Official 16 Set 2

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Lechuguilla Cave

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What is the main purpose of the lecture?
  • A. To explain the various ways that sulfuric acid is involved in the formation of caves

  • B. To describe caves and other geologic formations in U.S. national parks

  • C. To use Lechuguilla Cave as an example of how most caves form

  • D. To discuss the formation and characteristics of an unusual type of cave

显示答案 正确答案: D

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    NARRATOR:Listen to part of a lecture in a geology class.

    MALE PROFESSOR:Now, there are some pretty interesting caves in parts of the western United States, especially 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, so can anyone remember what it said?Ellen?

    FEMALE STUDENT:[upspeak]It’s the deepest limestone cave in the U.S.

    MALE PROFESSOR:That’s right—It’s one of the longest and deepest limestone caves not just in the country, but in the world. What else?

    FEMALE STUDENT:Well, it was formed because of sulfuric acid, right?

    MALE PROFESSOR: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, this 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? [short pause]Sulfuric acid eats away at limestone—very aggressively—so you get bigger cracks, and then passageways being formed along the openings in the rock. And it’s all underground.Yes, Paul?

    MALE STUDENT:So that water, it’s not flowing, right? It’s still?

    MALE PROFESSOR:Yes! So there’s 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 the limestone.It’s the moving water itself that wears away at the rock and makes passageways.Also, in surface water, there’s 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.They were all very wet, you know? From streams and rivers.This flowing water carved out the caves and the structures inside them.

    MALE STUDENT:But not Lechuguilla?

    MALE PROFESSOR:[matter-of-factly] Dry as a bone.[hesitates] 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 itIn fact, there’s no evidence that flowing water has ever gone in or out of the cave.So it’s like a maze, you have passageways all around.There’re 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 of these passageways have an opening to the outside world.

    And there’s 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.

    MALE STUDENT:It leaves a chemical residue. Um, [interrupted]

    FEMALE STUDENT:[interrupting]Gypsum, right?

    MALE PROFESSOR: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 anymore… but there’s other ones like it, for example in Mexico, that are forming, and when cave researchers go to explore them, they see—and smell—the sulfuric acid and gases at work.Whew! It’s 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?

    MALE STUDENT:Yeah, how about what these caves look like, uh, on the inside?

    MALE PROFESSOR:Oh, well, the formations are really something.And there’s such a 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 can be up to twenty feet long.It’s pretty impressive.

  • 旁白:听下面一段关于地质学的讲座。

    教授:现在在美国西部某些地方发现了一些有趣的洞穴,尤其是在国家公园一带,有一个地方发现了超过 100 个洞穴,其中一个的规模为世界最大。这些洞穴中,比较有趣的一个名叫拉曲吉亚洞穴。

    拉曲吉亚在近几十年被发掘过几次。我觉得它是一处让人激动的遗址。你们的书中只是大概提了一下,有人记得书里是怎么描述它的吗?艾伦

    学生:书里说它是美国最深的石灰岩洞穴。

    教授:正确。不过它的深度、长度不仅是我国第一,而且更是世界第一。还有别的补充吗?

    学生:嗯,它是由于硫酸的腐蚀而形成的,对吗?

    教授:是那样的。对,事情是,会有腐蚀。当地下深处有油类囤积和细菌微生物时,我来画张图吧!

    地下石灰岩的一部分被地下水淹没。我们假设这些波浪线代表了岩石的裂缝。石油出现的位置,在地下水位线和岩石之下。细菌靠分解石油维持生命,在这个过程中,就会有硫化氢释放出来。硫化氢上升,与地下水中溶解的氧气混合,渗入石灰岩的缝隙中。并且当硫化氢与水中的氧气反应时,会产生硫酸。对吧?硫酸对石灰岩的腐蚀性是极强的,所以这个裂缝就会越来越大,逐渐地就会有一条通道沿着岩石的断裂层形成。这个过程全部是发生在地下。保罗有什么问题吗?

    学生:这样的话,地下水…难道它是不流动的,对吗?它是静止的?

    教授:是的,所以世界上有两种类型的石灰岩洞穴。百分之九十的洞穴的表面都有水,比如小溪,瀑布等各种沿着岩石缝隙流动的水流。腐蚀岩石,形成通道的因素就在于这些水流自身。而且,这些地表水流中,存在弱酸,比如碳酸,而不是硫酸,这有助于溶解岩石。

    这种酸也能腐蚀岩层,依靠这种弱酸的帮助和水流的冲击力,大部分石灰岩洞穴形成了。几年前我做这项研究时,我探访了其中几个比较典型的洞穴。它们都很潮湿,你们也知道这是因为水流的缘故。总之,这些水流凿穿了岩石,造成了这些洞穴及其内部结构。

    学生:不过这些洞穴中不包括拉曲吉亚?

    教授:拉曲吉亚干燥得像块硬骨头。嗯,这个比喻有些夸张了。不过,我们可以很稳妥地说,是硫酸而非水流使得拉曲吉亚等洞穴成形,并且很少有这样的。实际上,没有证据表明水流曾在拉曲吉亚附近进出过。所以,拉曲吉亚洞穴就像一座迷宫,里面到处都是通道。宽的,窄的,各种深度都有...就像石灰岩中的地下通道。并且由于它们是在地下形成的,而不是由地表水流形成,某些通道完全没有与地面接通。

    还有其他证据表明水流与拉曲吉亚没有丝毫关系。我们讲过,硫酸可以溶解石灰岩,是吧?并且这种腐蚀形成了通道。硫酸还有什么别的功能呢?保罗?

    学生:嗯,它会遗留下一些残渣,并且…

    学生:这些残渣就是石膏,对吗?

    教授:正确,你们会在拉曲吉亚洞穴中发现许多沉积下来的石膏。同时,我们知道,石膏是可以溶解在水中的。所以如果拉曲吉亚附近有水流存在的话,它就会使石膏溶解。这就是为什么我们会得出拉曲吉亚是为数不多的、并非因水流形成的洞穴之一。

    现在拉曲吉亚洞穴的形成已经停滞了。它现在已经不再有进一步变化了。不过,也有其他与之相似的洞穴依旧进行着这个过程,比如,在墨西哥就有一个。当研究人员探索这些洞穴时,他们看到、闻到硫酸,和像…的气体。嗯…,想到臭鸡蛋的味道吧!而且,不仅是只有这种味道,研究员还得带上专门的面具,使自己和这种气体隔绝开。保罗,你有问题吗?

    学生:嗯,这些洞穴里面是什么样子的?

    教授:嗯,里面形成的物质…里面的确有些东西。不过这些物质各式各样,好像世界其他地方都没有。有些看上去很像精心设计的装饰物,其中相当一部分都是由石膏构成的,高度达到了 20 英尺。非常壮观

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    题型分类:主旨题

    题干分析:main purpose主旨题啦

    音频定位:文章开头无标志

    选项分析:本文讲是Lechugmlla Cave的形成和特点,A sulfuric acid参与山洞形成的不同方式,太片面;B介绍国家公园的山洞和其他地形,太大;CLC当做一个例子,而LC是很特殊的,不应成为山洞形成的一个例子。

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