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板凳

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发表于 2011-5-9 21:49:31
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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 am going to talk about particular dating technique, why? Good dating is a key to good analysis. In other words, if you want know how a land formation was formed, the first thing you probably want to know is how old it is. It is fundamental.
Take the GC for instance, now we geologist thought we had a pretty good idea of how the GC in the southwestern united state was formed. we knew that it was formed from sandstone that solidified somewhere between 150 and 300 million years ago. Before it is solidified, it was just regular sand , essentially it was part of the 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 is been the conventional wisdom among geologist for quite some time . but now we have learned something different and quite surprising using a technique called U L dating.
I should say that UL dating has been around for quite a while, but there have been some recent refinements. I will get into this in a minute, anyway UL dating has produced some surprises. Two geologists discovered that about half of the sand from the GC was actually once part of the apolition mountains. That is really eye-opening news since the apolition mountain range is ,of course , thousands of kilometers to east of the GC. Sounds pretty unbelievable ,right? Of course ,the obvious question is how did that sand end up so far west? 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 pretty revolutionary finding, and it was basically because of UD dating, why, well as everyone in this class should know we usually look at the grain type within sand stone, meaning the actual particles of the sand stone to determine where it came from. You can do other things too, like look at the wind or the water that brought the grains to their location and figure out which way it was flowing. But That is only useful up to a point and that is not what these two geologists did,
UL dating allowed them to go about it in an entirely different way. What they did was: they looked at the grains of the ZC in the sandstone. ZC is material that contains radio active U , which makes it very useful for dating purposes, ZC starts off molten magma the hot lava of volcanoes, this magma then crystallizes. And when ZC crystallizes, the U inside it begins to change into lead so if you measure the amount of lead in the ZC grain, you can figure out when the grain was formed. After that , you can determine age of ZC from different mountain ranges. Once you do that , you can compare the age of the ZC in the sand stone in your sample to the age of ZC in the mountains. If the age the ZC matches the age of one of your mountain ranges, then it means the sand stone actually used to be part of that particular mountain range . Is everybody with me on that ? good .
so in this case UL dating was used to establish that half of sand stone in the samples was formed at the same time the granite in the apolition mountains was formed. So because of this , this new way of doing UL dating we have able to determine that one of our major assumptions about the GC was wrong.
Like I said before , U L dating has been with us for a while ,but 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 was not very efficient. and it was not 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 will predict that UL dating is going to become a popular dating method.
There are a few pretty exciting possibilities of UL dating . here is the one that come to mind , you know the theory that earth’s continents were once joined together and only split apart relative recently? Well, with UL dating we can prove that more conclusively. If they show evidence of once having been joined, that could really tell us a lot of the early history of the planet’s geology. |
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