Geologists have known for about 100 years that the Earth is composed of four layers; the Crust, Mantle, Outer Core, and the Inner Core . Scientists still argue about the makeup of these layers and exactly how each layer interact.
A geologist, by the name of Andrija , 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!!! (see diagram A page 3) He decided 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. Waves of all kinds move faster and straighter through denser, more solid objects.Today scientists believe that the crust and the rigid, outer zone of the mantle makes up a layer that is called the Lithosphere . The lithosphere is broken into 12 large pieces that are called plates. The zone directly under the lithosphere is made of a flowing, denser layer called the Asthenosphere. Scientists believe that the plates ride on the asthenosphere, which flows due to convection currents.