托福听力TPO1原文 Lecture 2

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托福听力TPO1 原文 Lecture 2

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TPO 1 Lecture2GeologyListen to part of a lecture in a geology class.

ProfessorOk, 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.

Uh… 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 quite 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 f inding. 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,