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.
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.
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?