Into: How do we actually know how the inside of the Earth is structured? Three indirect lines of evidence:

But these are limited. Xenoliths come from modest depth in mantle, and meteorites are, after all, not fragments of Earth. Seismic waves, however, allow direct exploration of the Earth's interior.

Seismic waves as emmissaries to Earth's interior: Early study of earthquake waves revealed different "versions" of P and S waves arriving at a given location at different times. How? Waves undergo reflection and refraction when they pass interface between media with different properties. The abundance of "extra" seismic waves indicated that this was happening in the Earth. Thus, Earth contained regions with different physical properties:

Depth/velocity relationships: thousands of observations have allowed construction of reliable charts showing relationship between depth and speed of seismic waves. Generally speaking, velocity increases with depth in mantle, drops off slightly in outer core (for P waves!) and achieves a speed similar to that of outer mantle in inner core. Crust: Remember, volumetrically igneous rocks predominate:

Laboratory tests tell us the speeds at which seismic waves move through different materials. Most continents seem to be made up of Si and Al rich rocks to a great depth. In them, P waves generally move at 6-7 km/s

Mantle: Two issues:

Core Mantle Boundary (CMB): Enigmatic zone in bottom 20 km of mantle: S wave deceleration indicates partial melting. Area of intensive study. Apparent source of at least some plumes of hot rock causing hot-spot volcanism.

The Core:

Earth as Magnet: Apparantly, currents in liquid outer core produce Earth's magnetic field. Because currents can vary, the orientation and intensity of the magnetic field varies with time.


To conclude, the application of algorithms developed for CT X-ray tomography have been applied to seismic waves to create illuminating cross-sectional denisity maps of the deep earth, showing such things as subducting slabs.

Key concepts and vocabulary:

  • Xenolith
  • Gravitational anomaly
  • Meteorites
  • S wave shadow
  • P wave shadow
  • P wave acceleration at 5100 km.
  • Reflected waves
  • Mohorovicic discontinuity (the Moho)
  • Andrija Mohorovicic
  • Asthenosphere
  • 400 km and 670 km olivine transitions
  • Core - mantle boundary (CMB)
  • Core composition - iron, nickel, trace elements