Interview
The Earth, The interior and the earth's changing
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An explanation about the earth's interior and the earth changing from time to time.
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Jakarta, Perguruan Cikini, Geography ClassThe Earth part 2, interior and earth’s changing
How scientist learn about the interior
Volcanic eruptions bring up hot gases and molten rock. This shows that the interior of the earth is hot.
Earthquake waves tell much more about the inside of the earth. These waves give us something like an
X-ray picture of the interior.
When an earthquake occurs, different type of vibrations spread out in all directions through the rocks.
These waves, which are called seismic waves, can be detected with a very delicate instrument called a
seismometer. The first instruments were called seismographs because they themselves wrote the record
on a chart.
Seismic waves travel at different speeds in different kinds of material. And their direction changes as
they go from one kind of rock to another. Scientist can find the boundary of the crust by the way seismic
waves suddenly change direction at a certain depth. This is the Moho.
At a depth of 1,800 miles there is another sudden change in the way seismic waves travel. One kind of
wave, the P wave, slows down and changes its direction. Another kind, the S wave, stops entirely. There
must be a great change in the material of the interior at that depth. It is the boundary between the
mantle and the core.
S waves cannot travel through a liquid. The fact that they stop shows that the outer core must be liquid.
The fact that they stop shows that the outer core must be liquid. But the P waves continue on. At a
depth of about 3,160 miles, they seem once more to change their speed and direction. This may show
that they have reached a solid inner core.
The shock waves from an earthquake reach different seismic stations at different times. The time
depends on how far the station is from the earthquake and the material through which the waves have
travelled. This is the raw information which seismologists use to find out about the interior of the earth.
When several stations record and earthquake, scientists compare the patterns of the waves and also
their travel times. From these they can calculate by what path the waves have come through the interior
of the earth.
There is a third type of seismic wave, called a surface wave. The speed of the surface waves gives
information about the kind of rock near the surface of the earth.
The floating crust
Seismic surface wave show that the rock of the mantle is denser than the rock of the crust. Geologists
think that when the earth was first formed, lighter rock material rose to the top and “floated” on the
heavier mantle. This lighter material became the crust.
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