Official 49 Set 2

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Alaska Geologists

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What is the main purpose of the lecture?
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  • A. To contrast how different kinds of thaw lakes grow

  • B. To explain why a new theory of thaw lakes is gaining acceptance

  • C. To explain how processes in permafrost lead to the formation of thaw lakes

  • D. To describe two competing theories about the growth of thaw lakes

显示答案 正确答案: C 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:Alaska is fascinating to geologists because of its incredible landscapes.Uh, permafrost has a lot to do with this. That is, the areas where the ground—the soil—is always frozen, except for the very top layer—what we call the active layer of permafrost—which melts in the summer and refreezes again in the winter.The northern part of Alaska is covered in lakes—thousands of them—and most of these are what we call thaw lakes. [slowly] T-h-a-w. Thaw lakes.I'm gonna show you a few sketches of them in a minute, so you'll have a good idea of what I'm talking about.So, how these thaw lakes are formed has to do with...OK, it starts with ice wedges. The top part of the ice wedge melts—Should I back up?Ice wedges form when water runs into cracks in the ground, the permafrost, then freezes.You ever see mud after it dries?Dried mud has cracks, because when it dries, it contracts, it shrinks.Well, in winter, permafrost behaves similarly.It shrinks in winter, because it freezes even more thoroughly then, and as it shrinks, it forms deep, deep cracks.Then in the summer, when the active layer—the top layer of the permafrost—melts, the melt water runs into those cracks in the permafrost, then freezes again— because that ground, the ground beneath the active layer, is still below freezing.So, you have wedges of ice in the permafrost.

    Now the ice wedges widen the original cracks in the permafrost, because water expands when it freezes.All right? OK, then in autumn, the active layer on top freezes again.Then in winter, the permafrost starts contracting again and the cracks open up even wider.So the next summer, when the active layer melts again and flows into the widened cracks...and...freezes... it makes the cracks even wider.So it's sort of a cycle through which the cracks and the wedges grow wider and wider.

    So when the ice wedge reaches a certain size, its top part—in the active layer—turns into a little pond when it melts in the summer.And that's the beginning of your thaw lake.There are thousands of them in northern Alaska.One of the most fascinating things about these lakes—and this is important—is that they mostly have the same shape.Like an elongated oval, or egg shape. And what's more, all the ovals are oriented in the same way.Here's an idea of what they look like, what the landscape looks like from an aerial view, with the lakes side by side.

    There's been considerable research done to try to figure out what causes them to be shaped and oriented this way.We know that the shape and orientation are caused by the way the lakes grow once they're formed, but the question is, what makes them grow this way?One theory sees winds as the cause. This region of Alaska has strong winds that blow perpendicular to the lakes.What happens is, wind blows straight into the longer side of the lakes.Now, wouldn't that erode the lake bank in that direction? The same direction as the wind?Well...no.

    Actually, what happens is that the waves caused by the winds build a sorta protective layer of sediment—it's called a "protective shelf"—along the bank of the lake directly in front of them; so that bank is shielded from erosion, and the waves are diverted to the sides, to the left and to the right, and that's why the left and the right banks start eroding.Get it? The bank straight ahead is protected, but the lake currents--the waves-- erode the banks to the sides.

    That's the current model, um, the wind erosion model, which is generally accepted.But, there's a new theory that says that [deliberately] thaw slumping, not wind, is what shapes the thaw lakes.Thaw slumping, um...OK. Sometimes, in the summer, the temperature rises pretty quickly.So the active layer of permafrost thaws faster than the water can drain from the soil.So the sides of the thaw lakes get, like, mushy, and slump, or slide, into the lake.Then, the lake water spreads out more, and the lake gets bigger, OK?Also, in that part of Alaska, the terrain is gently sloped, so the lakes are all on an incline.Here.

    Now, this is an exaggeration of the angle—the hill isn't this steep—but see how with the lake's banks, the side that is farther downhill...it's smaller, lower.This short bank thaws faster than the tall one does, so it falls into the lake—it slumps, much more and much faster than the other bank.

    When the short banks of many lakes slump, they move farther downhill and the lakes grow—all in the same downhill direction.This is a new theory, so it hasn't been tested much yet.In field studies, when we've looked at the banks of these thaw lakes, there's not much evidence of slumping.We'd expect to see cliff-like formations there, from the slumping, but we haven't really found many of those.

