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Continents and ocean floors are always moving
http://www.chinadaily.com.cn/html/media/200810/29/000b5f500db65f4993.mp3
This is SCIENCE IN THE NEWS, in VOA Special English. I'm Steve Ember.
VOICE TWO:
And I'm Barbara Klein. 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. Today, we examine the process that causes earthquakes.
(MUSIC)
VOICE ONE:
The first pictures of Earth taken from space showed a solid ball covered by brown and green landmasses 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 1960s 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.
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 split as the plates move.
(MUSIC)
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.
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 1994 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 pointed 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 fossil remains 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 fossils were evidence that all the land on Earth was united at some time in the distant past.
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[ 本帖最后由 我爱英语 于 2008-11-1 11:01 编辑 ]
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