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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, and the way they flow needs some explaining. Now, the first type of the glaciers flow is called: basal slip. Basal slip or 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 a great deal of pressure-- the pressure coming from the weight of the overlaying 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 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 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 the ice is brittle, if you hit it with hammer, it will shatter like glass. But ice is also plastic, it can change the shape without breaking. If you leave, for example, a bar of ice supported only at one end, they end, the unsupported end will deform under its own way due---kind of flatten out one into get stored it deformed it. Think deformation as 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 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 the factors that affect the amount 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 of the gravity of the weight of the ice. And temperature also plays 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 had a glacier in a slightly warmer region, it will flow faster than the glacier in a cooler region. Ok, um… Now, let’s touch briefly on extension and compression. You textbook includes these 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 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 layer of the glacier. When the glacier comes into contact with bedrock walls or is otherwise under some kinds 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 of the layer of ice, and that brittle 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 speeds 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.
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