Official 01 Set 3

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Uranium-Lead Dating

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What does the professor mainly discuss?
  • A. The difference in age among American mountain ranges

  • B. The importance of a technique used for dating geological materials

  • C. The recent discovery of an ancient canyon

  • D. A comparison of various minerals used for dating

显示答案 正确答案: B
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    NARRATOR:Listen to part of a lecture in a geology class.

    MALE PROFESSOR:Ok, let's get started.Great—today I want to talk about a way in which we are able to determine how old a piece of land or some other geologic feature is. Dating techniques.I'm gonna to talk about a particular dating technique. Why?Good dating is key to good analysis.In other words, if you want to know how a land formation was formed, the first thing you probably want to know is how old it is. It's fundamental.

    Uh... Take the Grand Canyon for instance. Now, we geologists thought we had a pretty good idea of how the Grand Canyon in the southwestern United States was formed. We knew that it was formed from sandstone that solidified somewhere between 150 and 300 million years ago.Before it solidified, it was just regular sand—essentially, it was part of a vast desert.And until just recently, most of us thought the sand had come from an ancient mountain range fairly close by that flattened out over time.That's been the conventional wisdom among geologists for quite some time.

    But now we've learned something different and quite surprising using a technique called Uranium-Lead Dating.I should say that Uranium-Lead Dating has been around for quite a while. But there have been some recent refinements—I'll get into this in a minute.Anyway, Uranium-Lead Dating has produced some surprises.Two geologists discovered that about half of the sand from the Grand Canyon was actually once part of the Appalachian Mountains. That's really eye-opening news, since the Appalachian Mountain Range is, of course, thousands of kilometers to the east of the Grand Canyon. Sounds pretty unbelievable, right? Of course, the obvious question is how did that sand end up so far west?The theory is that huge rivers and wind carried the sand west where it mixed in with the sand that was already there.

    Well, this was a pretty revolutionary finding. uh, and it was basically because of uranium-lead dating.Why? [pause] Well, as everyone in this class should know, we usually look at the grain type within sandstone, meaning the actual particles in the sandstone, to determine where it came from.You can do other things too, like look at the wind or water that brought the grains to their location and figure out which way it was flowing. But that's only useful up to a point, and that's not what these two geologists did. Uranium-Lead Dating allowed them to go about it in an entirely different way. What they did was: they looked at the grains of Zircon in the sandstone. Zircon is a material that contains radioactive Uranium, which makes it very useful for dating purposes. Zircon starts off as molten magma, the hot lava from volcanoes. This magma then crystallizes. And when Zircon crystallizes, the Uranium inside it begins to change into Lead.So if you measure the amount of Lead in the Zircon grain, you can figure out when the grain was formed. After that, you can determine the age of Zircon from different mountain ranges.Once you do that, you can compare the age of the Zircon in the sandstone in your sample to the age of the Zircon in the mountains. If the age of the Zircon matches the age of one of the mountain ranges, then it means the sandstone actually used to be part of that particular mountain range. Is everybody with me on that? Good. So, in this case, Uranium-Lead Dating was used to establish that half of the sandstone in the samples was formed at the same time the granite in the Appalachian Mountains was formed. So because of this, this new way of doing Uranium-Lead Dating, we've been able to determine that one of our major assumptions about the Grand Canyon was wrong.

    Like I said before, Uranium-Lead Dating has been with us for a while. But, um... until recently, in order to do it, you really had to study many individual grains. And it took a long time before you got results.It just wasn't very efficient. And it wasn't very accurate.But technical advances have cut down on the number of grains you have to study, so you get your results faster. So I'll predict that Uranium-Lead Dating is going to become an increasingly popular dating method.

    There are a few pretty exciting possibilities for Uranium-Lead Dating. Here is one that comes to mind. You know the theory that earth's continents were once joined together and only split apart relatively recently?Well, with Uranium-Lead Dating, we can prove that more conclusively.If they show evidence of once having been joined, that could really tell us a lot about the early history of the planet's geology.

