Official 35 Set 6

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

Permian Extinction

  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6
What does the professor mainly discuss?
  • A. New evidence about what happened to dinosaurs

  • B. New methods for gathering evidence about mass extinctions

  • C. The link between two mascons from the Permian period

  • D. A possible cause of the Permian extinction

显示答案 正确答案: D

我的笔记 编辑笔记

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    NARRATOR:Listen to part of a lecture in an Earth science class.

    MALE PROFESSOR:Let's review something from last week.We talked about an event that happened 65 million years ago... Anyone?

    FEMALE STUDENT:An asteroid hit Earth; um, well, we think an asteroid hit Earth... near the Yucatan peninsula in Mexico, and that wiped out all the dinosaurs.

    MALE PROFESSOR:Right, uh, I wouldn't say that we've got 100 percent proof, but there's very strong evidence that this is why that mass extinction occurred.OK, but did you know there was an earlier extinction- far greater than the one that killed off the dinosaurs?It was what we call the Permian extinction.

    Now, way back about 290 million years ago-at the beginning of the Permian period, there was just one big continent, a supercontinent.And as the climate warmed up, plant and animal species began to diversify profusely.So life during the Permian period was abundant and diverse.

    But about 250 million years ago, the Permian period ended with a rapid mass extinction.Something happened that wiped out 75 percent of the land animals and over 95 percent of ocean life.

    So what was it?What could have caused this?Well, with all the evidence that it was an asteroid that led to the dinosaur extinction, we began asking ourselves: Is it possible that another asteroid... much earlier... caused the Permian extinction?And so researchers have been looking for an impact crater.

    FEMALE STUDENT:I thought the Permian extinction was caused by a decline in seawater oxygen levels.Isn't that what's in the textbook?

    MALE PROFESSOR:But don't forget, the textbook makes it very clear-that's only a theory.

    MALE STUDENT:And it mentions something about volcanic eruptions, too.

    MALE PROFESSOR:It does. But now, this new theory has led to a search for evidence of an asteroid impact.And one place of interest is a region called Wilkes Land in eastern Antarctica.

    A few years ago, a researcher reported a strange anomaly beneath the ice in Wilkes Land- evidence of what may be a mascon.That's just short for "mass concentration."

    When an asteroid hits Earth, when it slams into Earth's crust, we think that causes molten rock from deep below the surface to rise up into the impact area- sort of like if you bump your head, you get a big lump under the skin... fluid makes the area swell.Anyway, the material flowing up from below the crust is more dense than the crust itself.So that's how we get a mascon- a spot in the crust with newer crust material that's more dense than the material all around it.

    There are lots of mascons on the Moon too, where a mascon's density causes a small increase in the local gravity that can be measured and mapped by orbiting spacecraft.And where do these mascons tend to be found?In the centers of impact craters on the Moon's surface.

    But back to Wilkes Land...We're not certain that the mascon there... what might be a mascon... was actually caused by the impact of an asteroid, but there does seem to be evidence: researchers noticed a gravity anomaly similar to those on the Moon.And the spot where the gravity readings are especially high- this is right in the middle of a 500-kilometer-wide circular ridge- what could be part of an old impact crater.

    And if there was an asteroid impact there in Wilkes Land, the next question is- Did it happen 250 million years ago?Because that would put it when in geologic history?

    At the end of the Permian period, right when all those animals went extinct.


    But... can't researchers figure that out by studying the rocks there in Wilkes Land... where this impact supposedly took place?

    Well, to get to anything from that long ago, we'd have to drill down through about a mile... about 1.6 kilometers... of solid ice that covers the area today.And that's not likely to happen.

    But speaking of rocks, I should mention that Wilkes Land is not the only place of interest here.There's another called the Bedout the coast of Australia.

    And we have rock samples from the Bedout High- some apparently of extraterrestrial origin.I mean, they show the effects of extreme temperatures and pressures- the level of extremes produced only by an impact.And as for their age..., well, they do, in fact, date back to about 250 million years ago!

  • 旁白:请听一段地球科学课的讲座的节选。


    学生:小行星撞击了地球。嗯……我们认为是有一个小行星撞击了地球,在墨西哥的尤卡坦半岛(Yucatan Peninsula)附近,这个事情使得恐龙灭绝了。








    教授:是的,但是现在的新理论导致了一场寻找陨石坑证据的活动。有趣的是,一个南极洲东边叫做威尔克斯区(Wilkes Land)的地方。

    几年前,一个研究者报告说威尔克斯区(Wilkes Land)下面的冰层有异常情况,可能是一个质量密集现象。这是质量浓度的简写。



    让我们回到威尔克斯区(Wilkes Land)。我们并不确定有陨石坑……不确定什么可能是一个陨石坑……不确定这个坑确实是因为小行星撞击形成的,但是我们的确有一些证据,研究者们注意到了类似于月球上的重力异常现象。在那些重力特别大的地点……刚好在500千米的一个圆形山脊的中间,那就可能是一个老的陨石坑了。

    如果真的在威尔克斯区(Wilkes Land)里有个陨石坑,那么下一个问题就是:它是在25000万年以前形成的吗?因为这个问题会帮我们判断它所处的地质历史位置。



    但是研究者不能通过研究威尔克斯区(Wilkes Land)里面的岩石来确定这个时间……这个小行星撞击发生的时间?


    但是说起岩石,我必须提到,威尔克斯区(Wilkes Land)并不是唯一让研究者感兴趣的地点。还有一个叫做Bedout High的地方,远离澳大利亚的海岸。


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    OK, but did you know there was an earlier extinction—far greater than the one that killed off the dinosaurs? It was what we call the Permian extinction…


    文章先用恐龙灭绝来引入较其更早的大灭绝称之为二叠纪大灭绝(Permian extinction且后文述二叠灭绝产生可能的原因,选项D全文主旨最准确的概括。





Permian Extinction