托福模拟试题听力 - TPO24 - 讲座4

时间 : 2013-11-03 18:12来源 : VOA官网 收听下载次数 :
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TPO 24 Lecture4-Astronomy (Shield Volcanoes on Venus)

Professor:Many people have been fascinated about Venus for centuries because of its thick cloud cover, this so-called planet of mystery and all of that. Well, what's under those clouds? What's the surface of the planet like? Some questions about the surface are still unresolved but, but we have learned a lot about it in the past several years.First of all, let me talk about how we have been able to get past those clouds. First, there were Soviet modules2 that landed directly on the surface and sent back some images ofwhat was around them. Second, we did some radar imaging from satellites from above. Radar can get through the clouds. So what have we learned? Yes, Karen?

Student:Well, I remember reading that there's not really a lot going on, that the surface of Venus is just flat and smooth in a lot of places.

Professor:Yeah, smooth in a lot of places. But that's not, um... that's not the whole picture. In other areas, you've got canyons, ripped valleys, meteo craters, uh, lava domes, these lava formations that look like giant pancakes. And also volcanoes.Well, one of the most interesting features on the surface are in fact the shield volcanoes. Shield volcanoes formed when magma comes out of the ground in the same spot over and over again. Remember, magma is hot molten rock that's underground, and it is called lava when it reaches the surface. Uh, so the lava builds up, and hardens, and a volcano forms.Now, the lava on Venus is thin. It spreads out easily. So shield volcanoes have very gentle sloping sides. They are called shield volcanoes, because viewed from above, they kind of resemble shields, you know, like a warrior's shield.But what's particularly interesting about these volcanoes is that most of the volcanoes here on Earth are not shield volcanoes. Instead, they are other volcano types, like strata volcanoes, for example, which are a result of tectonic plate movement. Remember tectonic plates?Underneath the Earth's crust, there are a number of shifting slabs or plates that are slowly moving. And in the zones on the edges of the plates where different plates meet and interact, that's where we get most of Earth's volcanoesOn Venus, however, volcanoes are not clustered in discrete zones like they are on Earth. Instead, they are more or less randomly scattered over Venus's surface. Well, that's significant. Venus has mostly shield volcanoes, and they are randomly scattered, that indicates that Venus does not have moving tectonic plates, and that's a big difference compared to Earth. Here on Earth, moving tectonic plates are a major geological element, just crucial for the whole surface dynamic, right?So why doesn't Venus have them? Well, there are a few theories. One of them is that this has to do with the fact that Venus has no surface water that's needed to kind of lubricate the movement of the plates, you know, like oceans on Earth. Yeah, I forgot to spell that out. Uh, Venus has no surface water.

Student:Wait a second. Did you say we have shield volcanoes on Earth? Can you give an example?

Professor:Sure. The volcanoes in the Hawaii islands, in the Pacific Ocean are shield volcanoes. They are formed over a hot spot of magma. So while on Earth we have several types of volcanoes, on Venus there's mostly the one type. Uh, Eric?

Student:Are the volcanoes on Venus still active?

Professor:Well, that's an interesting question. There is still some discussion on that point. But here's what we do now. First, the level of sulfur dioxide gas above Venus's clouds shows large and very frequent fluctuations. It is quite possible that these fluctuations, the huge increase and decrease of sulfur dioxide, happening again and again. It's quite possible that this is due to volcanic eruptions, because volcanic eruptions often emit gases. If that's the case, volcanism could very well be the root cause of Venus's thick cloud cover. And also we have observed bursts of radio energy from the planet's surface. These bursts are similar to what we see when volcanoes erupt on Earth. So this too suggests ongoing volcanic activity. But although this is intriguing evidence, no one's actually observed a Venus volcano erupting yet, so we can't be positive.

 

1 What is the lecture mainly about?

A. Conclusions about the surface of Venus drawn from recent observations

B. The different types of volcanoes that have been found on Venus

C. Similarities between geologic processes on Venus and on other planets

D. New methods used to observe the surface of Venus



2 How were scientists able to learn new information about Venus' surface? Click on 2 answers

A. More powerful telescopes allowed astronomers to see more surface details

B. Satellites were used to get radar images of Venus' surface.

C. Space modules sent photographs after landing on Venus.

D. A space probe was able to locate gaps among Venus' clouds
 


3  What differences between volcanoes on Venus and those on Earth does the professor mention?  Click on 2 answers

A. Volcanoes on Venus are mostly shield volcanoes

B. Volcanoes on Venus emit thicker lava

C. Many volcanoes on Earth are concentrated inspecific zones.

D. Many volcanoes on Earth show no volcanic activity



4 Why does the professor mention the fact that Venus has no surface water?

A. To give an example of a discovery that was unexpected

B. To illustrate the consequences of intense volcanic activity

C. To question a theory about the origin of shield volcanoes

D. To suggest an explanation for why Venus does not have moving tectonic plates



5 According to the professor, what is a possible origin of Venus' clouds?

A. Gases released as a result of volcanic activity

B. Chemical reactions caused by high surface temperatures

C. Bursts of radio energy from the plane's surface

D. Strong winds that blow dust into the atmosphere



6 What is the professor’s opinion about the theory that Venus' volcanoes are active?

A. The theory has recently been proved by direct observation

B. At least two kinds of evidence support the theory

C. The data collected so far about the theory are contradictory

D. New findings have made the theory quite unlikely.