托福听力TPO7文本(Conversation+Lecture)

　　Lecture 4

　　Geology

　　Last time, we started to talk about the glaciers, and how these masses less forms from crystallized snow, and some of you were amazed at how huge some of the these glaciers are. Now, even though it may be difficult to understand how a huge mass less can move or flow, in another word for it, it’s really known that the secret that the glaciers flow, because of gravity. But how they flow, and why they flow needs some explaining. Now, the first type of the glaciers flow is called: basal slip. Basal slip or its sliding as it’s often called, basically refers to the slipping or sliding of glacier across bedrock, actually across the 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 of the base of the glacier is under gradual depression-- the depression coming from the weights of the overlaying ice. And you probably know that the under pressure, the melting temperature of water as the ice I mean, is reduced. So, ice at the basis of glacier melts, even though it’s below zero degree thaws. And this results in thin layer of water between the glacier and ground. This layer of water reduces friction is... is like a lubricant.

　　And it allows the glacier to slat or slip over the bedrock. Ok, now the next type movement we will talk about is called: deformation. You already known that the ice brittle, if you heated with hammer, it will shatterly glass. But ice also plastic, you can change the shapes without breaking. If you leave, for example, a bar of ice supported only at one end, they end, they unsupported end will deform under its own way due---kind of flatten out one in to get stored it deformed it. Think deformation a very slow oozing. Depending on the stresses on the glacier, the ice crystal was in the re-organized. And during this re-organization the ice crystal re-allied 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 the factors that affects the amounts of deformation that takes place or the speed of the glaciers movement for example. Deformation is more likely to occur the thicker the ices, because at the gravity of the weight its ice. And temperature also plays part here, in that XX does not moves easily. As the ice that is close to the mounting points, in fact, it is not to different from… the weight oil is, thicker at the lower temperature. So, if you had a glacier in the slightly warmer region, it will flow faster than the glacier in the cooler region. Ok, um… Now, let’s touch briefly on extension and compression. You textbook includes this as type as a particular type of glacier movement, but you will see that these are … cause many textbooks that omitted as type of movement as included. And I might not include right now, if there won’t in your textbooks. But, basically, the upper parts of the glacier have less pressure on them. So, they don’t deform easily, they tend to be more brittle. And crevasses can form in this upper layer of glacier. When the glacier comes into contact with bedrock walls or the otherwise under some kinds of stresses, but can deform quickly enough. So, the ice would expand or constrict, and that can cause XXX be crack to form in the surface of the layer of ice, and that brittle the surface ice moving, is sometimes considered a type of glacier movement depending on which source you can thaw to. 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 its speeds as high as 7000 meters per year. Now, a speed like that are pretty unusual, 100 of times faster than the regular movement of glaciers, but you can actually see glacier move during these surges, though it is rare.