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### Course: Cosmology and astronomy>Unit 3

Lesson 2: Seismic waves and how we know earth's structure

# The mohorovicic seismic discontinuity

The Mohorovicic Seismic Discontinuity- boundary between crust and mantle (or Moho). Created by Sal Khan.

## Want to join the conversation?

• The wave arcs drawn at around are constantly bending up towards the surface. In the video on refraction, Sal showed how increasing density caused a bend toward the surface. However, once the wave is moving parallel to the curvature of the earth, why doesn't the angle decrease or stop? On the second half of the arc, while it is moving closer to the surface of the earth, the density is decreasing, so why doesn't the wave start curving the other direction?
• If the wave started curing in the other direction it would mean the layers of density would have reversed, the layers would be getting more dense. But they don't. Going up, the wave would be traveling into less dense material, thus, turning toward the left. At first I didn't get it either, but I drew it out the way Sal did in Refraction of Seismic Waves. You should also try it!! It all made sense to me after that. Good luck!!
• Why does the change in density happen in a discrete manner, not gradual? i.e why the graph is 2 straight lines, the first half steeper than the second, instead of being a curve tending toward a horizontal?
Yes, it's because we have different layers. But that begs the question, why is the layering discrete instead of a gradient? After all, I presume neither the pressure on the rocks nor the temperature of the rocks happen in discrete fashions?
• The reason is that the chemical composition of the mantle is different than that of the crust. Oceanic crust mainly consists of basalt rocks, which has a density of about 3 g/cm^3.
Continental crust mainly consists of silicates (minerals that contain silicon and oxygen) and the overall density is about 2.7 g/cm^3.
The mantle has a much higher content of Mg and Fe and a lower content of Si and therefore it's made up of denser minerals (mainly olivine and pyroxenes) with a density generally greater than 3.2 g/cm^3.
So within the crust the density increases gradually and slowly, at the Mohorovici discontinuity there is a jump in the density due to a change in chemical/mineralogic composition.
P.S.:
Within the mantle there are also discontinuities, which are not based on a change in chemical composition, but the atomic ordering (i.e. the crystal structure of the minerals), which changes when the pressure becomes too high a mineral becomes unstable (pressure of the phase transition). The mineral olivine is responsible for discontinuities at 410 km, 520 km and 660 km below sea level. The latter one is actually a decomposition into two minerals, of which one received its name "Bridgmanite" in 2014 after being observed for the first time in a meteorite.
• I get that the waves speed up when they get to denser material, but then don't they also slow back down as they come back through the crust?
• I have this same question. At Sal shows the refraction of the wave going through the denser material, but when the wave then passes back into the less dense material why is it not refracted again?
• So when we feel an earthquake, and we're 100 miles from the epicenter, we're actually feeling body waves and not surface waves? Or are we feeling both?
• The body wave can affect the surface, but that does not characterize the body wave as a surface wave. So the earthquake described is a body wave, from start to end. At least, that's how I interpreted it.
• Where is the next video ?
• This video is out of order. He talks in length about the "innards" of the earth and how we know what we know from seismic waves.
(1 vote)
• The wave speed equations for both P- and S-waves both have density as a term in the denominator.
https://en.wikipedia.org/wiki/P-wave
Sal says that as density increases, speed does also, but wouldn't the equation suggest that body waves travel slower in more dense materials and not faster? As density increases, the output to this equation would decrease as well. Why does it speed up?
• I don't really know the correct answer to this but I am curious and started reading the wikipedia article you attached in your comment. The formula you are looking at (where density appears as a denominator) applies for homogeneous isotropic mediums which means uniform mediums with out irregularities. That is not the case to calculate the velocity of these p-waves on earth but it is the right formula to calculate the velocity of p-waves on a uniform body, where density would be constant for that specific body. I believe that in order to calculate the velocity of the waves through the different mediums of Earth you would have to use Birch's law. In this formula the density is multiplied and then added. Please if someone can correct me I would appreciate it.
• The seismic waves being measured, are they P waves or S waves?
• He only specifies body waves, which can be either P or S waves. Since P waves would be the first to arrive at the seismograph (they're faster), that might be the answer, but it was probably the case that both waves were being measured and compared.
• at when he draws the curved in in what he called crust , why this lines are curved although he said that a uniform layer ?
• The MATERIAL is uniform, but the DENSITY of the material CHANGES. That's what confused me at first. It is also what causes the gradual refraction. You have to pay close attention to specifically what he's talking about when he mentions uniform mass.