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Current time:0:00Total duration:5:49

Video transcript

so how do we know that there's a core and that the core is made up of a liquid outer core and a solid inner core and the answer there comes from the same technique that we saw mejor Ovie cheeked use in 1909 to essentially see how the behavior or when you measure the seismic waves or whether you can even measure the seismic waves the different distances from an earthquake so if there's an earthquake right here and we're talk calling that zero degrees let's remember a couple of things here let's remember that P waves P waves can travel through anything they can travel through solid or liquid or air for that matter so they can travel through anything anything but S waves can only ask for secondary S waves can only these are the transverse waves these can only only travel through solids so it turns out that if an earthquake happens at zero degrees and you had seismograph stations all over the world and these are extremely sensitive in order to be able to measure earthquakes that are happening thousands of kilometers away it turns out that there's something called an S shadow you can an S wave a shadow you can measure if these are S waves you can measure them here you can measure them here they can go all the way over here they can go over here they can go over there you can measure them over here you could so you can measure them at all of these points but then all of a sudden all of a sudden at 105 degrees and so we're measuring zero degrees here and we're going outwards like that all of a sudden at 105 degrees and further you stop measuring S waves they don't get for some reason you would think that some that the S waves would get over here they would get over here maybe they would be a little bit weaker but they would be able to get all the way over here but they just abruptly stop no more S waves so in this whole area right over here you get no S waves you get no S waves and obviously I could flip this picture over and you would see a symmetric thing on the other side of the globe that all of all of this area over here you also would not see you would also not see S waves you would only see them all from 105 degrees in this direction and 105 degrees in that direction and the only reasonable explanation that can give is that there must be some material that an s-wave cannot travel through that it would have to travel through to get to these points beyond 105 degrees and we know that s waves only travel in solids so the assumption there is that saw at some point beyond 105 degrees it's hitting liquid so that's what tells us this right here is probably this right here is probably a liquid it's hitting some layer that is liquid so that tells us that there's a core and at least the outer part of that core is liquid enough to stop enough to stop S waves so the S waves because it only travels in solids it leads to this S wave S wave shadow and this tells us that we have a core that we have a core and that core at least the outer part is liquid we don't know we we don't know yet whether the inner part is liquid or solid now the next point of engine is how do we know that there's an inner corner we can use P waves for that the P wave a few wave can travel through anything but remember as you get denser material as you get in general for the same type of material if you get denser material it's going to move faster source we're gonna refract outwards like we've seen over here but if it goes into a liquid in general sound waves or I should say P waves seismic waves move slower moves slower in liquids and so the refraction patterns we get when we do measure from seismograph stations around the world is that it looks like the P waves are there kind of doing this what you would expect in the mantle but then they're getting refracted as if they're going into a slower medium as they go through the outer core and we see that right over here and then they get refracted again to get to some point on the other side now that is just what you would expect if it was all liquid but if you go to kind of if you go to stations that are even further out it looks like if you just look at the refraction patterns and you can now model this with fancy computers and get all the data points but you could say well the only way to the only way that reality can fit the data that we get based on when things reach here is if the P waves are being first refracted through the outer core but then there are four a fracture in a way that they're going through denser material significantly denser material in the inner core in the inner core and then they're just continuing to refract the way you would expect so it's really the refraction pattern of the p-waves and frankly the fact that there's this what you call a p-wave shadow the p-wave shadow by itself all that tells you is that kind of roughly crazy things are happening someplace in the core but the real way to know that we have an inner core that's solid as opposed to the whole thing being liquid is that the p-waves is the pattern of when and how the p-waves reach essentially the other side of the globe then you can kind of based on modeling how how waves would travel through different densities and different types of mediums you could say well there's gotta be an inner core right over here and obviously it's a lot more math than I'm going into but if you if you do the math based on the shadow and you know the speed of the material and all of that type of thing then you can figure out the the the the depth at which these transitions occur we know that we have a transition from mantle to outer core here and then a transition from outer core to core there so hopefully that that satiates your questions about how do we know what the composition of the earth is without ever having digging without without having to dig down there because we've never even gotten below our crust