  • 旁白:听一段地质学的讲座。

    教授:阿拉斯加对于地质学家很有吸引力,因为它的地形令人难以置信。永久冻土与这个有很大关系。这些地方的地面、土壤始终是冻结的,除了最顶层,我们称之为活土层,活土层在夏天融化,在冬季再次冻结。阿拉斯加北部被湖泊覆盖,数以千计的湖泊。其中大多数的湖泊是融湖,t-h-a-w,融湖。我马上给你看几张草图,这样你就可以很清楚地知道我在说什么了。那么这些融湖是如何形成的呢?...好的,它是由冰楔发展而来的。冰楔顶部融化。需要我补充一下么?当水流入地面的裂缝时,冰楔开始形成,然后永久冻土冻结。你们看过干了的泥浆吧?干泥浆会有裂缝,因为当它变干的时候,它会收缩。在冬季,冻土有着和泥浆类似的情况。它在冬天收缩,因为它冻结得更彻底,当它收缩时,形成很深的裂缝。然后在夏天,当活土层(顶层的永久冻土层)融化,融化的水运行到这些永久冻土的裂缝中,然后再次冻结,因为在活土层一下的土地仍然低于冰点。所以冻土带中有冰楔。

    冰楔将永久冻土最初的裂缝扩大,因为当水结冰时会膨胀。然后在秋天,顶层的活土层再次冻结。然后在冬季,永久冻土开始收缩,裂缝变得更大了。因此,到下一个夏天,当活土层再次融化,流入扩大的裂缝并且冻结起来,它使裂缝变得宽。所以它是一种循环,通过这种循环,裂缝和楔变得越来越宽。

    所以当冰楔达到一定的大小时,当它在夏天融化时,活土层的顶部变成一个小池塘。那就是融湖的开端。阿拉斯加北部有成千上万这样的湖泊。这些湖泊最迷人最重要的特点就是,他们大多具有相同的形状。像一个拉长的椭圆形或卵状;而且,所有的椭圆形都朝着相同的方向。这就是他们看起来像什么(的基本情况),也是鸟瞰这些并排湖泊所得到的景观。

    已经有相当多的研究试图找出是什么原因使他们形成如此形状和朝向。我们知道,形状和方向取决于湖泊形成后的增长方式,但问题是,是什么让他们这样成长?一种理论认为风是原因。阿拉斯加的这一地区有着垂直于湖泊的强风。发生的情况是,风直接吹进湖泊较长的那边。那会不会侵蚀那个方向的湖岸呢?同风向一致(的那边)?并不会。

    事实上,发生的是风引起的波浪形成了一种沉积物保护层,这是所谓的保护架,沿着湖岸,直接在它们面前。因此,湖岸得以免受侵蚀,波浪被转移到其他方向,到左边和右边,这就是为什么左右湖岸开始被侵蚀。明白了吗?正前方的湖岸受到保护,但湖流和海浪侵蚀两边的湖岸。

    这是目前的模型,风蚀模型,这是被普遍接受的。但有一个新的理论,这个理论认为热融滑塌是铸就融湖形态的原因,不是风。热融滑塌…好的。有时在夏天,气温上升很快。所以,永久冻土的活土层融化的速度比睡从土壤中流失的更快。所以融湖的边缘变成糊状,并且坍落或滑入湖中。随后,湖水继续扩张,湖泊越来越大。明白了吗?并且,阿拉斯加的那些区域,地形比较平缓,所以湖泊都在斜坡上。这里

    角度有些夸大,山并没有这么陡。但是,看看湖岸,往下坡的那一边更小、更低。更短的湖岸比高的那边解冻的快,所以它掉进湖中,这一边比其他湖岸掉落地更多更快。

    当许多湖泊的短河岸塌下来时,它们向下坡进一步移动,湖泊就沿着同一个下坡方向生长。这是一个新的理论,因此还没有被很好地验证过。在实地研究中,当我们看着这些融湖的岸边时,没有很多和滑落相关的证据。我们认为从那里可以看到由于坍塌而产生类似悬崖的形成。但我们没有类似的发现。

  • 官方解析
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  • 本题对应音频:
    1 感谢 不懂
    音频1
    音频2
    解析
    题型分类: 主旨题
    原文定位:
    So, how these thaw lakes are formed has to do with…OK, it starts with ice wedges……
    That’s the current model, um, the wind erosion model, which is generally accepted. But, there’s a new theory that says that [deliberately] thaw slumping, not wind, is what shapes the thaw lakes.
    选项分析:教授首先引入thaw lakes 现象,从ice wedges开始解释其形成,继而用风侵蚀和土滑坡两个理论解释其生长。结合第一个音频可确定C选项,结合第二个音频可确定D选项。

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