Author : Researcher of Geodynamics, email From the earlier discussion, we came to know that convection is only possible in a fluid medium. The idea and theory of plate-tectonics support that mantle motion contribute a major part to to sustain the tectonic movements. This mantle motion occurs by means of convection within mantle. So correlating this two facts, one may come to a conclusion that mantle is in fluid state. However, mantle is much more complicated than that of this simple idea. Let us explore why!
We are at least familiar with the term earthquake, fortunately, if not with the phenomenon itself. Earthquake has various reasons. For the time being, we will not go into those and think about the horror experience of earthquakes, that destroys everything. From the visual impact of the earthquake terror, we may get an idea that earthquake must have a huge energy that can cause such a large scale destruction. Now, earthquakes begin at depth (may range from few km to maximum 700 km), but we feel the energy pound on the surface. Who carries the energy? If we see any sort of energy, like sound, light etc, they propagate as wave. Earthquake also produces waves which carry the energy from interior of the earth to surface. There are two types of wave that propagate through the interior of the earth. One is called longitudinal wave or in terms of earthquake science (known as Seismology) it is P-Wave. This is same as sound wave that can propagate through any medium, regardless it is solid or fluid. There is another kind of wave called transverse wave, or in terms of seismology it is S-wave. This is same as the circular waves produced when a stone is thrown into a pond (Figure 1). If we observe, these waves do not propagate inside the water-body of the pond, but only remain on the air-water interface. Such is happened because, transverse wave cannot propagate through a fluid medium. (Although light is also a transverse wave and can propagate through fluid medium, but light has electromagnetic properties that must not be confused with non EM waves). Hence, if we get a transverse wave propagating through a medium without any interface with other medium, the simple conclusion is the medium is solid.
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