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英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
n. 听,倾听

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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

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TPO 16 -Lechuguilla Cave

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Listen to a part of lecturer in a geology class.

Now there are some pretty interesting caves in parts of the western United States, especially in national parks. There is one part that has over a hundred caves, including some of the largest ones in the world. One of the more interesting ones is called Lechuguilla Cave. Lechuguilla has been explored a lot in recent decades, it's a pretty exciting place I think. It was mentioned only briefly in your books. So can anyone remember what it said? Ellen?

It's the deepest limestone cave in the U.S.?

That's right. It's one of the longest and deepest limestone caves not just in the country but in the world. Now, what else?

Well, it was formed because of sulfuric acid, right?

That's it. Yeah, what happens is you have deep underground oil deposits and there are bacteria. Here let me draw a diagram.

Part of the limestone rock layer is permeated by water from below. Those curly lines are supposed to be cracks in the rock. Below the water table and rock is oil. Bacteria feed on this oil and release hydrogen sulfide gas. This gas is hydrogen sulfide, rises up and mixes with oxygen in the underground water that sits in the cracks and fissures in the limestone.And when hydrogen sulfide reacts with the oxygen in the water, the result of that is sulfuric acid, Ok? Sulfuric acid eats away at limestone very aggressively.So you get bigger cracks and then passage way is being formed along the openings in the rock and it's all underground. Ah yes, Paul?

So that water... It's not flowing, right? It's still?

Yes, so there are two kinds of limestone caves. In about 90 percent of them, you have water from the surface, streams, waterfall or whatever - moving water that flows through cracks found in limestone. It's the moving water itself that wears away at the rock and makes passageways. Also, in surface water, there is a weak acid, carbonic acid, not sulfuric acid but carbonic acid that helps dissolve the rock. With a little help from this carbonic acid, moving water forms most of the world's limestone caves. When I was researching this for a study a few years ago, I visited a couple of these typical limestone caves, and they were all very wet, you know, from streams and rivers. This flowing water carved out the caves and the structures inside them.

But not Lechuguilla?

Dry as a bone. Well, that might be a bit of an exaggeration. But it's safe to say that it's sulfuric acid and not moving water that formed Lechuguilla cave and those few other ones like it. In fact, there is no evidence that flowing water has even gone in or out of the cave. So, it's like a maze. You have passageways all around. There are wide passages narrow ones at all different depths, like underground tunnels in the limestone. And, since they were created underground and not from flowing surface water, not all these passageways have an opening to the outside world. And... and there is other evidence that flowing water wasn't involved in Lechuguilla. We've said that sulfuric acid dissolves limestone, right, and forms the passageways? What else does sulfuric acid do? Paul?

Ah, leaves a chemical residue and... 

Gypsum, right?

Yep, you'll find lots of gypsum deposited at Lechuguilla. And, as we know, gypsum is soluble in water. So if there were flowing water in the cave, it would dissolve the gypsum. This is part of what led us to the realization that Lechuguilla is in that small group of waterless caves. And Lechuguilla is pretty much dormant now. It's not really forming any more. But, there is other ones like it, for example, in Mexico, they are forming. And when cave researchers go to explore them, they see and smell, the sulfuric acid and gases of... er... phew... now, something else, think of rotten eggs. And, it's not just the smell.Explorers even need to wear special masks to protect themselves from the gases in these caves. OK? Paul.

Yeah, how about what these caves look like on the inside?

Well, the formations... there is really something. There's such variety there like nothing anywhere else in the world, some of them are elaborate looking, like decorations.  And a lot of them are made of gypsum and could be up to 20 feet long. It's pretty impressive. 


Question 1
What is the main purpose of the lecture?
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    小托君 发表于 2018-08-31 15:58:48

    原文出处:Professor: Now, there’re some pretty interesting caves in parts of the western United States, um, specially in National Parks. There’s one park that has over a hundred caves including some of the largest ones in the world. One of the more interesting ones is called Lechuguilla Cave. Lechuguilla’s been explored a lot in recent decades. It’s a pretty exciting place I think. It was mentioned only briefly in your books. 解析:从演讲中得知本文主要讲的是Lechugmlla Cave的形成与特点,A 是硫酸参 与山洞形成的不同方式,太片面;B的介绍国家公园的山洞和其他地形,范围太广;C的把LC 当做一个例子,介绍大多数山洞的形成,而首先LC是很特殊的,是少数而非大多数的例子,其次虽然介绍了由水冲击的山洞的形成,不是文章的 main idea。

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