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VOICE ONE:

This is Science in the News, in VOA Special English. I'm SarahLong.

VOICE TWO:

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

VOICE ONE:

Today we examine the process that causes earthquakes.

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The first pictures of Earth taken from space showed a solid ballcovered by brown and green land masses and blue-green oceans. Itappeared as if the Earth had always looked that way -- and alwayswould. Scientists now know, however, that the surface of the Earthis not as permanent as had been thought.

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

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Only since the nineteen-sixties have scientists begun tounderstand that the Earth is a great, living structure. Some expertssay this new understanding is one of the most important revolutionsin scientific thought. The revolution is based on the work ofscientists who study the movement of the continents - a processcalled plate tectonics.

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

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

When the plates move, the continents move with them. Sometimesthe continents are above two plates. The continents split as theplates move.

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Tectonic plates can cause earthquakes as they move. Moderninstruments show that about ninety percent of all earthquakes takeplace along a few lines in several places around the Earth. Theselines follow underwater mountains where hot liquid rock flows upfrom deep inside the planet.

Sometimes, the melted rock comes out with a great burst ofpressure. This forces apart pieces of the Earth's surface in aviolent earthquake.

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

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One example of this is found in California, on the West Coast ofthe United States. One part of California is on what is known as thePacific plate. The other part of the state is on what is known asthe 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 SanAndreas Fault. It is along or near this line that most ofCalifornia's earthquakes take place, as the two tectonic plates movein different directions.

The city of Los Angeles in Southern California is about fiftykilometers from the San Andreas Fault. Many smaller fault lines canbe found throughout the area around Los Angeles. A major earthquakein nineteen-ninety-four was centered along one of these smallerfault lines.

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VOICE TWO:

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

He said the idea came to him when he observed that the coasts ofSouth America and Africa could fit together like two pieces of apuzzle.He proposed that the two continents might have been one, thensplit apart.

Later, Alfred Wegener said the continents had once been part of ahuge area of land he called Pangaea. He said the huge continent hadsplit more than two-hundred-million years ago. He said the pieceswere still floating apart.

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Wegener investigated the idea that continents move. He pointedout a line of mountains that appears from east to west in SouthAfrica. Then he pointed out another line of mountains that looksalmost exactly the same in Argentina, on the other side of theAtlantic Ocean. He found fossil remains of the same kind of an earlyplant in areas of Africa, South America, India, Australia and evenAntarctica.

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

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Wegener also noted differences between the continents and theocean floor. He said the oceans were more than just low places thathad filled with water. Even if the water was removed, he said, aperson would still see differences between the continents and theocean floor.

Also, the continents and the ocean floor are not made of the samekind of rock. The continents are made of a granite-like rock, amixture of silicon and aluminum. The ocean floor is basalt rock, amixture of silicon and magnesium. Mister Wegener said the lightercontinental rock floated up through the heavier basalt rock of theocean floor.

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Support for Alfred Wegener's ideas did not come until the earlynineteen-fifties. American scientists Harry Hess and Robert Dietzsaid the continents moved as new sea floor was created under theAtlantic Ocean.

They said a thin valley in the Atlantic Ocean was a place wherethe ocean floor splits. They said hot melted material flows up fromdeep inside the Earth through the split. As the hot material reachesthe ocean floor, it spreads out, cools and hardens. It becomes newocean floor.

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

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

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

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The two scientists also said Alfred Wegener was correct. Thecontinents 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 theoryexplains that as the sea floor spreads, the tectonic plates arepushed and pulled in different directions.

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VOICE ONE:

The idea of plate tectonics explains volcanoes as well asearthquakes. Many of the world's volcanoes are found at the edges ofplates, where geologic activity is intense. The large number ofvolcanoes around the Pacific plate has earned the name "Ring ofFire."

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

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

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Volcanoes and earthquakes are among the most frightening eventsthat nature can produce. The earthquake in the Iranian city of Bamat the end of two-thousand-three, for example, killed more thanforty-thousand people. At times like these, we remember that theground is not as solid and unchanging as people might like to think.

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VOICE ONE:

SCIENCE IN THE NEWS was written by Marilyn Christiano and NancySteinbach. Our producer was Cynthia Kirk. This is Sarah Long.

VOICE TWO:

And this is Bob Doughty. Join us again next week for more newsabout science in Special English on the Voice of America.