This is SCIENCE IN THE NEWS in VOA Special
English. I'm Bob Doughty.
I'm Steve Ember. 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.
<!-- IMAGE -->
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
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
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.
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.
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.
the plates move, the continents move with them. Sometimes the continents are above two plates. The continents split as the
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
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.
<!-- IMAGE -->
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 line.
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.
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
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.
<!-- IMAGE -->
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.
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.
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.
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.
Hess and Robert Dietz said this spreading does not make the Earth bigger. As new ocean floor is created, an equal
amount is destroyed.
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.
called their theory "sea floor spreading." The theory explains that as the sea
floor spreads, the tectonic plates are pushed and pulled in
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 geologic activity is
intense. The large number of volcanoes around the Pacific plate has earned
the name "Ring of Fire."
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.
and earthquakes are among the most frightening events that nature can
produce. More than one thousand people
were killed when a powerful earthquake struck western Indonesia at the end of
September. Thousands 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 Bob Doughty.
And I'm Steve Ember. 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 firstname.lastname@example.org. Join us again next week for more news about science in Special English
on the Voice of America.