【正文】 and Canyon in the southwestern United States was formed. We knew that it was formed from sandstone that solidified somewhere between 150 and 300 million years ago. Before it solidified, it was just regular sand. Essentially it was part of a vast desert. And until just recently, most of us thought the sand had e from an ancient mountain range fairly close by that flattened out over time. That’s been the conventional wisdom among geologists for quite some time. But now we’ve learned something different, and quite surprising, using a technique called UraniumLead Dating. I should say that UraniumLead Dating has been around for quite a while. But there have been some recent refinements. I will get into this in a minute. Anyway, UraniumLead Dating has produced some surprises. Two geologists discovered that about half of the sand from the Grand Canyon was actually once part of the Appalachian Mountains. That’s really eyeopening news, since the Appalachian Mountain Range is, of course, thousands of kilometers to the east of the Grand Canyon. Sounds pretty unbelievable, right? Of course, the obvious question is how did that sand end up so far west? The theory is that huge rivers and wind carried the sand west where it mixed in with the sand that was already there. Well, this was a pretty revolutionary finding. Um… and it was basically because of UraniumLead Dating. Why? Well, as everyone in this class should know, we usually look at the grain type within sandstone, meaning the actual particles in the sandstone, to determine where it came from. You can do other things too, like look at the wind or water that brought the grains to their location and figure out which way it was flowing. But that’s only useful up to a point, and that’s not what these two geologists did. UraniumLead Dating allowed them to go about it in an entirely different way. What they did was: they looked at the grains of Zircon in the sandstone. Zircon is a material that contains radioactive Uranium, which makes it very useful for dating purposes. Zircon starts off as molten magma, the hot lava from volcanoes. This magma then crystallizes. And when Zircon crystallizes, the Uranium inside it begins to change into Lead. So if you measure the amount of Lead in the Zircon grain, you can figure out when the grain was formed. After that, you can determine the age of Zircon from different mountain ranges. Once you do that, you can pare the age of the Zircon in the sandstone in your sample to the age of the Zircon in the mountains. If the age of the Zircon matches the age of one of the mountain ranges, then it means the sandstone actually used to be part of that particular mountain range. Is everybody with me on that? Good. So, in this case, UraniumLead Dating was used to establish that half of the sandstone in the samples was formed at the same time the granite in the Appalachian Mountains was formed. So because of this, this new way of doing UraniumLead Dating, we’ve been able to determine that one of our major assumptions about the Grand Canyon was wrong. Like I said before, UraniumLead Dating has been with us for a while. But, um… until recently, in order to do it, you really had to study many individual grains. And it took a long time before you got results. It just wasn’t very efficient. And it wasn’t very accurate. But technical advances have cut down on the number of grains you have to study, so you get your results faster. So I’ll predict that UraniumLead Dating is going to bee an increasingly popular dating method. There are a few pretty exciting possibilities for UraniumLead Dating. Here is one that es to mind. You know the theory that earth’s continents were once joined together and only split apart relatively recently? Well, with UraniumLead Dating, we could prove that more conclusively. If they show evidence of once having been joined, that could really tell us a lot about the early history of the planet’s geology. TPO 1 Conversation 2NarratorListen to part of a conversation between a student and his professor.ProfessorHi Mathew, I’m glad you could e in today. You’ve been observing Mr. Grable’s thirdgrade class for your approaches to education paper, right?Student,Um, yes. I go over to Johnson Elementary School, you know, to watch Mr. Grable teach the children in class. It’s been amazing, I mean, I’m just learning so much from just watching him. I’m so glad the classroom observations are a requirement for the education program. I mean it’s like the best thing ever to prepare you to be a good teacher.ProfessorWell, I’m glad to see you feel that way, Mathew. You know, that’s the goal. So, I’ve been reading over your observation notes and I’m quite interested in what’s going on, in particular with the astronomy unit he’s been teaching.StudentThe astronomy unit?ProfessorIt seems that Mr. Grable has mastered the interdisciplinary approach to teaching —that we’ve been talking about in class.StudentOh! OK, yeah, so like when he was teaching them astronomy, he didn39。s the amount of time it takes for them to grow, right? So it would be five months? ProfessorUmm? Oh, uh… I’m sorry but no. It has nothing to do with that. It39。Librarian 238TPO20 Lecture1Linguistics(Gricean Maxims) 240TPO20 Lecture2Environmental Science(Interglacial Periods) 242TPO20 Section2 Conversation2Studentamp。 Clerk in the Bookstore 276TPO 24 Lecture lBiology (Crocodile Vocalization) 277TPO 24 Lecture2Art History (Modern Dance) 278TPO 24 Conversation 2一Student amp。t need to stay together as a family to survive either.ProfessorUhha. Any contrast, the Olympic marmots? What about them?StudentWell, they live together as a family and take care of their young until they are at least two years old. They’re really friendly with each other. And what I really like is that they even have greeting ceremonies. And they are not at all aggressive and territorial l