VOA慢速英語:地球表面一直在運動

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From VOA Learning English, this is Science in theNews.

I’m Anna Matteo.

And I’m Christopher Cruise.

Scientists who study the Earth tell us the continentsand ocean floors are always moving. This movementsometimes can be violent, causing death anddestruction. Today, we examine what causesearthquakes and volcanic activity.

The Earth seems to melt into the sea.

The first pictures of Earth taken from space showed asolid ball covered by brown and green landmasses andblue-green oceans. It appeared as if the Earth had always looked that way -- and always would.

Yet the surface of the Earth is not as solid or as permanent as had beenthought. Scientists found that the surface of our planet is always in motion.Continents move about the Earth like huge ships at sea, floating on pieces of the Earth’s outer skin, or crust. New crust is created as melted rock pushesup from inside the planet. Old crust is destroyed as it moves toward the hotrock and melts.

In the 20th century, scientists began to understand that the Earth is a great, living and moving structure. Someexperts say this understanding is one of the mostimportant revolutions in scientific thought.

The knowledge of the Earth’s continual motion is basedon the work of scientists who study the movement of the continents. This process is called “plate tectonics.”Earthquakes and volcanic activity are a result of thatprocess.

Alfred Wegener at work.

Scientists say the surface of the Earth is cracked like a huge eggshell. Theycall these pieces “tectonic plates.” As many as 20 such plates cover theEarth. They sometimes hit each other, and sometimes move away fromeach other. Because some continents are above two plates, the continentsmove when the plates do.

The movement of tectonic plates can cause earthquakes and volcanoes.Understanding this movement can help predict where earthquakes will takeplace. Research shows that about 90 percent of all earthquakes happen alonga few lines in several places around the Earth. These lines follow underwatermountains, where hot liquid rock flows up from deep inside the Earth.Sometimes, the melted rock comes out with a great burst of pressure. Thisforces apart pieces of the Earth’s surface in a violent earthquake.

Some earthquakes take place at the edges of continents. Pressure increasesas two plates move against each other. When this happens, one plate movespast the other, suddenly causing the Earth’s surface to split open.

One example of this pressure is found on the west coast of the United States.Part of California is on what is known as the Pacific plate. The other part of thestate is on what is known as the North American plate.

Earh's Crust

Scientists say the Pacific plate is moving toward the northwest, while theNorth American plate is moving toward the southeast. These two huge platescome together at what is called a fault line. This line between the plates inCalifornia is called the San Andreas Fault. It is along or near this fault line thatmost of California’s earthquakes take place, as the two tectonic plates movein different directions.

The city of Los Angeles is about 50 kilometers from the San Andreas Fault.Many smaller fault lines can be found throughout the Los Angeles area.

Scientists began making major discoveries aboutplate tectonics in the 20th century. One of thosescientists was Alfred Wegener of Germany. Onehundred years ago, he proposed that the continentshad moved and were still moving.

Mr. Wegener said the idea came to him when he sawthat the coasts of South America and Africa fit togetherlike two pieces of a puzzle. He suspected that the twocontinents might have once been one, and then splitapart.

Giant tsunami waves hit coast of Japan.

He believed the continents had once been part of a huge area of land that hecalled “Pangaea.” He said the huge continent had split more than 200 millionyears ago. And, he said the pieces were still floating apart.

Alfred Wegener investigated the idea that continents move. He noted that aline of mountains that appears from east to west in South Africa looks almostexactly the same as a line of mountains in Argentina -- on the other side of theAtlantic Ocean. He found fossil remains of the same plant in parts of Africa,South America, India, Australia and even Antarctica.

Mr. Wegener said the mountains and fossils were evidence that all the land onEarth was united at some time in the distant past.

Mr. Wegener also noted differences between the continents and the oceanfloor. He said the oceans were more than just low places that had filled withwater. Even if the water

was removed, he said, a person would still see differences between thecontinents and the ocean floor.

Kilauea volcano on Hawaii erupts.

Also, the continents and the ocean floor are not made of the same kind ofrock.

The continents are made of a granite-like rock. Graniteis made when hot, liquid rock cools and hardens underthe Earth’s surface. The ocean floor is basalt rock, amixture of silicon and magnesium.

The German scientist said the lighter continental rockfloated up through the heavier basalt rock of the oceanfloor.

Support for Mr. Wegener’s ideas did not come until the 1950s. Two American scientists found that the continents moved as new seafloor was created under the Atlantic Ocean. Harry Hess and Robert Dietz saida thin valley in the Atlantic was a place where the ocean floor splits. They saidhot 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. Itbecomes new ocean floor.

The Americans proposed that the floor of the Atlantic Ocean is moving awayfrom each side of the split and expanding. The movement is very slow -- a fewcentimeters a year. In time, they said, the moving ocean floor is blocked whenit comes up against the edge of a continent. Then it is forced down under thecontinent, deep into the Earth, where it is melted again. Harry Hess said thePacific Ocean was getting smaller.

He and Robert Dietz said this spreading does not make the Earth bigger. Asnew ocean floor is created, an equal amount is destroyed.

The two scientists said Alfred Wegener was correct. The continents do moveas new material from the center of the Earth rises, hardens and pushes olderpieces of the Earth away from each other.

They called their theory “sea floor spreading.” The theory explains that as thesea floor spreads, the tectonic plates are pushed and pulled indifferent directions.

New research also supports Mr. Wegener’s ideas. Scientists in Britainrecently reported that large amounts of water may be trapped under the oceanplates near northern Japan. Their findings were published in the journalGeology.

?A strong earthquake hit Japan in March of 2011. Thequake and resulting tsunami waves killed almost 16,000people. A team of researchers from the University ofLiverpool studied the tectonic plates in the area wherethe earthquake took place. Their report says that whentwo plates meet, one may bend and end up underneaththe other. During this process, it says, ocean watergets trapped below the plates and goes down past theEarth’s crust to its mantle.

The researchers said a large hole lies near the fault lines in Japan. They saidthe hole could be as much as 150 kilometers deep. Water gets carried downthe hole. The report says there could be 3½ times more water in the Earth’smantle than there is in all the oceans.

The idea of plate tectonics explains both volcanoes and earthquakes. Many of the world’s volcanoes are found at the edges of plates. The large number ofvolcanoes around the Pacific plate has earned this area the name “Ring ofFire.”

Last week a powerful earthquake hits the coast of Chile.

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

The Hawaiian Islands were created in the PacificOcean as the plate moved slowly over a hot spot. Thisprocess is continuing, as the plate continues to move.

Similar to earthquakes, volcanos can cause destructionand displace populations. Because of this, scientists are also hoping to learnmore about volcanic activity.

Recently, the United States National Science Foundation provided financialsupport for research on volcanos. Geoscientists from the University ofOregon and the University of California Davis studied Mt. Hood in Oregon.

The scientists say that volcanos become active when the rock or magmainside becomes as hot as 750 degrees Celsius. The volcano becomes activewhen the hot magma from the Earth’s crust rises and meets cooler solidmagma. This can happen in as little as a few months’ time.

The American scientists believe the magma at Mt. Hood had been stored in the volcano for at least 20,000 years, or even as long as 100,000 years. Theysay that modern technology should be able to sense when the magma isgetting warmer, and could possibly explode. This could prevent disasterswhen volcanos erupt around the world.

This Science in the News was written by Kim Varzi and Christopher Cruise.

I’m Anna Matteo.

And I’m Christopher Cruise.

Join us again next week for more news about science on the Voice ofAmerica.