Content Center (Lesson #1)
The Earth's layers

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 Mohorovicic, 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. See diagram on page 2.



Beno Gutenberg

, a German geologist, believed that the Outer Core must be made of a liquid because the slower S (secondary waves) could not pass through this layer and in fact "bounced off" and were deflected many degrees off course. Study diagram A on page 3.



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. Earthquake waves that reach this layer move at the greatest speeds because waves move through solids faster than through gases and liquids.
To honor Mohorovicic, scientists have named the boundary between the crust and the mantle the Mohorovicic discontinuity or the "MOHO" for short. Beno Gutenberg discovered the boundary or discontinuity between the mantle and the outer core. This boundary was named after him, the Gutenberg discontinuity. See diagram B.



Discussion Questions


  1. How did Andrija Mohorovicic discover that the Earth's crust was made of less dense rock than the mantle.
  2. Write in your own words the definition of a discontinuity.
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