Internal Structure of the Earth

Simon Abbott

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