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TPO 7 Lecture 4
Geology
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 known that 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’v 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, there are a couple of factors that affect the amount of deformation that takes place or the speed of the glacier’s movement for example. Deformation is more likely to occur the thicker the ice is, because of the gravity of the weight of the ice. And temperature also plays a part here, in that cold ice does not move as easily as ice that is close to the melting point, in fact, it is not too different from… the way oil is, thicker at lower temperatures. So, if you have a glacier in a slightly warmer region, it will flow faster than a glacier in a cooler region.
Ok, um… Now, I’d like to touch briefly on extension and compression. Your textbook includes these as types, as a particular type of glacier movement, but you will see that there are …as many textbooks that omit it as a type of movement as include it. And I might not include it right now, if it weren’t in your textbook. But, basically, the upper parts of glaciers have less pressure on them. So, they don’t deform easily, they tend to be more brittle. And crevasses can form in this upper layers of the glacier. When the glacier comes into contact with bedrock walls or is otherwise under some kind of stress, but can’t deform quickly enough. So, the ice would expand or constrict, and that can cause big fissures big cracks to form in the surface layers of the ice, and that brittle surface ice moving is sometimes considered a type of glacier movement depending on which source you are consulting.
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.
《地理学》
独白：听一段关于地理学的讲座
教授：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，很多学生都为一些大型的冰川所震惊。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，而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，第一种漂浮的冰川叫做基面滑移。
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渐增的重量压力。那么你们可能知道，冰块在压力下熔解温度就会降低。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前那层水存在的原因。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环境。
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。但是冰同样也有塑料的特性，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支撑的冰，那么未被支撑的这边冰将会以自己的方式重新塑形，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由于重力因素，变形更容易发生在厚冰层的区域。而且温度也起到了一定的作用，因为冰川不会轻易的移动。事实上，冰块与增长点很接近，和油不一样，温度月底冰层越厚。因此，在稍温暖地带的冰川，就会比更冰冷地带的冰川更容易移动。
好了。现在我们简单的提及一下拉张与挤压。你们的教科书上把这种类型列为冰川移动的特殊类型，但是你们会看见许多教科书把这部分省略掉了，不认为这是一种冰川移动。既然你们的书上不会有，我也就不多讲了。但是，基本上来讲，就是冰川上层的压力小，因此它们不会很容易重新塑形，而是很脆，那么裂缝就会在此部分产生。当冰川涉及到基岩墙或者承受其它压力的时候，就很容易变形。因此，冰块就会拉张或挤压，这就导致冰块破裂并重新形成表面的冰层，这些易碎的表面冰层的移动有时候就被看做是一种冰川移动，主要还是看融化源在哪里。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 100 倍，你还可以看见冰川的移动，尽管并不常见。
以上就是托福听力TPO7原文中Lecture 4的文本内容，希望大家能够用心体会，更多TPO文本内容小编稍后为您呈现。最后，小编祝大家在托福考试中取得好成绩！
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