Internal structure of earth and basis of study

Internal structure

The Earth consists of three concentric shells: crust, mantle and core.

  Crust   : 5-70 km    (granitic/basaltic)
  ---------- Moho ----------
  Mantle  : to 2900 km (peridotite)
  ------- Gutenberg -------
  Outer core: 2900-5150 km (liquid Fe-Ni)
  Inner core: 5150-6371 km (solid Fe-Ni)

• Crust: thin outer rock layer; continental crust is granitic and thick (30-70 km), oceanic crust is basaltic and thin (5-10 km).
• Mantle: thick layer extending to about 2900 km depth; ultramafic rocks; the upper part (asthenosphere) is plastic and allows plate movement.
• Core: iron-nickel; outer core liquid, inner core solid; highest density and temperature.

Bases of study

The interior cannot be seen directly, so it is studied by:

1. Seismic waves: change of speed and direction at boundaries; S-waves stop at the outer core (liquid), P-waves are refracted and make shadow zones.
2. Density: the mean density of the Earth (5.5 g/cm³) is much higher than that of surface rocks (2.7 g/cm³), so the interior must be made of heavier material.
3. Meteorites: believed to represent the material of the Earth's interior.
4. Volcanic eruptions and xenoliths: samples from the mantle.
5. Deep drilling and mines: show temperature rise with depth (geothermal gradient).
6. Magnetism and gravity: the magnetic field indicates a liquid iron core.