Seismic waves are used to study the composition of the interior of the Earth.
The Earth is composed of three major zones:
crust
mantle
core
The core is the innermost sphere of the Earth composed of an iron and nickel alloy. THe core is 2,161 miles in diameter and is under extreme pressure.
The mantle is a solid rocky shell extending 1800 miles. The uppermost mantle is composed of peridotite and increases in density with depth.
The crust is a thin outer skin divided into continental crust and oceanic crust. The continental crust is approximately 25 miles thick and as much as 40 miles in some areas while the dense oceanic crust is five miles thick. The composition of the older continental crust is granitic in upper layers while the lower layers are of younger basaltic composition like oceanic crust.
The Earth can also be divided into layers based upon the physical properties and mechanical strength.
Lithosphere
outermost layer consisting of the crust and uppermost mantle
differing chemical composition
100 to 250 km thick
detached from the layer below allowing the lithosphere to move
Asthenosphere
soft layer
the top portion has a temperature/pressure regime resulting in a small amount of melting
rocks are easily deformed, but their strength depends upon their composition and the temperature and pressure of the environment
Mesosphere (Lower Mantle)
increased pressure gives the rock greater strength, but the temperature still allows gradual flow
Outer Core
liquid layer
convective flow of iron causes the Earth's magnetic field
Inner Core
immense pressure outweighs the temperature so the material is solid
The layers were discovered by studying seismographs. Seismologist, Andrija Mohorovicic found that the velocity of seismic waves increased below 50 km of depth. The boundary separates the crust from the mantle and is known as the Moho boundary.
Ben Gutenberg found that P waves diminish and die at 105° around the globe from an earthquake and reappear 140° and two minutes later than expected. The zone where the P waves disappear is called the shadow zone. The P waves hit the core and bend because of the rock composition giving evidence to a dense core. Further it was observed that S waves did not travel through the Earth indicating the core had a liquid portion.
Inge Lehmann discovered the inner core within the Earth's core in 1936
