
B.To explain why volcanoes form both on land and in the ocean
C.To explain variations in the amounts of magma produced by volcanoes
D.To explain why volcanoes may become inactive after millions of years
Q2
正确答案：A
定位原文：
But some volcanoes occur not at the edges, but in the middle of a continental or oceanic plate… So, how do we explain these volcanic anomalies, these exceptions to the general rule? ... and they're the cause of these volcanoes.
解析：热点理论原本是因为什么提出的呢?hot spot theory这个信息最先出现在professor提出的问题：So, how do we explain these volcanic anomalies, these exceptions to the general rule? 之后，违背上文所述的火山爆发一般理论的，特例的火山爆发该如何解释呢?是hot spot theory。前文说到一般火山爆发出现在地壳板块的边界，但是特例的火山爆发却离地壳板块边界很远。对应A选项的解释。
3.Why does the professor describe moving a sheet of heavy paper over a candle?
A.To clarify that plumes do not produce great amounts of heat
B.To describe an experiment he would like the students to conduct
C.To illustrate one hypothesis for the way some volcanic anomalies are formed
D.To emphasize the thinness of some of Earth’s tectonic plates
Q3
正确答案：C
定位原文：
According to this hypothesis, plumes, uh, basically columns of extremely hot magma. These plumes well up from deep inside … in the paper.
解析：这其实是原文中的为了解释plume hypothesis理论的一个比喻，把plumes，也就是columns of extremely hot magma比作burning candle，把a sheet of heavy paper比作tectonic plate。
4.Why does the professor discuss how high one of the Hawaiian Islands rises above the ocean floor?
A.To provide evidence supporting the plume hypothesis
B.To compare the Hawaiian Island to other volcanic islands in the Pacific Ocean
C.To point out a common difference between volcanic mountains and other types of mountains
D.To emphasize that hotspot volcanoes can produce large amounts of magma
Q4
正确答案：D
定位原文：
Incidentally, volcanic islands … whole chain of islands.
解析：在说到这部分信息的最后一句提到：So, you can imagine the huge amounts of magma, or lava, they've flowed up to form even just this one island, much less the whole chain of islands. 可见hotspot volcanoes产生的岩浆量之多。
5.According to the crack hypothesis, what causes a hot spot?
A.Hot magma rises from deep in Earth to melt a piece of the crust.
B.Hot magma flows out through a break in the side of an active volcano.
C.Hot magma flows up through spaces created between tectonic plates as they move.
D.Hot magma flows up at a point where a tectonic plate has been stretched thin.
Q5
正确答案：D
定位原文：
But one well-regarded theory is the crack hypothesis, which assumes that hot spots are created when a piece of the crust gets stretched thinner and thinner and the resulting stress causes small cracks to open up at weak spots in the crust, and it's through these cracks that magma pushes up to form volcanoes.
解析：crack hypothesis的解释就是从which assumes 开始，地壳壁因为拉伸变得越来越薄，单位面积承受的压力引起了裂缝，这样的裂缝就变成了地壳的薄弱点，然后岩浆从这样的裂缝冲出来就形成了火山。
6.What is the professor’s opinion of the two main hypotheses he presents?
A.Neither hypothesis can explain the formation of the Hawaiian Islands.
B.Each hypothesis accounts for some, but not all volcanic anomalies.
C.Each hypothesis explains the formation of more volcanoes than plate tectonics does.
D.The studies supporting the crack hypothesis are more convincing than those supporting the plume hypothesis.
Q6
正确答案：B
定位原文：
Proponents of the crack hypothesis consider this a widespread phenomenon and believe that magma's not coming up from deep within the Earth's interior, … Could a series of random cracks produce that same particular string of Islands that's sequenced so neatly from old to young? You know, it worries me when a theory depends on coincidence to produce results.
解析：作者的态度重点在最后三句，意思是理论能完美解释我们已经知道的板块构造论，但是怎么用来解释Hawaiian Islands呢?随机产生的裂缝怎么能形成排列如此整齐的特定群岛呢?理论总是依据偶然产生的结果是很让人担心的。意思就是两种理论都在某一层面能够说得通，但是没有一种理论能够用来解释所有特例，所以都不是足以能让人信服的。结合选项，只能选择B。
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