四六级

　　英语六级听力这部分在2018年下半年考试后得到不少考生吐槽，大部分内容是说“六级听力太难了”，所以这部分弱的考生要早做准备，下面是小编准备的六级听力练习，主要讲的是冰川现象。

　　Now listen to the following recording and answer questions 23 to 25

　　1) A. explanations of how glaciers move

　　B. landscape changes caused by glacial movement

　　C. climate changes that influence glacial movement

　　D. causes of glacial formation

　　2) A. the thickness of glacial ice

　　B. the slipping or sliding of a glacier across bedrock

　　C. the amount of water beneath the glacial ice

　　D. the temperature of the glacial ice

　　3) A. to explain the process of deformation

　　B. to provide the background information

　　C. to give a brief description of glacier formation

　　D. to draw the students’ attention

　　4) A. It affects the amount of glacial ice that forms.

　　B. Sometimes it is fast enough to be noticed

　　C. It is reduced by cracks in the ice.

　　D. It is unusually high in colder regions.

　　听力原文：

　　Last time, we started to talk about glaciers and how these masses of ice form from crystallized snow, and some of you were amazed at how huge some of these glaciers are. Now, even though it may be difficult to understand how a huge mass of ice can move or flow, it's another word for it, it's really no secret that the glaciers flow because of gravity. But how they flow, the way they flow, needs some explaining.

　　Now, the first type of glacier flow is called: basal slip. Basal slip or sliding as it's often called, basically refers to the slipping or sliding of a glacier across bedrock, actually across a thin layer of water on top of the bedrock. So, this process shouldn't be too hard to imagine. What happens is that the ice at the base of the glacier is under a great deal of pressure -- the pressure coming from the weight of the overlying ice. And you probably know that under pressure, the melting temperature of water, of the ice I mean, is reduced. So, ice at the base of the glacier melts, even though it's below zero degree Celsius. And this results in a thin layer of water between the glacier and the ground. This layer of water reduces friction is...is like a lubricant. And it allows the glacier to slide or slip over the bedrock. OK?

　　Now the next type of movement we will talk about is called: deformation. You've already known that ice is brittle, if you hit it with a hammer, it will shatter like glass. But ice is also plastic, it can change shape without breaking. If you leave, for example, a bar of ice supported only at one end, the end, the unsupported end will deform under its own weight, it'll kind of flatten out at one end, get distorted, deformed. Think of deformation as a very slow oozing. Depending on the stresses on the glacier, the ice crystal within it reorganize. And during this re-organization the ice crystals realign in a way that allows them to slide pass each other. And so the glacier oozes downhill without any ice actually melting.

　　Now, as you probably know, glaciers generally move really slowly. But sometimes, they experience surges, and during these surges, in some places, they can move at speeds as high as 7000 meters per year. Now, speeds like that are pretty unusual, hundreds of times faster than the regular movement of glaciers, but you can actually see glacier move during these surges, though it is rare.

　　Questions:

　　1. What is the lecture mainly about?

　　2. According to the professor, what is basal slip?

　　3. Why did the professor give the example of “a bar of ice supported only at one end”?

　　4. What does the professor say about the speed of glaciers?

　　听力译文：

　　上次我们谈论了冰川，以及大型的冰川如何从结晶雪形成而来，很多学生都为一些冰川之大感到惊讶。现在，即便了解大型冰川如何移动或漂浮这个现象有些困难，但这已经不再是一个秘密，冰川其实是靠地心力来漂浮的。但是冰川怎样浮动，为什么会浮动却仍然需要解释一下。

　　那么，第一种冰川漂移叫做基面滑移。基面滑移，或者也被称作底部滑动，其实就是指冰川在基岩上的滑动，细致来讲应该是在基岩顶上的一层水上滑动的现象。因此这个过程应该就不难理解了。实际上就是冰川底部的冰块承受来自上方冰块重量的巨大压力。那么你们可能知道，冰块在压力下融化温度就会降低。所以说，冰川底部的冰块就会融化，即便它的温度还是在零度以下。这就是地面和冰川之间那层薄薄的水存在的原因。这层水能减少摩擦，就像…就像润滑剂一样。这就给冰川在基岩上滑动创造了条件。

　　好了，下面一种滑动我们称之为变形。大家都知道冰块易碎，倘若你拿锤子敲击，它会立即如玻璃一样碎裂。但是冰同样也有可塑性，你无需打碎它也能重新塑形。比如，如果你放置一块只有一面有支撑的冰，那么未被支撑的这边冰在重力下将会重新塑形，通过扭曲变形，使冰块变得平稳。变形是一种缓慢的过程，取决于冰川的压力，冰晶需要重新组织构架。在此期间，冰晶会在允许它们相互移动的情况下重新联合在一起。因此冰川往下坡移动时是没有冰块融化的。

　　现在，你们也许已经知道了，冰川一般移动起来很慢。但是有时候，它们也会移动很快，有些地方的冰川移动速度甚至可以达到每年7000米。那么这种速度往往是不寻常的，是一般冰川移动速度的数百倍快，在这样的速度下，你还可以看见冰川的移动，尽管并不常见。

　　参考答案

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