The Earth's Interior
Size of Earth:
Polar Radius = 6356.75km
Equatorial Radius = 6378.14km

Rotation about its spin axis causes the earth to be a very slightly flattened sphere

There are two basic ways of dividing the Earth into layers: Chemical Compostion and Physical Properties

Chemical Composition
Compositional Sorting - Overall Density Sorting of the Earth, Iron and Nickel sink to the center - lighter elements float upwards. Called the "Iron Catastrophe"

Chemical differentiation - chemical affinities of elements for each other. Uranuium has an affinity for oxygen, not iron, so it formed compounds that were relatively light and rose up rather than sinking to the core.

Compositional Layered structure:

• Crust
  • Continental (about 35 km thick; 60 km in mountain ranges)
    Granitic composition (2.7 g/cm3)
    Non-ferromagnesian minerals dominate

  • Oceanic (about 5 km thick)
    Basaltic composition (3.0 g/cm3)
    Ferromagnesian minerals dominate

• Mantle (2885 km thick)

  The mantle has the biggest volume of all the layers. It is primarily made up of Iron (Fe) and Aluminium (Al) rich silicate minerals (silicates are minerals made of Silicon (Si) and Oxygen (O) ).

  Olivine (3.38 g/cc), Spinel (3.66 g/cc), Perovskite (4.27 g/cc)
  Solid that flows; plastic behavior

• Core - chemically homogeneous (all the same)
  Outer core (2270 km thick)
  Molten Fe with some Ni

  Inner core (1216 km radius)
  Solid Fe with some Ni

Physical Properties - Mechanical Behavior

The layers are defined by their physical properties
With increasing depth, Earth's interior is characterized by gradual increases in temperature, pressure, and density

Depending on the temperature and depth, a particular Earth material may behave like a brittle solid, deform in a plasticÐlike manner, or melt and become liquid

Mechanical layers of Earth's interior are based on physical properties and hence mechanical strength

Mechanically Layered Structure

• Lithosphere (sphere of rock)
  Earth's outermost layer
  Consists of the crust and uppermost mantle
  Relatively cool, rigid shell (BRITTLE)
  Averages about 100 kilometers in thickness, but may be 250 kilometers or more thick beneath the older portions of the continents

• Asthenosphere (weak sphere)
  Beneath the lithosphere, in the upper mantle to a depth of about 600 kilometers
  Small amount of melting in the upper portion mechanically detaches the lithosphere from the layer below allowing the lithosphere to move independently of the asthenosphere

• Mesosphere or lower mantle
  Rigid layer between the depths of 660 kilometers and 2900 kilometers Rocks are very hot and capable of very gradual flow

• Outer core
  Composed mostly of an iron-nickel alloy
  Liquid layer
  2270 kilometers (1410 miles) thick
  Convective flow within generates EarthÕs magnetic field

• Inner core
  Sphere with a radius of 3486 kilometers (2161 miles)
  Material is stronger than the outer core
  Behaves like a solid

So - How do we know this ?

1. Drilling Wells drilled into Earth are mostly in the upper 7 km of the crust. Deepest well is only a little under 8mi deep
2. Volcanic activity Xenoliths = foreign rock (pieces of the mantle in lava). Only useful to depth of about 200 km
3. High pressure laboratory experiments
4. Samples of the solar system (meteorites)
5. Study of seismic waves generated by earthquakes and nuclear explosions. Seismic waves are one of the main keys used to understand the EarthÕs Interior

Important Notes

• Velocity depends on the density and elasticity of the transmitting material. The denser the material, the faster the waves travel. Crystalline rock transmits waves faster than mud.
• Pressure increases within the Earth - therefore density increases. The deeper inside the Earth the faster waves travel.
• P-waves travel in all mediums
• S-waves travel only in solids
• P-waves travel faster than S-waves

Seismic Energy Wave energy gets reflected and refracted at seismic boundaries.