[00:00.00]Narrator: Listen to part of a lecture in a geology class.
[00:04.84]MALE PROFESSOR: OK, let's get started. [00:07.91]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. [00:18.71]I'm gonna talk about a particular dating technique. [00:22.04]Why? Good dating is key to good analysis. [00:26.75]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. [00:34.89]It's fundamental.
[00:36.42]Uh, take the Grand Canyon for instance.[00:39.45]Now, we geologists thought we had a pretty good idea of how the Grand Canyon in the southwestern United States was formed. [00:48.45]We knew that it was formed from sandstone that solidified somewhere between 150 and 300 million years ago. [00:56.12]Before it solidified, it was just regular sand—[01:00.24]essentially, it was part of a vast desert.
[01:03.17]And, uh, until just recently most of us thought the sand had come from an ancient mountain range fairly close by that flattened out over time. [01:12.61]That's been the conventional wisdom among geologists for quite some time. [01:17.54]But now we've learned something different and quite surprising using a technique called uranium-lead dating.
[01:26.07]I should say that uranium-lead dating has been around for quite a while, [01:31.06]but there have been some recent refinements—[01:33.91]I'll get into this in a minute. [01:35.75]Anyway, uranium-lead dating has produced some surprises. [01:40.49]Two geologists discovered that about half of the sand from the Grand Canyon was actually once part of the Appalachian Mountains.[01:49.16]That's really eye-opening news, since the Appalachian Mountain range is, of course, thousands of kilometers to the east of the Grand Canyon. [01:58.52]Sounds pretty unbelievable, right?
[02:01.10]Of course, the obvious question is, how did that sand end up so far west? [02:07.69]The theory is that huge rivers and wind carried the sand west, where it mixed in with the sand that was already there.[02:16.05]Well, this was a pretty revolutionary finding, [02:19.29]uh, and it was basically because of uranium-lead dating. [02:23.41]Why?
[02:24.56]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. [02:36.16]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. [02:45.20]But that's only useful up to a point, and that's not what these two geologists did.
[02:50.56]Uranium-lead dating allowed them to go about it in an entirely different way.[02:55.51]What they did was they looked at the grains of zircon in the sandstone.[03:00.26]Zircon is a material that contains radioactive uranium, which makes it very useful for dating purposes.
[03:07.93]Uh, zircon starts off as molten magma, the hot lava from volcanoes. [03:13.84]This magma then crystallizes. [03:16.30]And when zircon crystallizes, the uranium inside it begins to change into lead. [03:21.90]So, if you measure the amount of lead in a zircon grain, you can figure out when the grain was formed. [03:28.76]After that, you can determine the age of zircon from different mountain ranges.
[03:33.96]Once you do that, you can compare the age of the zircon in the sandstone in your sample to the age of the zircon in the mountains. [03:41.75]If the age of the zircon matches the age of one of your mountain ranges, then it means the sandstone actually used to be part of that particular mountain range. [03:51.25]Is everybody with me on that? [03:53.30]Good.So in this case, uranium-lead 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.
[04:05.71]So, because of this—this new way of doing uranium-lead dating, we've been able to determine that one of our major assumptions about the Grand Canyon was wrong.[04:17.14]Like I said before, uranium-lead dating has been with us for a while, [04:21.72]but, uh, until recently, in order to do it, you really had to study many individual grains, [04:28.14]and it took a long time before you got results. [04:31.20]It just wasn't very efficient [04:33.59]and it wasn't very accurate.
[04:35.62]But technical advances have cut down on the number of grains you have to study, so you get your results faster. [04:42.33]So I'll predict that, uh, uranium-lead dating is going to become an increasingly popular dating method.[04:49.29]There are a few pretty exciting possibilities for uranium-lead dating.
[04:54.33]Here's one that comes to mind. [04:56.67]You know, the theory that Earth's continents were once joined together and only split apart relatively recently? [05:03.12]Well, with uranium-lead dating, we could prove that more conclusively. [05:07.99]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.
托福14天冲刺微信战团