Cosmology and astronomy
The mohorovicic seismic discontinuity
The Mohorovicic Seismic Discontinuity- boundary between crust and mantle (or Moho). Created by Sal Khan.
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- The wave arcs drawn at around1:30are 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?(23 votes)
- 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!!(24 votes)
- 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?(8 votes)
- 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.
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.(11 votes)
- 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?(8 votes)
- I have this same question. At4:21Sal 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?(2 votes)
- Where is the next video ?(8 votes)
- 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)
- 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?(6 votes)
- 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.(1 vote)
- The wave speed equations for both P- and S-waves both have density as a term in the denominator.
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?(4 votes)
- 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.(1 vote)
- The seismic waves being measured, are they P waves or S waves?(3 votes)
- 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.(3 votes)
- how do the waves of the earthquake travel underground without grinding up the ground(3 votes)
- I think they do travel on the surface and what´s actually measured is a sum of superficial and deep waves. I assume that shows how fast the underground waves are going is the delay between the detection of the superficial and the deep ones(1 vote)
- at4:05when he draws the curved in in what he called crust , why this lines are curved although he said that a uniform layer ?(3 votes)
- 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.(1 vote)
- How did they accurately measure the speed of the seismic wave back then? As far as I know accurate clocks are a relatively new thing, and I'd suspect you need quite accurate (and synchronous!) clocks for measuring the speed of seismic waves.(2 votes)
- Seismic speeds of rocks can be measured in a laboratory. Also, if you know where exactly an earthquake happened, you can use the same equations shown in the video to work out the speed of the seismic waves.(3 votes)
The first realization that there were actually distinct layers of the earth came from this guy right over here, Andrija Mohorovicic. And I apologize ahead of time to any Croatians for butchering any of the pronunciation. And he was a meteorologist and a seismologist. And he was the first one to notice, in 1909, when there was an earthquake. There was an earthquake in Croatia, a little bit southeast of Zagreb. So the earthquake was roughly over here. And lucky for him and lucky for us, before that earthquake there was actually a bunch of seismographic stations already in the area. And all these seismographic stations are are, essentially, instruments were installed so that if there was any essentially seismic waves passing, they would be able to measure it when the waves got there. And what was interesting about this, Andrija realized that if the entire earth was just kind of a uniform materials-- let's draw that scenario-- it would get denser as you go down. And so you would have kind of this refraction, this continuous refraction, or these curved pats, happening. But he realized that, let's say we had an earthquake right over here, so this is the uniform case. Uniform. Uniform layer, only one layer, although it does get denser. Then the closer you are to the earthquake-- so waves would get there first, then waves would get over there, then waves would you get over there-- and these are the body waves. These are the ones that are traveling through the earth's crust. But in general, the further you are away from the earthquake, or the time it takes for the waves to get to a point, is going to be proportional to the distance that point is away from the earthquake. So you would expect to see something like this. So if you were to plot on the horizontal axis, if you were to plot distance, and on the vertical axis you were to plot time, you should see something like this. You should see a straight line. And that's just because it's traveling roughly the same velocity along any of these arcs. It's maybe getting a little bit faster as it's getting deeper. But roughly the same velocity as it's traveling along these arcs. And the distance of these arcs are proportional to the distance along the surface, along the distance of the surface. So essentially, the time is, they're all traveling roughly at the same velocity, and their just traveling different distances, so the time it takes is just going to be proportional to the distance. But he noticed something interesting. When he actually measured when the waves from that earthquake reached different seismographic stations, he saw something interesting. So this is in the theoretical, if we had a kind of this uniform one-layered earth. But he saw something interesting. So once again, this is the distance, and this right over here is time. And at 200 kilometers, at 200 kilometers away from the earthquake-- so until 200 kilometers, he saw exactly what you would expect from a uniform earth. It was just the time took was proportional to the distance. But at 200 kilometers, he saw something interesting. All of a sudden, the waves were reaching there faster. The slope of this line changed. It took less time for each incremental distance. So for some reason, the waves that we're going at these farther stations, the stations that were more than 200 kilometers away, somehow they were accelerated. Somehow they were able to move faster. And he's the one that realized that this was because the waves that were getting to these further stations must have traveled through a more dense layer of the earth. So let's just think about it. So if we have a more dense layer, it will fit this information right over here. So if we have a layer like this, which we now know to be the crust, and then you have a denser layer, which we now know to be the mantle, then what you would have is-- so you have your earthquake right over here, closer by, while you're still within the crust, it would be proportional. It would be proportional. And then let's say that this is exactly, this right here is 200 kilometers away. But then if you go any further, the waves would have to travel. They would travel, so they would go like this. And then they would get refracted even harder. So they would get refracted. So they would be a little bit curved ahead of time. But then they're going to a much denser material. Or it's not gradually dense, it's actually kind of a all of a sudden a considerably more dense material, so it will get refracted even more. And then it'll go over here. And since it was able to travel all of this distance in a denser material, it would have traveled faster along this path. And so it would get to this distance on the surface that's more than 200 kilometers away, it would get there faster. And so he said that there must be a denser layer that those waves are traveling through, which we now know to be the mantle. And the boundary between what we now know to be the crust and this denser layer, which we now to be the mantle, is actually named after him. It's called the Mohorovicic discontinuity. And sometimes this is called the Moho for short. So that boundary between the crust and the mantle is now named for him. But this was a huge discovery, because not only was he able to tell us, based on the data-- based on, kind of, indirect data, just based on earthquakes happening, and measuring when the earthquakes reach different points of the earth-- that there probably is a denser layer. And if you do the math, under continental crust that denser layer is about 35 kilometers down. He was able to tell us that there is that layer. But even more importantly, he was able to give the clue that just using information from earthquakes, we could essentially figure out the actual composition of the earth. Because no one has ever dug that deep. No one has ever dug into the mantle, much less the outer core or the inner core. In the next few videos, we're going to kind of take this insight, that we can use information from earthquakes, to actually think about how we know that there is an outer liquid core and that there's an inner core, as well. And then, obviously, you could keep going and think about all the different densities within the mantle and all of that. I won't go into that much detail, but I'll see you in the next video.