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Current time:0:00Total duration:6:13

Video transcript

the first realization that there were actually distinct layers of the earth came from this guy right over here Andrea mejor ovie chick 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 south east 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 they 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 you had a if the entire earth was just kind of a uniform material so let me 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 Pat's 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 the closer you are to the earthquake so waves would get there first then waves would get over there then waves would get over there and these are the 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 time you should see something like this you should see a straight line and that's just because you're going it's traveling roughly the same velocity along any of these arcs as maybe getting a little bit faster as it's getting deeper but roughly the same velocity is traveling along these arcs and the dish of these arcs are proportional to the distance along the surface along the distance of the surface so essentially the time is going they're all traveling roughly at the same velocity and they're 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 different when the waves from that earthquake reached different seismographic stations he saw something interesting let me so this is the theoretical if we had a kind of this uniform one layer at 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 were going at these farther stations the stations that were more than 200 kilometers away the station's somehow they were accelerated somehow they were able to move faster and he is 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 there 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 so that they would travel so they would go like this and then they would get refracted even harder so they would get refracted even so they would be a little bit curved ahead of time but then they're going into a much denser material or it's not gradually dense it's actually kind of a all of a sudden considerably more dense materials 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 this distance on the surface that's more than 200 kilometers away it would get there it would get there faster and so he said that there must be there must be a denser a denser layer that those waves are traveling through which we now know to be the mantle and and the boundary between what we now know to be the crust and this denser layer which is we now know to be the mantle is actually named after him it's called the Moho mejor ovv chick discontinuity and sometimes it's just called the Moho for short so that boundary between the cusp 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 you know based on earthquakes happening and measuring when the earthquakes reached different points of the earth that there probably is a denser layer and if you do that Matt 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 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 and the next few videos were kind of 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 of the different densities within the mantle and all of that I won't go into that much detail but I'll see in the next video