TOEFL Listening: TPO-TOEFL听力TPO - B439RL_PGQ6506ZIS$

Lecture: Different minerals in the Earth's crust: Narrator: Listen to part of a lecture in a geology class. The professor has been discussing minerals. Professor: OK so let's quickly sum up what we've said so far. There are over 2000 different minerals in the Earth's crust, things like, uh, quartz and feldspar and olivine and such. And these minerals come from, well, most minerals form out of-liquids, from fluids ... they come from solidification of material in fluids. When this material solidifies, under certain conditions, as we'll see, it forms crystals-mineral crystals. Now, these crystals are usually found inside rocks. I mean, most often what you see are rocks that contain crystals, rocks like, like granite, which is made up of crystals of different minerals that are bonded together. Now, there are four ways that minerals form that need to be covered, ok? And these are...uh, actually, you know what? It occurs to me now, we can't possibly do justice to them all in the time we have left. So maybe instead-let's just focus on one of them-it's called crystallization from magma. OK. Magma-uh, molten rock material deep in the Earth. It is from this magma that many minerals form. So, how do you get crystallization happening in molten rock? Well, magma is a very, very hot liquid, but as it rises up toward the Earth's surface, it cools. And as it cools, the energy from heat that kept the atoms in the magma apart, the heat that was keeping it all liquid, begins to be lost. And as that happens, as it rises and cools, the movement of atoms and groups of atoms slows down and they begin to arrange themselves into orderly patterns, begin to fit together to form solid crystal structures. The rate of cooling is important. Under normal cooling of magma, a normal rate, mineral crystals will form. -But, if the rate of cooling is very rapid, much faster than normal, then it is too fast for an orderly arrangement to happen. So you don't get crystals. Instead, you get-well you get what looks like a frozen liquid mixture, a kind of glass material-not the kind you have in your windows, but it's like a naturally occurring glass material. This glass is very brittle, it breaks easily-it doesn't have any structure like crystals do. OK? So these effects of cooling are important to our next point. OK. So mineral crystals form when magma cools underground as it slowly rises. But why do we get lots of different kinds of mineral crystals in the same rock? How do you get minerals like quartz and feldspar and olivine all forming from the same starting magma? One of the first scientists to look at this, in the early 1920's, was Norman Bowen. What Bowen did was create a "magma" in his laboratory: He took particular amounts of elements found in rocks, then he melted it all down into a "magma" at a very, very high temperature. Then he let this magma mixture cool down-at a slow enough rate, so that solid mineral crystals would form in it. When the mixture reached a certain temperature, he stopped the crystallization by instantly freezing the mixture. Now he didn't literally freeze it of course, he just cooled it so fast that it all suddenly turned from molten material into a solid "glass" material. Very rapid cooling, OK? We said crystallization can't take place in that case. So then what he had was mineral crystals inside the solid "frozen magma glass". Bowen could then analyze the crystals, that is exactly which type of minerals had formed at that point of cooling. And he could also analyze the "glass," which represented what was left in the magma after some minerals had crystallized out. Then Bowen repeated this procedure many times. He'd create more magma with the same starting composition, but each time, he'd let the slow cooling go on a little bit longer before he froze it, stopping the crystallization. Then he'd analyze what additional minerals had crystallized at the end of the longer cooling period. And he found that, for example, if he let the magma cool this long, then a mineral like feldspar would form, and when he let the magma cool longer, then another mineral like quartz would form from the remaining elements in the magma. So by repeating this procedure, Bowen showed that there was a specific sequence in which the different minerals formed as the magma became cooler.