Name
A CAT Scan of Earth
By Patti Hutchison
If a doctor needs to see
what's wrong inside you, he
doesn't have to cut a big
hole in you and look in. He
uses other methods such as
CAT scans and X-rays. The
earth's interior is too hot,
and there is too much
pressure for humans to
explore it directly. So, like
doctors, scientists rely on
other methods to find out
what's inside the earth.
There are many ways of finding out what the earth is like below the
surface. One method is drilling. Large drills bore holes in the crust of
the earth for oil and gas wells. This method only touches the surface,
however. Wells have only been able to be drilled about ten kilometers
below the surface. However, they do give us information about what
lies at those depths.
Rarely, volcanic and tectonic actions give us clues to the interior of
the earth. Erupting lava sometimes brings xenoliths. These are parts of
the earth's mantle. Earthquakes sometimes expose huge pieces of
mantle rock. Scientists then study these materials to know what the
mantle is made of.
Scientists know the earth has a magnetic field. It is what makes
compasses work. It also helps animals on their paths of migration.
Scientists also know that in order to have this magnetic field, there
must be a high concentration of metal inside the earth. They also
know that this metal must be able to be in a liquid state at high
pressures. This information gives clues as to what the core of the earth
is made of.
Geophysicists are scientists who study the interior of the earth. They
conduct laboratory experiments to help solve these mysteries. They
expose surface materials to high temperature and pressure that is
similar to the earth's interior. Then they study the changes in these
materials. They compare these to changes that take place inside the
earth.
Mathematical calculations also help us to know what the earth is like
inside. We can't measure the temperature of the core directly. But
scientists know that temperature and pressure increase with depth.
They also know the rate at which they increase. By knowing the
thickness of each layer, they can calculate the temperature of the core.
All these methods give us glimpses of what the interior of the earth is
like. However, scientists have learned the most by studying
earthquake waves. Since many earthquakes occur every day, it is easy
to study this data.
When an earthquake occurs, seismic waves travel throughout the
earth. Sensitive seismographs can detect these waves very far from the
epicenter. These waves move faster through denser rock. Scientists
measure the time it takes for the waves to reach locations far from the
epicenter. They can then determine the density of the material they
have moved through.
There are two types of waves, P-waves and S-waves. Scientists study
the characteristics of these waves and how they travel through the
earth. P-waves travel most rapidly through solids. They can also move
through liquids. However, S-waves cannot travel through liquids.
Scientists studied the speed of these waves as they moved through the
earth. They found that at a depth of about 60 kilometers, the waves
increased their speed. The boundary between the crust and the mantle
was discovered. The waves moved faster through this denser layer.
At a depth of 2900 kilometers below the earth's surface, P-waves
slow down quickly. S-waves disappear. The changes in these waves
tell scientists that a liquid layer begins at this depth. This is the outer
core. As the P-waves move to a depth of about 5100 kilometers, the
speed of the P-waves increases. They are no longer traveling through
liquid. They are now passing through the solid layer called the inner
core.
Scientists use many methods to study the interior of the earth. Most
of the information they have gathered has come from studying
earthquakes. The seismic waves travel to the depths of the earth and
then return to its surface. As they move, the speed and direction of the
waves change. They allow us to "see" inside the earth, much like a
Name
CAT scan allows a doctor to see inside you.
A CAT Scan of Earth
Questions
1. Name three methods scientist use to study the interior of the
earth.
2. Scientists figure out the temperature of the core by using:
A. CAT scans
B. laboratory experiments
C. mathematical calculations
3. What method has given scientists the most information about
the interior of the earth?
4. Which seismic waves can move through liquid?
A. P-waves
B. S-waves
C. both P-waves and S-waves
5. What happens to S-waves at a depth of 2900 kilometers below
the earth's surface?
A. They slow down.
B. They disappear.
C. They speed up.
6. In what state of matter is the outer core?
A. gas
B. liquid
C. solid
Explain how scientists know the inner core is solid metal.
Name
Explain how scientists use information about seismic waves to learn
about the interior of the earth.