SCIENCE IN THE NEWS, in VOA Special English.
I'm Steve Ember.
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.
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
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.
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
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.
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
When the plates move, the continents
move with them. Sometimes the
continents are above two plates. The
continents split as the plates move.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
Wegener said the mountains and fossils were evidence that all the land on Earth
was united at some time in the distant past.
Wegener also noted 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.
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 silicon and aluminum.
The ocean floor is basalt rock, a mixture of silicon and magnesium. Mister Wegener said the lighter continental
rock floated up through the heavier basalt rock of the ocean floor.
Support for Alfred Wegener's ideas did
not come until the early nineteen-fifties.
American scientists Harry 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 splits. 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.
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.
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
called their theory "sea floor spreading." The theory explains that as the sea
floor spreads, the tectonic plates are pushed and pulled in
of plate tectonics explains volcanoes as well as earthquakes. Many of the world's volcanoes are found at
the edges of plates, where geologic activity is intense. The large number
of volcanoes around the Pacific plate has earned the name "Ring of
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.
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.
Volcanoes and earthquakes are among the
most frightening events that nature can produce. The major earthquake in China's Sichuan Province in May, two
thousand eight, killed almost seventy thousand people. Many more were injured or left without homes
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.
This SCIENCE IN THE NEWS program was
written by Nancy Steinbach. Our
producer was Brianna Blake. I'm Steve
I'm Barbara Klein. We would like to
hear from you. Write to us at Special
English, Voice of America, Washington, D-C, two-zero-two-three-seven, U-S-A. Or send your e-mails to
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