OFFICIAL7 What does the professor imply about compression and extension?

[00:00.00]Narrator: Listen to part of a lecture in a geology class.
[00:04.50]MALE PROFESSOR: 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. [00:17.48]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 glaciers flow because of gravity. [00:30.04]But how they flow, the way they flow needs some explaining.
[00:33.49]Now, the first type of glacier flow is called basal slip. [00:38.14]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.[00:51.15]Uh, so this process shouldn’t be too hard to imagine. [00:55.54]What happens is that the ice at the base of a glacier is under a great deal of pressure, the pressure coming from the weight of the overlying ice.
[01:03.74]And you probably know that under pressure, the melting temperature of water, uh of the ice I mean, is reduced. [01:11.99]So ice at the base of the glacier melts, even though it's below zero degrees Celsius, [01:17.68]and this results in a thin layer of water between the glacier and the ground. [01:22.96]This layer of water reduces friction, it's, it's like a lubricant, [01:28.09]and it allows the glacier to slide or slip over the bedrock. OK?
[01:33.00]Now, the next type of movement we’ll talk about is called deformation. [01:37.44]You already know that ice is brittle—if you hit it with a hammer, it will shatter like glass. [01:42.45]But ice is, ah, also plastic—it can change shape without breaking. [01:47.55]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—
[01:57.20]it'll kind of flatten out at one end, get distorted, deformed. [02:01.65]Think of deformation as a very slow oozing. [02:05.73]Depending on the stresses on a glacier, the ice crystals within it reorganize. [02:11.27]And during this…uh reorganization, uh, the ice crystals realign in a way that allows them to slide past each other. [02:18.74]And so the glacier oozes downhill without any ice actually melting.
[02:23.44]Now there are a couple of factors that affect the amount of deformation that takes place or the speed of the ah glacier’s movement. For example, [02:35.25]deformation is more likely to occur the thicker the ice is—because of the gravity of the weight of the ice.
[02:42.04]And temperature also plays a part here, in that cold ice does not move as easily as ice that is closer to the melting point—[02:50.55]in fact, it's not too different from hm the way oil is, uh, thicker, at low temperatures. [02:57.71]So if you have a glacier in a slightly warmer region, it will flow faster than a glacier in a colder region.
[03:04.55]OK, hm now I’d like to touch briefly on extension and compression. [03:10.39]Your textbook includes these as types—as a particular type—of glacial movement, but you'll see that there are as many textbooks that omit it as a type of movement as include it. [03:21.54]And I might not include it right now if it weren’t in your textbook.
[03:25.45]But hm basically, the upper parts of glaciers have less pressure on them, [03:31.62]so they don't deform as easily, they tend to be more brittle. [03:34.87]And crevasses can form in these upper layers of the glaciers [03:39.06]when the glacier comes into contact with bedrock walls or ah is otherwise under some kind of stress but can't deform quickly enough.
[03:49.15]So the ice will 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 glacial movement, depending on which source you're consulting.
[04:07.90]Now, as you probably know, glaciers generally move really slowly, [04:12.73]but sometimes they experience surges, and during these surges, in some places they can move at speeds as high as 7,000 meters per year. [04:22.03]Now speeds like that are pretty unusual, hundreds of times faster than the regular movement of glaciers—but you can actually see glaciers move during these surges, though it is rare.