VOA慢速英语20060418c

SCIENCE IN THE NEWS - Plate Tectonics: Nature's Way of StretchingBy Nancy Steinbach

Broadcast: Tuesday, April 18, 2006

(MUSIC)

VOICE ONE:

This is SCIENCE IN THE NEWS, in VOA Special English. I'm Barbara Klein.

VOICE TWO:

And I'm Bob Doughty¹. Scientists who study the Earth tell us that the continents and ocean floors are always moving. Sometimes, this movement is violent and might result in great destruction.

VOICE ONE:

Today we examine the process that causes earthquakes.

(MUSIC)




The first pictures of Earth taken from space showed a solid ball covered by brown and green land masses and blue-green oceans. It appeared as if the Earth had always looked that way -- and always would. Scientists now know, however, that the surface of the Earth is not as permanent as had been thought.

Scientists explain that the surface of our planet is always in motion. Continents move about the Earth like huge ships at sea. They float on pieces of the Earth's outer skin, or crust. New crust is created as melted rock pushes up from inside the planet. Old crust is destroyed as it rolls down into the hot area and melts again.

VOICE TWO:

Only since the nineteen-sixties have scientists begun to understand that the Earth is a great, living structure. Some experts say this new understanding is one of the most important revolutions in scientific thought. The revolution is based on the work of scientists who study the movement of the continents -- a process called plate tectonics.

Earthquakes are a result of that process. Plate tectonics is the area of science that explains why the surface of the Earth changes and how those changes cause earthquakes.


Photograph showing destruction caused by the 1999 Izmit, Turkey, earthquake

VOICE ONE:

Scientists say the surface of the Earth is cracked like a giant eggshell. They call the pieces tectonic plates. As many as twenty of them cover the Earth. The plates float about slowly, sometimes crashing into each other, and sometimes moving away from each other.

When the plates move, the continents move with them. Sometimes the continents are above two plates. The continents split² as the plates move.

VOICE TWO:

Tectonic plates can cause earthquakes as they move. Modern instruments show that about ninety percent of all earthquakes take place along a few lines in several places around the Earth.

These lines follow underwater mountains where hot liquid rock flows up from deep inside the planet. Sometimes, the melted rock comes out with a great burst of pressure. This forces apart pieces of the Earth's surface in a violent earthquake.

Other earthquakes take place at the edges of continents. Pressure increases as two plates move against each other. When this happens, one plate moves past the other, suddenly causing the Earth's surface to split.

VOICE ONE:

One example of this is found in California, on the West Coast of the United States. One part of California is on what is known as the Pacific plate. The other part of the state is on what is known as the North American plate.


San Francisco lies in rubble⁴ following the 1906 earthquake

Scientists say the Pacific plate is moving toward the northwest, while the North American plate is moving more to the southeast. Where these two huge plates come together is called a fault line.

The name of this line between the plates in California is the San Andreas Fault. It is along or near this line that most of California's earthquakes take place, as the two tectonic plates move in different directions.

The city of Los Angeles in Southern California is about fifty kilometers from the San Andreas Fault. Many smaller fault lines can be found throughout the area around Los Angeles. A major earthquake in nineteen ninety-four was centered along one of these smaller fault lines.

(MUSIC)

VOICE TWO:

The story of plate tectonics begins with the German scientist Alfred Wegener in the early part of the twentieth century. He first proposed that the continents had moved and were still moving.

He said the idea came to him when he observed that the coasts of South America and Africa could fit together like two pieces of a puzzle. He proposed that the two continents might have been one, then split apart.

Later, Alfred Wegener said the continents had once been part of a huge area of land he called Pangaea. He said the huge continent had split more than two hundred million years ago. He said the pieces were still floating apart.

VOICE ONE:

Wegener investigated the idea that continents move. He pointed⁵ out a line of mountains that appears from east to west in South Africa. Then he pointed out another line of mountains that looks almost exactly the same in Argentina, on the other side of the Atlantic Ocean. He found fossil⁶ remains⁷ of the same kind of an early plant in areas of Africa, South America, India, Australia and even Antarctica.

