How do we see into the earth? Geophysics as remote sensing. Nonunique solutions. What determines density?

Simple argument of comparative densities (gm/cc).

• crustal material: sediments 2.1-2.8, granites 2.6, basalts 2.95.
• mantle material: peridotite 3.25, dunite 3.25.
• bulk density of earth 5.5.
• what does this tell you?

Seismic waves:

• surface vs. body waves.
• P vs. S waves.

Sesimic imaging:

• seismic velocities and properties of various rock/sediment types.
• reflection and refraction at velocity discontinuities (see adjacent diagram).
• globally consistent velocity boundaries: Moho (oceanic vs. continental crust), lithosphere asthenosphere contact, 670 km discontinuity, core-mantle boundary.
• shadow zones and the molten core.
• seismic anisotropy and tomography.

Other types of data to use in constructing a model for the interior of the earth?

• far traveled xenoliths.
• meteorites.
• behavior of magnetic field.
• theoretical studies of phase changes in P-T space.

Models for the large scale structure of the earth:

• compositional boundaries and layers: crust - mantle - core
  • Moho and crust/mantle boundary at 30-70 km for continents and 5-15 km for oceanic crust.
  • mantle vs. core contact at 2900 km, much more interesting and complex than thought before.
• mantle phase change boundaries.
  • lithosphere-asthenosphere at roughly 70-150 km depth - below conditions create a small percent partial melt in mantle material that weakens it considerably. Boundary known as LVZ (for low velocity zone).
  • circa 400 km there is an olivine to spinel mineral transition.
  • 670 km there is a spinel to perovskite mineral transition boundary between upper and lower mantle.
• liquid outer core and solid inner core at 5100 km depth. Core is composed primarily of iron with some impurities. Rapid convection in outer core creates a magnetic dynamo affect that produces our magnetic field. With time inner core should be getting larger.

Mantle convection:

• lines of evidence that interior convection goes on inside the earth:
  • processes of seafloor spreading and subduction where crust forms from and then is recycled into the mantle.
  • hot spots - areas of concentrated volcanism located above mantle plumes.
  • physics that suggests under mantle conditions the rocks can convect.
• overall driving force is gravity. Convection is in response to density differences and bouyancy forces due to temperature and phase changes.
• does interior convection drag the plate above about, or do the plates do their own thing and thus influence underlying mantle convection.
• convection movies from Caltech.
• Los Alamos convection movies.

A bit on comparative planetology.

Harmon D. Maher Jr. reserves copyrights to the materials in this site. Material may be used for non-profit educational purposes as long as proper attribution is given. For permission for any other use please contact author. Thank you.

Return to Physical Geology index page.

Return to my home page index.