Official 54 Set 3

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New Evidence of River Hypothesis

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What does the professor mainly discuss?
  • A. A hypothesis that rivers formed before the rise of plant life

  • B. A study of the effects that rivers have on soil formation

  • C. A debate surrounding two opposing hypotheses

  • D. New evidence in support of a previously proposed hypothesis

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 a geology class.

    MALE PROFESSOR: About 30 years ago a geologist named Edward Cotter—that’s C-O-T-T-E-R— uh, published a paper that contained a very interesting hypothesis.He was studying ancient rivers in a North American mountain chain.And he noticed that about 450 million years ago, rivers started to behave differently.

    Before then, rivers were wide, shallow, and straight.But after that time, they became deeper and had more curves; they became increasingly meandering.And that’s actually how rivers behave to this day.So, why might this change have happened?

    FEMALE STUDENT: Maybe there was some kind of climate shift?

    <-MALE PROFESSOR:-> Well…lots of climate shifts have happened since then.

    MALE STUDENT: Was the change worldwide, or just in that geographical area?

    MALE PROFESSOR: Well, Cotter speculated that rivers changed worldwide, but he couldn’t prove it… because he only had evidence from the one North American mountain chain.But his studies gave him an idea about why rivers started to change—he hypothesized it had to do with the spread of plant life on Earth.

    FEMALE STUDENT: So … there was no plant life before 450 million years ago?

    MALE PROFESSOR: Very little according to fossil records.Anyway, geologists were intrigued by this hypothesis, which claims that as plants evolved and spread, they had an effect on the terrain and rivers.In the past 30 years, more studies have been done, and now we have a lot of data about river systems from around 450 million years ago from all over the world.

    In a recent study, a couple of researchers gathered together the existing data and combined them with their own new field data to get a comprehensive picture of the situation.Their study was specifically designed to identify changes in the shapes of rivers during the time period when vegetation was evolving.And when the researchers compared the data about river shapes with data they had collected about plant life from the same period, the data seemed to prove Cotter’s hypothesis.

    MALE STUDENT: OK, but how did plant life affect rivers?

    MALE PROFESSOR: Well, in order to answer that question, we need to look at the geological evidence.You see, as rivers flow, they leave layers of sediment behind that eventually fossilize.The content, thickness, and shape of these fossilized layers in rocks give us information about how rivers flowed.The earliest records from 500 million years ago show that the sediment in river deposits was largely composed of coarse grains of sand and gravel.That tells us that rivers weren’t defined—they were very shallow and wide, almost like floods.

    But around the time of the rise of plant life, the content of those sediment layers began to change.The coarse grains became much finer, and we see evidence of mud.This suggests that plants promoted the preservation of mud when they sent their roots into the ground.The roots helped to reinforce the ground, which in turn allowed for the creation of riverbanks.And we also see evidence of a process called lateral accretion.

    Lateral accretion happens when water flows around a curve, a bend, in a riverbed.Now, the speed of the flow on the outside of the bend is fastest and slowest on the inside of the bend.This sets up what’s called a secondary flow across the river bottom.The fast-flowing water on the outside of the bend digs out material from the riverbank and pushes this material laterally across the bottom, and it gets deposited on the other side of the river, on the inner side of the bend.So when we see in the sediment layers evidence of lateral accretion— erosion on one side and deposits on the other—that’s an indicator that a meandering river existed.And according to the study, strong evidence of lateral accretion appears in the geological record…at the same time that there’s also evidence of plants with underground root systems.This suggests that plants promoted the development of modern rivers by creating stable banks, which resulted in the flow of water in single, meandering channels.

    FEMALE STUDENT: So… It looks like the researchers were able to prove the hypothesis…

    MALE PROFESSOR: Well, there’s no denying that the study presents a very strong case.But some questions about this hypothesis remain.For example, it’s well-known that on other planets, like Mars, there’s clear evidence of meandering rivers.But is there evidence of vegetation on Mars? I think not.