Alfred Wegener said the mountains and fossils⁸ were evidence that all the land on Earth was united at some time in the distant past.

VOICE TWO:

Wegener also noted⁹ differences between the continents and the ocean floor. He said the oceans were more than just low places that had filled with water. Even if the water was removed, he said, a person would still see differences between the continents and the ocean floor.

Also, the continents and the ocean floor are not made of the same kind of rock. The continents are made of a granite-like rock, a mixture of silicon¹⁰ and aluminum¹¹. The ocean floor is basalt rock, a mixture of silicon and magnesium¹². Mister Wegener said the lighter¹³ continental¹⁴ rock floated up through the heavier basalt rock of the ocean floor.

VOICE ONE:

Support for Alfred Wegener's ideas did not come until the early nineteen-fifties. American scientists Harry¹⁵ Hess and Robert Dietz said the continents moved as new sea floor was created under the Atlantic Ocean.

They said a thin valley in the Atlantic Ocean was a place where the ocean floor splits¹⁶. They said hot melted material flows up from deep inside the Earth through the split. As the hot material reaches the ocean floor, it spreads out, cools and hardens. It becomes new ocean floor.

The two scientists proposed that the floor of the Atlantic Ocean is moving away from each side of the split. The movement is very slow -- a few centimeters a year.

In time, they said, the moving ocean floor is blocked when it comes up against the edge of a continent. Then it is forced down under the continent, deep into the Earth, where it is melted again.

Harry Hess and Robert Dietz said this spreading does not make the Earth bigger. As new ocean floor is created, an equal amount is destroyed.

VOICE TWO:

The two scientists also said Alfred Wegener was correct. The continents move as new material from the center of the Earth rises, hardens and pushes older pieces of the Earth away from each other. The continents are moving all the time, although we cannot feel it.

They called their theory sea floor spreading. The theory explains that as the sea floor spreads, the tectonic plates are pushed and pulled in different directions.

(MUSIC)

VOICE ONE:

The idea of plate tectonics explains volcanoes as well as earthquakes. Many of the world's volcanoes are found at the edges of plates, where geologic¹⁷ activity is intense. The large number of volcanoes around the Pacific plate has earned the name Ring of Fire.


Mount³ St. Helen's exploding, May 18, 1980.

Volcanoes also are found in the middle of plates, where there is a well of melted rock. Scientists call these wells hot spots. A hot spot does not move. However, as the plate moves over it, a line of volcanoes is formed.

The Hawaiian Islands were created in the middle of the Pacific Ocean as the plate moved slowly over a hot spot. This process is continuing, as the plate continues to move.

VOICE TWO:

Volcanoes and earthquakes are among the most frightening events that nature can produce. The major earthquake in South Asia in October of two thousand five, for example, killed more than seventy thousand people. More than three million people were made homeless because of the earthquake. At times like these, we remember that the ground is not as solid and unchanging as people might like to think.

(MUSIC)

VOICE ONE:

SCIENCE IN THE NEWS was written by Nancy Steinbach. Cynthia Kirk was our producer was Cynthia Kirk. I'm Barbara Klein.

VOICE TWO:

And I'm Bob Doughty. Internet users can read and listen to our programs at www.unsv.com. Join us again next week for more news about science in Special English on the Voice of America.

1 doughty   

adj.勇猛的，坚强的

参考例句：

• Most of successful men have the characteristics of contumacy and doughty.绝大多数成功人士都有共同的特质：脾气倔强，性格刚强。
• The doughty old man battled his illness with fierce determination.坚强的老人用巨大毅力与疾病作斗争。

2 split   

n.劈开，裂片，裂口；adj.分散的；v.分离，分开，劈开

参考例句：

• Who told you that Mary and I had split up?谁告诉你玛丽和我已经离婚了？
• The teacher split the class up into six groups.老师把班级分成6个小组。

