Internal Structure of the Earth


Structure of the Earth

What is the evidence for our knowledge
of the internal structure of the Earth?

As we enter the twenty first century we
are beginning to learn more and more about the composition of the Earth.

Early predictions have thrown up some rather strange and peculiar thoughts
as to what is making up our Earth, but now day¡¦s scientists
can be confident that the Earth is made up of what they think. As from
experiments and other sources of information a picture to what is really
down there is becoming much clearer.

So how do these scientists know that the

Earth¡¦s sections are made up of different compositions, and
how do we know that the physical state of each layer is what it is?

The outmost layer of the Earth is the crust,
this is what we stand on and covers the earth entirely. It is made up of
many different rocks and minerals, we know that the composition of the

Earth¡¦s crust is generally the same due to the mines and
boreholes that humans have made down into it. Mines that have been dug
go down and still bring up valuable minerals that can be found just as
close to the Earth¡¦s surface. The deepest goes down around

3km into the earth, and the temperature is 70¢XC, the only way for
miners to work is because of the air conditioning, and still the type of
rock looks the same all around. Also boreholes that have been drilled as
far as half way into the Earth¡¦s crust bring up rocks that
look very similar to the ones on the surface. So scientists can safely
say that the Earth has a crust which is very similar in composition all
the way down until the mantle is reached.

When earthquakes happen they produce two
types of waves P-waves and S-waves. Primary waves (p-waves) are the fastest
waves, they travel away from a seismic event. Primary waves are longitudinal,
they can travel through solids, liquids and gases. The secondary waves
(s-waves) travel slower than the primary waves, and are traverse waves.

This type of wave can only travel through solids. Measuring these waves
is called seismology.

Scientists have known for a long time that
the lava, which comes out from volcanoes when they erupt, was from the
mantle. The asthenosphere is the probable source of much basaltic magma,
this is because the velocity in S-waves is slowed down and partially absorbed
in the asthenosphere. This gives the characteristics that the waves are
passing through a solid, which the mantle is, but that contains a small
amount of liquid. Also when the volcano erupt occasionally they shoot out
solid nodules that have come from the solid rock in the mantle, the so-called
plumbing of the mantle. These rocks have been broken of and carried out
with the flow of the lava, this type of rock is called peridotite and is
what mostly makes up the mantle.

It is a fairly recent discovery to prove
that the mantle of the earth is not the only part of the interior. Seismology
is a new discovery this century that enables observations of natural ground
vibration signals, basically the study of earthquakes. It can also be the
study of artificially generated seismic signals.

Scientists then started to record these
signals from earthquakes using seismographs, which are set in stations
around the world to record the signals. In all some 125 stations exist
around the world. They noticed that the P and S-waves did not travel in
a straight line through the Earth, they came to the presumption that the

Earth¡¦s mantle was made up of many different materials. This
caused the P and S-waves to travel at different speeds, because of the
way the materials conduct the waves at different velocities. The waves
also bent as they went from layer to layer, this caused their path through
the interior of the earth to be curved.

This was then put into practice, so after
an earthquake happened in a country the P and S-waves were sent out. Further
around the globe the waves were expected to be received after a couple
of minutes. Which they would, first the P-waves came followed by the S-waves,
with an interval time in the middle. This time could then be scaled up
to give the results of what they though would be the times for the P and
then the S-waves to arrive at other destinations. This theory was correct,
further away from the point of the epicentre, first the P-waves arrived
followed by the S-waves a few minutes later. So then they thought that
this would be