  • 请听一段地质学课上的讲座片段。

    教授:大概30年前,一个名叫爱德华•科特的地质学家,Cotter的拼写为C-O-T-T-E-R,他发表了一篇论文,论文里描述了一个非常有趣的假设。他那个时候正在研究位于北美山脉的古老河流。并且他注意到,大概4.5亿年以前,河流开始变得不同。

    在那以前,河流是宽的,浅的并且是笔直的。但是从那以后,河流开始变深,变曲折,变得越来越曲折。也就是变成现在河流的样子。所以为什么会发生这些改变呢?

    学生:可能是某种气候转变?

    教授:呃,但从那时之后又发生了很多的气候转变。

    学生:是全球性的改变吗?还是仅仅就是那个地理区域呢?

    教授:是的,科特推测河流的变迁是全球性的,但是他无法证明。因为他仅仅有来自于北美山脉的证据。但是他的研究告诉他为什么河流开始变迁。他假设是由于地球上植物生命的蔓延。

    学生:所以是说在4.5亿年以前是没有植物的是吗?

    教授:由化石记录可知,(植物)非常少。不管怎么说,地质学家都对这个假设非常感兴趣,这个假设是说,植物的进化和蔓延对地貌和河流的改变有影响。在过去30年,越来越多的研究被开展。现在我们有很多来自世界各地的关于大概4.5亿年以前的河流系统的数据。

    在最近的一次研究中,一些研究学者集中收集了这些存在的数据,并且将它们与自己新的领域的数据相结合,得到了一个综合的形势图。这个研究是特别设计出来为了证明植物正在进化的那个时期河流形状的改变的。并且当研究者把河流形状的数据与他们收集到的同时期的植物生命的数据相比较时,发现这些数据似乎证明了科特的假设。

    学生:好的,但是,植物是怎样影响河流的呢?

    教授:好的,为了回答这个问题,我们需要看这个地质证据。你们看,随着河流的流动,它们会留下沉积物,这些沉积物最终会石化。这些石化了的岩层和岩石的(构成物质)内容、厚度以及形状给我们提供了河流是怎样流动的信息。5亿年以前的最早的记录表明,河流沉积物大部分是由大颗粒的沙子和碎石组成的。那就告诉我们,河流不是轮廓分明的,它们非常浅、宽,几乎就像洪水一样。

    但是,大概在植物开始发展的时候,这些沉积物层的内容开始改变。粗糙颗粒变得更加精细。我们发现了淤泥的迹象。这就表明,当植物把它们的根扎进大地的时候,它们提高了泥土的储存量。植物的根能帮助加固土壤,转而使得河岸得以形成。我们也发现了一个被称为横向吸积过程的迹象。

    横向吸积发生在河水在河床的曲线或弯曲之处流动的时候。现在,河湾外部的水流速度是最快的,而其内部的水流速度是最慢的。这就形成了河底的二次流。河湾外部最快的水流把河床的物质挖掘出来并且推到了河底的另一侧,物质就在河流另一侧靠近河湾内部的地方沉积下来。所以,当我们看到,在沉积层里,这个横向吸积的证据,河道一侧被侵蚀而一边则有沉积物堆积,这就表明了弯曲河流的存在。根据这个研究,地质记录中显示出了横向吸积的强有力的证据。与此同时,也存在植物的地下根系统的证据。这就表明植物促进了现代河流的发展,植物通过巩固河床,使得河流在单一的蜿蜒的渠道中流动。

    学生:所以就是说研究者们已经能够证明那个假设了。

    教授:是的,不可否认,这个研究呈现出了一个很有力的证明。但是关于这个假说仍然存在一些问题。举个例子,大家都知道,在其它星球上,像火星,很明显,上面也有弯曲的河流。但是,火星上有植物存在的证据吗?我不那么认为。

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    题干分析:教授讨论的主要内容

    原文定位:

    But his studies gave him an idea about why rivers started to change—he hypothesized it had to do with the spread of plant life on Earth.

    选项分析:教授首先介绍地质学家Cotter,以及他观察到的现象:450M前后,北美山区的河流从宽浅直变得更深更弯。在定位句,教授介绍了Cotter的假设:和地表植被有关。后文介绍支持Cotter假设的新证据发现。D概括最准确。

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