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A、Geologists had a pretty good idea of the Grand Canyon. B、United States was formed 150 million years ago. C、United States was f
A、Geologists had a pretty good idea of the Grand Canyon. B、United States was formed 150 million years ago. C、United States was f
admin
2016-07-29
26
问题
Ok, let’s get started Great Today I want to talk about a way in which we are able to determine how old a piece of land, or some other geologic feature is—dating techniques. I’m going to talk about a particular dating technique. Why? Good dating is key to good analysis. In other words, if you want to know how a land formation was formed, the first thing you probably want to know is how old it is. It’s fundamental.
Take the Grand Canyon for instance. Now, we geologists thought we had a pretty good idea of how the Grand 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 come 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 qui e surprising, using a technique called Uranium-Lead Dating. I should say that Uranium-Lead Dating has been around for quite a while. But there have been some recent refinements. I will get into this in a minute.
Anyway, Uranium-Lead 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 eye-opening 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 Uranium-Lead 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.
There are a few pretty exciting possibilities for Uranium-Lead Dating. Here is one that comes to mind. You know the theory that earth’s continents were once joined together and only split apart relatively recently? Well, with Uranium-Lead 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.
22. What is the key to good analysis in geology?
23. What has been the conventional wisdom among geologists for quite some time?
24. What has produced some surprises among geologists?
25. What is one of the exciting possibilities for Uranium-Lead Dating?
选项
A、Geologists had a pretty good idea of the Grand Canyon.
B、United States was formed 150 million years ago.
C、United States was formed from sandstone.
D、The sand had come from an ancient mountain range.
答案
D
解析
录音提到,在相当长一段时间里,地质学家还认为沙子是由附近平坦分布的古老山脉退化形成的。D项是对该内容的概述,故为正确答案。A项“地质学家对大峡谷形成已经有足够的了解”是目前的情况,并非题目所问的在相当长时间内的conventional wisdom(普遍看法);录音没有提到美国形成的时间,B项“美国在1500万年前形成”错误;录音只提到大峡谷是由砂石形成,并没有提到C项“美国是由砂岩形成”.
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大学英语六级
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