3 mount   

n.山峰，乘用马，框，衬纸；vi.增长，骑上（马）；vt.提升，爬上，装备

参考例句：

• Their debts continued to mount up.他们的债务不断增加。
• She is the first woman who steps on the top of Mount Jolmo Lungma.她是第一个登上珠穆朗玛峰的女人。

4 rubble   

n.（一堆）碎石，瓦砾

参考例句：

• After the earthquake,it took months to clean up the rubble.地震后，花了数月才清理完瓦砾。
• After the war many cities were full of rubble.战后许多城市到处可见颓垣残壁。

5 pointed   

adj.尖的，直截了当的

参考例句：

• He gave me a very sharp pointed pencil.他给我一支削得非常尖的铅笔。
• She wished to show Mrs.John Dashwood by this pointed invitation to her brother.她想通过对达茨伍德夫人提出直截了当的邀请向她的哥哥表示出来。

6 fossil   

n.化石，食古不化的人，老顽固

参考例句：

• At this distance of time it is difficult to date the fossil.时间隔得这么久了，很难确定这化石的年代。
• The man is a fossil.那人是个老顽固。

7 remains   

n.剩余物，残留物；遗体，遗迹

参考例句：

• He ate the remains of food hungrily.他狼吞虎咽地吃剩余的食物。
• The remains of the meal were fed to the dog.残羹剩饭喂狗了。

8 fossils   

n.化石( fossil的名词复数 )；老顽固；食古不化的人；老古董（老人）

参考例句：

• fossils over two million years old 两百多万年的化石
• The geologist found many uncovered fossils in the valley. 在那山谷里，地质学家发现了许多裸露的化石。 来自《简明英汉词典》

9 noted   

adj.著名的，知名的

参考例句：

• The local hotel is noted for its good table.当地的那家酒店以餐食精美而著称。
• Jim is noted for arriving late for work.吉姆上班迟到出了名。

10 silicon   

n.硅(旧名矽)

参考例句：

• This company pioneered the use of silicon chip.这家公司开创了使用硅片的方法。
• A chip is a piece of silicon about the size of a postage stamp.芯片就是一枚邮票大小的硅片。

11 aluminum   

n.(aluminium)铝

参考例句：

• The aluminum sheets cannot be too much thicker than 0.04 inches.铝板厚度不能超过0.04英寸。
• During the launch phase,it would ride in a protective aluminum shell.在发射阶段，它盛在一只保护的铝壳里。

12 magnesium   

n.镁

参考例句：

• Magnesium is the nutrient element in plant growth.镁是植物生长的营养要素。
• The water contains high amounts of magnesium.这水含有大量的镁。

13 lighter   

n.打火机，点火器；驳船；v.用驳船运送；light的比较级

参考例句：

• The portrait was touched up so as to make it lighter.这张画经过润色，色调明朗了一些。
• The lighter works off the car battery.引燃器利用汽车蓄电池打火。

14 continental   

adj.大陆的，大陆性的，欧洲大陆的

参考例句：

• A continental climate is different from an insular one.大陆性气候不同于岛屿气候。
• The most ancient parts of the continental crust are 4000 million years old.大陆地壳最古老的部分有40亿年历史。

15 harry   

vt.掠夺，蹂躏，使苦恼

参考例句：

• Today,people feel more hurried and harried.今天，人们感到更加忙碌和苦恼。
• Obama harried business by Healthcare Reform plan.奥巴马用医改掠夺了商界。

16 splits   

劈叉； 划分( split的名词复数 )； 分歧； 裂缝； 劈叉

参考例句：

• The river splits into three smaller streams at this point. 这条河在此处分成三条小河。
• Usually only a trained dancer can do the splits. 通常只有训练有素的舞蹈演员才会劈叉。

17 geologic   

adj.地质的

参考例句：

• The Red Sea is a geologic continuation of the valley.红海就是一个峡谷在地质上的继续发展。
• Delineation of channels is the first step of geologic evaluation.勾划河道的轮廓是地质解译的第一步。