  • 旁白:听一段地质学课程。

    教授:好吧,我们开始上课。好,今天我要谈一些关于如何测定地质年份和其他关于地质特征的方法——年代测量法。为什么我要讲一些特殊的测定法呢?准确的年代测定是打开分析之门的钥匙。换句话说,如果你想知道这块地质形成的原因,你要做的第一件事就是要搞清楚它的“年纪”。这是基础。

    嗯,举个大峡谷的例子。现代地质学家认为我们已经形成了一个没有太多瑕疵的关于美国西南部大峡谷起源的假说。我们推测它的形成源于15 至30 亿年前砂岩的凝固。在成型之前,这些本质上是普通的沙子,沙漠的一部分。但最近,大多数人认为这些沙子来源于一个古代峰峦相近的山脉,随着时间的推移,它变得平缓了。这是相当长一段时间在地质学家中的传统观点。

    但是现在我们学到了一些不同的并令人惊讶的东西,使用一种被称为“铀铅年份测定法”的技术。虽然说铀铅年份测定法问世已经有一段时间,但它也经历过几次改良。等一下我们就会讲到。总之,铀铅年份测定法给我们带来了一些惊喜。两个地质学家发现大峡谷大约一半的沙子实际上源于阿巴拉契亚山脉。这个发现真的令人瞠目结舌,因为大峡谷往东几千公里才到阿巴拉契亚山脉。听起来很不可思议,是吧?的确是这样,问题是那些沙子怎么能西移得那么远?理论上是这样认为的:这些沙子被大河冲,狂风刮至大峡谷处汇集、混合并沉淀。

    这确实是一个革命性的发现。嗯…主要还是铀铅年份测试法的应用。为什么这样说?这样,正如在座的每一个同学应该知道的那样,我们通常关注沙砾内部的纹理类型,也就是沙砾里的微粒,来判定它来自哪里。我们也有别的方法,比如关注将沙砾带至该地的风或河流,然后推出它的流向。然而,这种方法只在一定程度上有用,那两个地质学家没有这样做。铀铅年份测试法给他们提供里全新的方式来推断沙砾的流向。他们做的是:他们研究沙砾里锆石的纹理。锆石是一种内含放射性铀的物质,而放射性铀对年代推算很有帮助。锆石源于熔岩,一种火山喷发的岩浆。这种岩浆随后会凝结。而当锆石凝结时,锆石里面的铀转化成铅。所以,如果你测量锆石里铅的含量,你可以推算纹理是何时形成的。之后,你就能从不同的山脉来断定锆石的年份。一旦你这样做,你就可以将样本沙石里锆石的年份和山脉里锆石的年份做比较。如果样本沙石里锆石的年份与山脉里锆石的年份一致,那就意味着这里的沙石曾经作为这个山脉的一部分而存在。大家同意这个看法吗?很好。这样,铀铅年份测试法用于确认样沙里一半沙石和阿巴拉契亚山脉花岗岩的成形。所以,正因为这种,这种进行铀铅年份测试的新方法,我们有能力判定我们对大峡谷的重大假设之一是错误的。

    像我之前说过的那样,我们已经使用铀铅年份测试法有一段时间了。但…,嗯,直到最近,要完成这样的测试,你还真得研究很多(沙砾的)纹理。当然,要得出结果是需要很长一段时间的。这样的做法其实效率并不高。同时,也谈不上精确。但是,技术的进步能缩减你需要研究的纹理的数量,这样你会更快地得到结果。我估计铀铅年份测试法将成为越发受人欢迎的年份测试法。

    在铀铅年份测试法里有几个令人兴奋的可能性。我想起的有这么一个。大家都知道地球大陆板块“始合近分”的理论吧?用铀铅年份测试法,我们能更决定性地证实这个理论。如果能有证据表明“始合”,那些证据就能给我们提供很多地球地质学的早期史料。

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  • 本题对应音频:
    6 感谢 6 不懂
    音频1
    解析

    题型分析:主旨题

    原文定位

    That’s been the conventional wisdom among geologists for quite some time. But now we’ve learned something different and quite surprising using a 

    technique called uranium-lead dating.

    选项分析

    文章开头讲到dating technique,又说it's foundamental。接着说了我们对Grand Canyon旧有的认知,然后转折说现在发现了一些令人惊讶的difference,然后引出,uranium-lead dating,并且之后的文本也都在说这种技术。因此答案选B。

    A选项,重点在difference上,没有提到不同山脉的年龄区别。C选项,并没有新发现一个canyon。D选项,minerals是细节,并且也没有对比。

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