Intro to Earth Sciences

Introduction to Earth Sciences I

4.4.2 Structure of Mantle Discontinuities

So far in these lectures you have learned about some of the major divisions within the Earth - the crust, the lithosphere (involved in plate tectonics), the inner and outer core - but there are a number of other boundaries that are much more subtle. The major boundaries involve significant changes in the physical properties within the Earth and some are often associated with major chemical changes also. Several "discontinuities" occur in the mantle that are most likely to be associated with phase transitions. It is believed that the bulk composition of the mantle is of the material peridotite that is composed of an iron and magnesium silicate (Mg, Fe) SiO4. This is the material that lies immediately beneath the crust and is sometimes exposed in oceanic fracture zones and in other tectonically active parts of the Earth. Deep in the Earth the increasing pressure squeezes the peridotite and cause the atoms to form a more compact structure. No chemical reaction occurs, but the re-packing of the atomic structure causes the same atoms to become a new material known as spinel or Ringwoodite. It is still made of Mg, Fe, Si and O (magnesium, iron, silicon and oxygen) but it is a different material because of the atomic packing. This may be a hard idea to understand at first but remember that graphite (the material in your "lead" pencil), coal, and diamonds are all made of carbon. They are just in different atomic arrangements associated with pressure due to burial (diamonds come from very deep within the Earth). A second phase transition occurs from spinel to different material known as perovskite. The first phase transition causes a discontinuity in the mantle about 410 km beneath the surface and the second at about 660 km (they actually vary in depth depending on the tectonic setting.
The physical property changes across these boundaries that occur over quite short distances (on the scale of the mantle) are not large enough to be detectable by seismic tomography which gives a fairly low resolution, smooth picture of the structure of the mantle. To resolve the discontinuities we need to study the very weak direct reflections they cause or from refractions along their surface. Even trickier, there appears to be a very subtle boundary at about 520 km. It too is most likely caused by a phase transition like the 410 and 660 discontinuities. In fact it may be a pair of boundaries, one at 500 and the other at 560 km.

The difficulty with using waves to detect the physical world is that the quality of your image is limited by the wavelength you use. When using a particle as a probe, we need to use particles with short wavelengths to get detailed information about small things. To probe down to smaller scales, the probe's wavelength has to be made smaller.

Things with long wavelengths are analogous to the 'throwing basketballs in the cave to discover what is in growling in front of you' story because neither can provide too much detail about what they hit. Things with short wavelengths are like the 'throwing marbles in the cave to discover what is in growling in front of you' story in that they can provide you with fairly detailed information about what they hit. The shorter the probe's wavelength is, the more information you can get about the target.

The resolution of your measurement is the wavelength of light divided by the diameter of the aperture. This means that you would either need a small wavelength or a very large diameter to have good resolution.

Check out this website for a more detailed explanation. http://pdg.web.cern.ch/pdg/cpep/cave.html

In an innovative investigation shown below seismologists used shear waves that bounced underneath the boundary as shown below.

Figure 4.4.8

They used two energy paths, one that bounced at the Earth's surface and one that bounced off the "520" discontinuity. Because the latter travels a shorter path length it takes less time to reach back to the surface (about 20 seconds less). This second path makes a very weak wiggle in seismograms, but it is definitely present. In some places we see a single "520" wiggle; in other places there appears to be two wiggles suggestive of a splitting of the boundary into one at 500 and one at 560 km.
Detecting these very subtle boundaries requires the analysis of very high quality seismogram data from many different parts of the Earth. Every year it seems, new information about the structure of the mantle comes to light from these studies. One reason they are important is that although they are subtle they represent a partitioning of the mantle into different zones with different physical properties. In particular these properties are thought to have an influence on the pattern of mantle convection and the way in which subducting slabs penetrate into the deep mantle.