- Xenoliths: Rocks from the sides of magma chambers and conduits are often torn loose and frozen inside an igneous rock. Occasionally xenoliths are brought up from the mantle by volcanic eruptions, allowing us to see the mantle directly.
- Gravitational anomalies: Long history. 19th cent. surveyers in north India consistently came up with persistent errors much greater than expected due to the gravitational influence of the Himalayas. We now use gravitational anomalies to search for density contrasts deep within the earth.
- Meteorites: Unaccreted material that falls to Earth falls into three basic catagories:
- Chondrites: Undifferentiated material thought to represent the material of the Solar Nebula. Contain homogeneously mixed rocky and metalic substances. Most meteroites fall into this catagory.
- Stony meteorites: Differentiated meteorites containing lighter silicate material.
- Iron meteorites: Meteorites consisting of metals, primarily iron and nickel, usually in interlocking crystals.
The latter two catagories are what we would expect from a differentiated body that was later smashed by a planetesimal. Their occurrance demonstrates that differentiation of planetary bodies into rocky and metallic regions definitely has occurred somewhere. (Occasionally, we find a stony iron meteorite that came from the interface of the original body's core and mantle.)
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:
- Earliest study of differential arrival time of S waves indicated the distinction between crust and a more dense mantle.
- Shadow zone - Discovery of the liquid core:
- S waves: These propagate through the Earth and emerge on far side from an angle of 0 deg to 105 deg. (Angle measured from center of Earth) Between 105 and 180 deg they don't pass through. Liquids don't support S waves, so shadow is presumed to be cast by a liquid core.
- P waves also cast a shadow from 105 to 142 deg. This indicates that they are being refracted, i.e. significantly slowed down at some deep interface. P waves move faster through solids than liquids, so this supports the idea that the core is liquid.
- Observation that P waves that reach depth of 5100 km suddenly speed up indicates that the core is divided into a liquid outer core and a solid inner core.
- Reflected waves: These allow independent verification of depth calculations. When velocity is known, time for reflected wave to reach the surface is indicator of depth of reflector.
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
- In 1910 Croatian seismologist Andrija Mohorovicic calculated the depth of a sudden discontinuity in seismic wave velocity, at which seismic waves rapidly accelerate to 8 km/s, the Mohorovicic discontinuity. (mercifully known as the Moho) This is the transition that distinguishes the crust from the mantle.
- Knowing this, we can use seismic waves to map the thickness of the crust. Result:
- Oceanic 5 km
- Continental (avg. 40 km)
- Large mountain ranges: up tp 65 km.
- The upper mantle is part of the lithosphere. Waves propagate quickly. Beneath it, the asthenosphere is a zone of partial melting and ductile flow in which waves are significantly slowed.
- Beneath this, as pressure and density increase, waves show steady acceleration marked by two discontinuities. What's up with that? Note: The upper mantle consists of the rock peridotite which contains large amounts of the mineral olivine.
- At depth of about 400 km, minerological change occurs in which olivine becomes a more close packed polymorph.
- At 650-700 km this happens again, further accelerating seismic waves.
- We've already seen that it has a liquid outer core and a solid inner core.
- So what's it made of? Had to be heavy to account for bulk mass of Earth. Why not Uranium?
- Needed to be something reasonably common in Solar System
- Needs to be compatible i.e. come in reasonably compact ions because of the close packing of materials under so much pressure. Paradoxically, because uranium is such a large ion, comparatively, it tends to be forced toward the surface, such that the crust is actually enriched in it.
- Tests of wave propagation through compressed iron in laboratory
- Iron meteorites.
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:
- Stony meteorites
- Iron meteorites