Internal
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 the case for all over the globe, but they found out something
else. They tried to predict the time they expected the waves to reach a
destination on exactly on the other side of the globe, so they scaled up
the time interval between the two waves arriving. First the P-wave arrived,
on time as they expected but the S-wave didn?¦t, this was
because the P-waves can travel through any physical state. However S-waves
can only travel through solids which is why they can pass through the mantle,
so a change of physical state must happen in the middle of the mantle somewhere.
The area where the S-waves enter and do not come back out is called the
shadow zone. The P-waves also have a shadow zone. This would be from about
105?X to 142?X marked from the Focus of the earthquake. This
is because when the P-waves enter the core they are bent downwards, they
are then bent down again when they leave the core-mantle boundary. So no
waves can emerge at the surface before 142?X.
From these results scientists are convinced
that inside the mantle there is a molten core that must be blocking out
the S-waves.
So scientists have very good evidence to
prove what they believe to be inside of the mantle. They even have their
ways to prove what they believe the core to be made up of, they think there
are two layers, an inner core surrounded by the outer core. The outer core
is believed to be made up of liquid iron and the actual centre of the core
is made of solid iron.
They have numerous reasons to back up this
theory.
Scientists can work out how big the mass
of the earth is, not by trying to weigh it because that is merely impossible.
Instead they used the gravity on the earth to help them. We know the velocity
at which objects fall to the earth, so from this scientists were able to
work out the mass of the earth. So they found the total mass of the earth,
and compared that to a mass made up of just the crust and the mantle. They
could get this mass reading because they know the density of the crust
and the mantle. However even after working this mass out the total amount
was well short, compared with the mass they found from using the velocity
at which objects fall to the earth. So they were convinced by these results
that the mantle was not the only thing down there, they put the lack of
mass to believing that something heavier and denser was in the middle.
When the earth formed it originally condensed,
by gravitational attraction of cosmic dust and gas. The continuing contraction
of these materials caused them to heat, as did some of the radioactivity
of some of the heavier elements. As this progressed the earth became very
hot and it began to melt. This caused the different layers to form in the
earth, because all the lighter materials moved up to the surface to produce
the crust. So the heavier materials like the metals iron and nickel sank
to form the core, the materials in the mantle were made form the silicates
that didn?¦t sink or float.
When meteorites reach the surface of our
planet before they are entirely consumed, they provide us with valuable
information. Meteorites are believed to be fragments from other planets,
formed some 4.6 billion years ago. Around the same time as the earth was
been formed. These meteorites are mostly made up of iron, this is what
excites scientists. As they believe that this is proof that the middle
core of our earth is made from solid iron. Thinking that meteorites are
part of planets that have broken up and sent fragments flying out into
space.
So they know that the inner core is solid
iron but why when the outer core is a liquid iron. Well as the depth increases
in the earth then so does the pressure. So scientists put the solid middle
down the fact that the pressure becomes too much for the liquid iron, so
the pressure solidifies the core.
Another point of evidence is the earth?¦s
magnetic field, again suggesting an iron core, because iron is a metal
that can be magnetised. The magnetic field is thought to be in the liquid
outer core, because of the readily movement, and iron been a good conductor.
This is what may be required for a dynamo with the capacity to generate
enough current to produce the earth?¦s magnetic field. The
liquid iron is thought be stirred in a motion by heat from the core. This
action is thought to produce an electric current and therefore the magnetic
field. This is another explanation as to the outer core been liquid, because
if it wasn?¦t then where would the magnetic field come from.
As permanent magnetism cannot be kept with temperatures exceeding 500?XC,
however the outer core may well produce electric currents because of the
free movement. The inner core would not be able to do this, as it is a
solid.
