Geologists have known for about 100 years that the Earth is composed of four layers; the Crust, the Mantle, the Outer Core, and the Inner Core . Scientists still argue about the makeup of these layers and exactly how each layer interact with the other layers. We are not even sure how the layers were formed but we have some theories.
Because we can not go to the center of the earth we have to find our answers otherwise.
<span>This is what a geologist by the name of Andrija Mohorovicic did. He discovered in 1909 that earthquake waves near the surface moved slower than earthquake waves that passed through the interior of the Earth. He also noticed that the P (primary, first and strongest) waves that passed through the interior of the Earth did not do so in a straight line. These waves were bent or deflected by something! </span> What the scientist knew was that waves of all kinds move faster and straighter through denser, more solid objects. <span>So Mohorovicic came to the conclusion that the outside layer or Crust was made of less dense material (Rock) and the next layer, the Mantle was much denser. This would explain why the earthquake waves moved slower through the crust. </span> <span>So by looking at the seismic waves from earthquakes the scientist learned about the crust and the mantle but they also learned about the outer and inner core. </span> To do this you have to look at a different kind of waves, the S (secondary waves) waves that also get released by an earthquake. These S waves are slower. <span>Beno Gutenberg , a German geologist, believed that the Outer Core must be made of a liquid because the slower S waves could not pass through this layer and in fact "bounced off" and were deflected many degrees off course. </span> <span>The fourth layer, the Inner Core, is composed of very, very hot metals (iron and nickel) with pressures so great that the metals do not flow as a liquid, but are forced to vibrate in place like a solid. </span> <span>Earthquake waves that reach this layer move at the greatest speeds because waves move through solids faster than through gases and liquids. </span> This is how we know that there have to be different layers. Otherwise the behavior of the different seismic waves would not make sense.
