Official 48 Set 3

  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6

Formation of Some Special Volcanos

  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6
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

显示答案 正确答案: C

我的笔记 编辑笔记

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    NARRATOR:Listen to part of the lecture in an Earth Science class. The class has been discussing volcanoes.

    MALE PROFESSOR: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’re 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, that'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.

  • 旁白:请听地球科学课上的部分内容。这节课讨论的是火山。






    已提出的理论中支持率最高的是地幔柱理论(Plume Hypothesis)。

    根据这个理论,地幔柱......基本上就是极其炙热的岩浆柱形物。 这些地幔柱从地球内部的深层区域涌出,也许甚至有地核那么深,然后一路上涌熔化地壳。





    人们进行了一些地震学、地球化学方面的研究,这些研究中得出的数据和模型是一致的,但是它们不是最可靠的证据。虽然这个模型的支持者们都不愿宣称这个理论能解释所有异常的火山, 像所有的流行理论一样,我认为它也有一些很坚决的批评家。

    这些批评家提出了大量的可替代理论,所有的这些理论目前都未得到证明。不过有一个很值得考虑的理论,那就是裂缝理论,这一理论认为当一块地壳被拉伸得越来越薄时就产生了热点,同时产生的压力导致小裂缝,在地壳不牢固的地点打开, 而岩浆正是通过这些裂缝上涌形成火山的。



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    题型分类: 主旨题
    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?


    教授首先介绍板块理论,大多数火山发生在板块边缘,接着在定位段引入夏威夷群岛火山,它是在大陆板块中央,需要板块理论之外的新理论解释,之后给出两种理论:地幔柱理论和裂缝理论,解释出现在板块中间的火山石如何形成的,对应选项C:Explanations for why volcanoes occur in some locations。





Formation of Some Special Volcanos